Full opinion text
BOWNES, Senior Circuit Judge. The defendant-intervenor Loon Mountain Recreation Corporation (“Loon Corp.”) operates a ski resort in the White Mountain National Forest in Lincoln, New Hampshire. In order to expand its skiing facilities, Loon Corp. sought and received a permit to do so from the United States Forest Service. Appellant Roland Dubois sued the Forest Service alleging violations of the National Environmental Policy Act (“NEPA”), 42 U.S.C. § 4321, et seq., the Clean Water Act (“CWA”), 33 U.S.C. § 1251, et seq., the Administrative Procedure Act, 5 U.S.C. § 501, et seq. (“APA”), and Executive Order 11,-990, 42 Fed.Reg. 26,961 (1977), reprinted as amended in 42 U.S.C.A. § 4321 (West 1994). Appellant RESTORE: The North Woods (“RESTORE”) intervened as a plaintiff claiming violations of the same statutes, and appellee Loon Corp. intervened as a defendant. Dubois and RESTORE (collectively referred to as “plaintiffs”) and the Forest Service filed cross-motions for summary judgment, and Loon moved to dismiss. The district court granted the Forest Service’s motion for summary judgment and denied the other motions. We affirm in part, reverse in part, and remand. I. STATEMENT OF THE CASE A. Facts The White Mountain National Forest (“WMNF”) is a public resource managed by the United States Forest Service for a wide range of competing public uses and purposes, including “outdoor recreation, range, timber, watershed, ... wildlife and fish purposes,” 16 U.S.C. § 528 (1994), and skiing, 16 U.S.C. § 497(b) (1994). Pursuant to the National Forest Management Act of 1976, the Forest Service makes long-term plans to coordinate these competing uses, 16 U.S.C. § 1604(e)(1) (1994), and issues “special use” permits authorizing private recreational services on national forest land, 36 C.F.R. §§ 251.50-.65 (1995). The Forest Service’s exercise of its permitting authority is legally constrained by environmental considerations emanating, inter alia, from NEPA, the CWA, and Executive Order 11,990. Loon Pond is located in the WMNF at an elevation of 2,400 feet. It has a surface area of 19 acres, with shallow areas around the perimeter and a central bowl 65 feet deep. It is unusual for its relatively pristine nature. There is virtually no human activity within the land it drains except skiing at the privately owned Loon Mountain Ski Area. New Hampshire Department of Environmental Services (“NHDES”) regulations classify Loon Pond as a Class A waterbody, protected by demanding water quality standards under a variety of criteria, see N.H.Code Admin. R. Env-Ws 432.03, and as an Outstanding Resource Water (“ORW’), protected against any measurable long-term degradation by the State’s anti-degradation rules, see id. 437.06; 40 C.F.R. § 131.12(a)(3) (1995). It ranks in the upper 95th percentile of all lakes and ponds in northern New England for low levels of phosphorus, which results in limited plant growth and therefore high water clarity and higher total biological production. The pond súpports a rich variety of life in its ecosystem. Loon Pond also constitutes a major source of drinking water for the town of Lincoln 1,600 feet below it. A dam across the outlet of the Pond regulates the flow of water from the Pond to Lincoln’s municipal reservoir. Loon Corp., defendant-intervenor herein, owns the Loon Mountain Ski Area, which has operated since the 1960s not far from Loon Pond. Prior to the permit revision that gave rise to this litigation, Loon Corp. held a special use permit to operate on 785 acres of WMNF land. That permit allowed Loon Corp. to draw water (“drawdown”) for snow-making from Loon Pond, as well as from the East Branch of the Pemigewasset River (“East Branch”) and from nearby Boyle Brook. In order to use water from Loon Pond, Loon Corp. also needed authorization from the Town of Lincoln and the State of New Hampshire. Beginning in 1974, Loon Corp. was authorized to pump snowmaking water from Loon Pond down to 18 inches below full level. A 1988 amendment to this agreement permitted drawdown below the 18-inch level on a case-by-case basis. Combined uses by Lincoln and Loon Corp. during the period governed by these agreements typically caused four- to six-foot fluctuations in the level of Loon Pond. In addition to being used as a source of water for snowmaking, Loon Pond has been the repository for disposal of water after it is pumped through the snowmaking system. This includes water that originally came from Loon Pond, as well as water that originated in the East Branch or in Boyle Brook. Approximately 250,000 gallons of East Branch water have been transferred into Loon Pond each year in this manner. Obviously the water discharged into Loon Pond contains at least the same pollutants that were present in the intake water. Evidence in the record indicates that intake water taken from the East Branch contains bacteria, other aquatic organisms such as Giardia lambía, phosphorus, turbidity and heat. Evidence was also introduced in court, but not available prior to the issuance of the Environmental Impact Statement (“EIS”), that oil and grease were present in the discharge water, although their source was disputed. In 1986, Loon Corp. applied to the Forest Service for an amendment to its special use permit to allow expansion of its facilities within the WMNF. Pursuant to NEPA, 42 U.S.C. § 4382, the Service developed a draft EIS, and a supplement to the draft. Responding to criticism of the adequacy of those documents, the Forest Service issued a revised draft EIS (“RDEIS”), which was published for public comment. The RDEIS set forth five alternatives to meet the perceived demand for additional alpine skiing. All five were located at the Loon Mountain site. Many individuals and groups, including both plaintiffs, filed comments pointing out various environmental problems with each alternative that involved expanding the ski area. One lengthy comment from the U.S. Environmental Protection Agency (“EPA”) expressed its concern that the use of Loon Pond for snowmaking purposes would “use Loon Pond like a cistern” instead of treating it “with care” because it is “acknowledged to be one of the rare high altitude ponds of its size in the White Mountains.” Joint Appendix (“JA”), vol. II, Response to Public Comment on RDEIS at A-78. Other eommen-ters suggested that Loon Corp. be required to build artificial water storage ponds, in order to eliminate the problem of depleting Loon Pond when withdrawing water for snowmaking as well as the problem of adding pollutants to Loon Pond when discharging water into the Pond after use. During the EIS process, Ron Buso, a hydrologist for the WMNF, expressed concern to another Forest Service hydrologist that the proposed drawdown of Loon Pond by twenty feet was likely to have a severe impact on the Pond. He explained that natural snowmelt in New Hampshire is extremely acidic and that, as a result of the planned drawdown, a substantial amount of acidic snowmelt would remain in Loon Pond, increasing the Pond’s acidity by a factor of two to three times what it would be without the planned drawdown. Without the drawdown, Loon Pond would be relatively full in the spring, and much of the snowmelt from surrounding higher elevations would glide over the surface of the Pond and down the mountain without significantly mixing with other Loon Pond water. According to Buso and a number of scientists whose affidavits were submitted to the district court, the increase in the Pond’s acidity due to the planned drawdown would change the chemistry of the Pond, cause toxic metals to be released from the sediment, and kill naturally occurring organisms. Without addressing the issues raised in the Buso memorandum or in the comments suggesting artificial storage ponds, the Forest Service prepared a Final EIS (“FEIS”). The FEIS added a sixth alternative, also on the Loon Mountain site. The new alternative provided for expansion of Loon Corp.’s permit area by 581 acres and for the construction of one new lift and approximately 70 acres of new ski trails, changes designed to accommodate 3,200 additional skiers per day (from the current 5,800 per day). The Forest Service deemed Alternative 6 as the preferred alternative. Under it, Loon Corp. would more than double the amount of water used for snowmaking, from 67 million gallons per year to 138 million gallons. Seventeen million gallons of the increase would be drawn from the East Branch, and 54 million gallons from Loon Pond. In addition, Loon Corp. was authorized to draw the Pond down for snowmaking by fifteen feet, compared to the current eighteen inches. The Forest Service assumed that the Town of Lincoln would need up to an additional five feet of Pond water, making a total of twenty feet that the Pond was expected to be drawn down each year. This would constitute approximately 63% of the Pond’s water. In March 1993, the Forest Service published a Record of Decision (ROD) adopting Alternative 6. As a mitigation measure to blunt the adverse environmental impact on Loon Pond, the Forest Service required Loon Corp. to pump water from the East Branch to Loon Pond in December and May of each year if the Pond was not otherwise full at those times. In its FEIS, the Forest Service recognized that the East Branch is a relatively unprotected Class B waterway under New Hampshire law, and that transfer of East Branch water to Loon Pond, a protected Class A waterbody and Outstanding Resource Water under state and federal law, would introduce pollutants into the Pond. Accordingly, it specified that this transfer of East Branch water could not occur if it ex-eéeded certain levels of turbidity, bacteria, or oil and grease. Neither the FEIS nor the ROD set any limits, however, on the level of non-bacterial organisms such as Giardia lam-bía or on pollutants such as phosphorus that may be present in the transferred water. Nor did the FEIS indicate an alternative means of refilling Loon Pond—with clean water—if conditions were such that the transfer of East Branch water would exceed the specified levels. It did, however, provide a series of restrictions and monitoring requirements for water levels and water quality, including daily testing of the transferred water for turbidity, bacteria, and oil and grease. Dubois and RESTORE appealed the ROD to the Regional Forester and, thereafter, to the Chief of the Forest Service. These appeals were denied. On March 16, 1994, the Forest Service issued a special use permit to Loon Corp., implementing the decision described in the ROD. B. Proceedings Below Plaintiff Dubois filed a complaint in the United States District Court for the District of Columbia, challenging the Forest Service’s approval of the Loon Mountain expansion project. He made three arguments. First, he argued that the Forest Service actions violated the CWA because they would lead to violations of state water quality standards which, he asserted, have the effect of federal law because they were approved by the federal EPA. Second, he argued that the Forest Service violated both NEPA and Executive Order 11,990 by failing to consider alternatives to the use of Loon Pond and failing to develop adequate mitigation measures. Finally, he argued that the Forest Service violated the CWA, 33 U.S.C. § 1311, by failing to obtain a National Pollutant Discharge Elimination System (“NPDES”) permit before approving Loon Corp.’s expansion plans, which entailed removing water from the East Branch, using it to pressurize and prevent freezing in its- snowmaking equipment, and then discharging the used water into Loon Pond. According to Dubois, an NPDES permit was required in order for Loon Corp. to discharge pollutants into Loon Pond, including the discharge from Loon Corp.’s snowmaking equipment. Plaintiff RESTORE, a membership organization, intervened on behalf of its members to challenge the project. RESTORE first reiterated Dubois’ claim that an NPDES permit was required. In addition, RESTORE claimed that the Forest Service violated NEPA by failing to prepare a Supplemental EIS after it developed Alternative 6 as the preferred alternative. According to RESTORE, this new alternative, not specifically mentioned in the previously published draft EIS or RDEIS, contained substantial changes to the proposed action that are relevant to environmental concerns, which required a supplemental EIS under NEPA and relevant implementing regulations. Finally, RESTORE claimed that a supplemental EIS was required because the Forest Service’s Final EIS failed to “rigorously explore and objectively evaluate all reasonable alternatives” that are capable of meeting the stated goals of the project, as required by 40 C.E.R. § 1502.14 (1995). According to RESTORE, the asserted goal of meeting skier demand could have been met by expanding ski areas other than Loon, in particular, ski areas located outside the White Mountain National Forest. The parties cross-moved for summary judgment. Loon Corp. intervened, and moved to dismiss on the ground that both plaintiffs lacked standing. The district court denied Loon Corp.’s motion to dismiss, granted summary' judgment for the Forest Service, and denied the plaintiffs’ cross-motions for summary judgment. II. DUBOIS’STANDING The ingredients of standing are imprecise and not easily susceptible to concrete definitions or mechanical applications. Allen v. Wright, 468 U.S. 737, 751, 104 S.Ct. 3315, 3324, 82 L.Ed.2d 556 (1984). In order to have standing to sue, a plaintiff must have “such a personal stake in the outcome of the controversy as to assure that concrete adverseness which sharpens the presentation of issues upon which the court so largely depends for illumination of difficult ... questions.” Baker v. Carr, 369 U.S. 186, 204, 82 S.Ct. 691, 703, 7 L.Ed.2d 663 (1962). Standing consists of both a constitutional aspect and a prudential aspect. The constitutional dimension derives from the requirement that federal courts can act only upon a justiciable case or controversy. U.S. Const. art. III. If a party lacks Article III standing to bring a matter before the court,, the court lacks subject matter jurisdiction to decide the merits of the underlying ease. FW/PBS, Inc. v. City of Dallas, 493 U.S. 215, 231, 110 S.Ct. 596,' 607-08, 107 L.Ed.2d 603 (1990). To satisfy the constitutional component of standing, a plaintiff must have suffered an “injury in fact,” i.e., an invasion of a legally protected interest. Lujan v. Defenders of Wildlife, 504 U.S. 555, 560, 112 S.Ct. 2130, 2136, 119 L.Ed.2d 351 (1992). That injury must be “concrete and particularized”; the latter term means the injury must be personal to the plaintiff. Id. at 560 & n. 1, 112 S.Ct. at 2136 & n. 1. It may be shared by many others, United States v. Students Challenging Regulatory Agency Procedures (SCRAP), 412 U.S. 669, 687-88, 93 S.Ct. 2405, 2415-16, 37 L.Ed.2d 254 (1973), but may not be common to everyone, see Warth v. Seldin, 422 U.S. 490, 499, 95 S.Ct. 2197, 2205, 45 L.Ed.2d 343 (1975). The injury must also be “actual or imminent, not conjectural or hypothetical,” Defenders of Wildlife, 504 U.S. at 560, 112 S.Ct. at 2136 (quotation omitted), and it must be “distinct and palpable,” Warth, 422 U.S. at 501, 95 S.Ct. at 2206. The latter requirement may be satisfied by environmental or aesthetic injuries. See SCRAP, 412 U.S. at 686, 93 S.Ct. at 2415; Sierra Club v. Morton, 405 U.S. 727, 734, 92 S.Ct. 1361, 1366, 31 L.Ed.2d 636 (1972). The injury need not be “significant”; a “small” stake in the outcome will suffice, if it is “direct.” SCRAP, 412 U.S. at 689 n. 14, 93 S.Ct. at 2417 n. 14. In addition, the injury must be fairly traceable to the defendant’s allegedly unlawful conduct and likely to be redressed by the requested relief. Defenders of Wildlife, 504 U.S. at 560-61, 112 S.Ct. at 2136. The doctrine of standing also includes prudential concerns relating to the proper exercise of federal jurisdiction. Among these concerns is the requirement that “a plaintiffs complaint fall within the zone of interests protected by the law invoked.” Allen, 468 U.S. at 751, 104 S.Ct. at 3324. In addition, as a general rule, a plaintiff “must assert his own legal rights and interests, and cannot rest his claim to relief on the legal rights or interests of third parties.” Warth, 422 U.S. at 499, 95 S.Ct. at 2205. A membership organization constitutes an exception to this general rule: it may assert the claims of its members, provided that one or more of its members would satisfy the individual requirements for standing in his or her own right. See UAW v. Brock, 477 U.S. 274, 281-82, 106 S.Ct. 2523, 2528-29, 91 L.Ed.2d 228 (1986). The burden falls on the plaintiff “clearly to allege facts demonstrating that he is a proper. party to invoke” federal jurisdiction. Warth, 422 U.S. at 518, 95 S.Ct. at 2215. The plaintiff must “set forth reasonably definite factual allegations, either direct or inferential, regarding each material element needed to sustain standing.” United States v. AVX Corp., 962 F.2d 108, 115 (1st Cir.1992). “[E]aeh element must be supported in the same-way as any other matter on which the plaintiff bears the burden of proof, i.e., -with the manner and degree of evidence required at the successive stages of the litigation.” Defenders of Wildlife, 504 U.S. at 561, 112 S.Ct. at 2136. At the pleading stage, “general factual allegations of injury resulting from the defendant’s conduct may suffice, for on a motion to dismiss we ‘presum[e] that general allegations embrace those specific facts that are necessary to support the claim.’ ” Id. (quoting. Lujan v. National Wildlife Federation, 497 U.S. 871, 889, 110 S.Ct. 3177, 3189, 111 L.Ed.2d 695 (1990)). The district court denied Loon Corp.’s motion to dismiss Dubois’ claims on standing grounds, relying on our precedent in Washington Legal Found, v. Massachusetts Bar Found., 993 F.2d 962, 971-72 (1st Cir.1993). In that case, we held that the court need not determine the standing of all plaintiffs if at least one plaintiff has standing to maintain each claim. The district court found that RESTORE had standing to bring all the claims at issue in this case, and, therefore, that the court could reach the merits of all claims without first addressing Dubois’ standing. We agree that RESTORE would have standing to raise, on behalf of its members, all the issues in dispute in this litigation. But the district court erred in concluding that it could therefore reach the merits of all claims, because the district court’s premise was incorrect: RESTORE did not, even at the 'district court level, raise the issues relating to Executive Order 11,990 and the state water quality standards, which only Dubois is pursuing here. The situation is not, therefore, analogous to Washington Legal Foundation; if Dubois has no standing, we cannot decide issues that RESTORE has never raised. We find, however, that Dubois does satisfy all requirements for standing to litigate the claims he seeks to pursue on appeal. His second amended complaint alleged that [his] principal residence from 1959-1977 was in Lincoln, New Hampshire. [He] has returned to the Lincoln area at least once per year-and occasionally up to twelve or more times per year —since 1977. During these trips, [he] has visited relatives and friends, collected botanical samples for scientific analysis, and engaged in recreational activities in and around the WMNF and the Loon Mountain Ski Area. Plaintiffs interest in the environmental, recreational and aesthetic quality of the WMNF are and will be adversely affected by the Defendants’ actions challenged in this Complaint. Second Amended Complaint, ¶ 5. The last sentence is rather conclusory, but the entire complaint, taken together with inferences reasonably drawn from its allegations, contains sufficient “reasonably definite factual allegations,” AVX, 962 F.2d at 115; to survive a motion to dismiss. “We are mindful that, under the notice pleading requirements of the federal rules, the allegations of the complaint should be construed favorably to the complainant on a motion to dismiss.” Papex Int’l Brokers v. Chase Manhattan Bank, 821 F.2d 883, 886 (1st Cir.1987). Moreover, as noted supra, at the pleading stage, “we presumfe] that general allegations embrace those specific facts that are necessary to support the claim.” Defenders of Wildlife, 504 U.S. at 561, 112 S.Ct. at 2137 (quotation omitted). Further, the record reveals that the district court had adduced additional information during its consideration of the standing issue. See AVX, 962 F.2d at 114 n. 6 (appellate court considering standing issue went beyond the complaint “in a record-wide search for facts supporting” the claim of standing). Dubois’ local counsel represented to the court that Dubois continues to return “regularly,” at least annually, to his parents’ home in Lincoln; that he drinks the water from the “Town of Lincoln water supply that comes down from Loon Pond”; that he “walks those mountains” in the WMNF. Transcript of Hearing, June 14, 1995, at 7-9. The court expressed its understanding of Dubois’ standing allegations as follows: Mr. Dubois’ injury in fact is he periodically comes back to the area and enjoys its natural beauty and will be injured by not being able to experience its natural beauty if the project is allowed to go forward? ... It’s not a case of someone who’s simply saying I’m an environmentalist and I want to protect the environment, which everybody presumably has an interest in doing. It’s somebody who says I’m back there a lot, I drink the water a lot, I’m up there in the woods a lot, and this is going to hurt me. Id. at 8,12. We think it useful to compare the facts here with those alleged in AVX, 962 F.2d at 116-17. In AVX, the plaintiff organization had simply made conclusory allegations that its “members have been and will continue to be harmed by the releases that [were] the subject of [that] litigation”; its “averment [had] no substance: the members [were] unidentified; their places of abode [were] not stated; the extent and frequency of any individual use of the affected resources [was] left open to surmise.” Id. This court in AVX pointed to the allegations in SCRAP, 412 U.S. 669, 93 S.Ct. 2405, as attenuated as they were, in which “there was a geographic nexus; all the association’s members resided in a single, defined metropolitan area, -directly affected by the challenged action.... In SCRAP, unlike [AVX], the claimed environmental injury was tied to the particular pursuits of particular persons.” AVX, 962 F.2d at 117. The instant case, in contrast with AVX, presents a particular person, whose family home is located squarely within the geographical area allegedly directly affected by the proposed project, who visits the area regularly, who drinks the water which will allegedly be tainted by pollutants, and who will allegedly be deprived of his environmental, aesthetic and scientific interests in ways directly tied to the project he challenges. These are the types of interests which the Supreme Court has held-when asserted by an organization such as RESTORE on behalf of its members — satisfy the constitutional requirements for standing. See SCRAP, 412 U.S. at 685-87, 93 S.Ct. at 2415-16; Sierra Club v. Morton, 405 U.S. at 734-35 & n. 8, 92 S.Ct. at 1366 & n. 8; see also supra, note 10. There is certainly no reason why an organization would have standing to raise these interests on behalf of its members, but an individual such as Dubois would not have standing to raise the same interests on his own behalf. Thus, with the degree of specificity necessary at the pleading stage, Dubois has articulated— directly and by inference-how his personal interests will be adversely affected by the Loon expansion proposal. Finally, his injuries are “likely to be redressed” by the relief he has requested in the complaint: inter alia, an injunction against the project’s proceeding. See Defenders of Wildlife, 504 U.S. at 560-61, 112 S.Ct. at 2136. As for the prudential standing requirements, there is no dispute that the violations and injuries alleged in the complaint are the sort that NEPA, the CWA, and the Executive Order were “specifically designed” to protect. See Lujan v. National Wildlife Federation, 497 U.S. at 886, 110 S.Ct. at 3187. Moreover, our discussion above related only to Dubois’ own legal rights and interests, not those of third parties. Accordingly, we find that Dubois has standing to litigate the claims he seeks to pursue on appeal. III. STANDARD OF REVIEW The district court’s order granting summary judgment is subject to de novo review. Borschow Hosp. and Medical Supplies v. Cesar Castillo, Inc., 96 F.3d 10, 14 (1st Cir. 1996); Lawrence v. Northrop Corp., 980 F.2d 66, 68 (1st Cir.1992). We independently weigh the merits of the summary judgment motions “without deference to the reasoning of the district court.” Hughes v. Boston Mut. Life Ins. Co., 26 F.3d 264, 268 (1st Cir.1994). Accordingly, we must reverse the court’s grant of summary judgment unless “there is no genuine issue as to any material fact and ... the moving party is entitled to a judgment as a matter of law.” Fed.R.CivJP. 56(c). In analyzing the issues, we will review the record in the light most favorable to the non-movants, and make all inferences in their favor. Borschow, 96 F.3d at 14; Petitti v. New England Tel. & Tel. Co., 909 F.2d 28, 31 (1st Cir.1990). It is well established that a reviewing court may not set aside administrative decisions “simply because the court is unhappy with the result reached.” Baltimore Gas & Elec. Co. v. Natural Resources Defense Council, Inc. (“NRDC”), 462 U.S. 87, 97, 103 S.Ct. 2246, 2252, 76 L.Ed.2d 437 (1983) (quoting Vermont Yankee Nuclear Power Corp. v. NRDC, 435 U.S. 519, 558, 98 S.Ct. 1197, 1219, 55 L.Ed.2d 460 (1978)). The fundamental policy questions are “appropriately resolved in Congress and in the state legislatures”; they “are not subject to reexamination in the federal courts under the guise of judicial review of agency action.” Vermont Yankee, 435 U.S. at 558, 98 S.Ct. at 1219. Courts may set aside agency decisions “only for substantial procedural or substantive reasons as mandated by statute.” Id. The applicable statutes here are NEPA and the CWA. NEPA requires that the agency take a “hard look” at the environmental consequences of a project before taking a major action. Baltimore Gas, 462 U.S. at 97, 103 S.Ct. at 2252 (quoting Kleppe v. Sierra Club, 427 U.S. 390, 410 n. 21, 96 S.Ct. 2718, 2730 n. 21, 49 L.Ed.2d 576 (1976)). It is the role of the courts on judicial review to ensure “that this legal duty is fulfilled.” Foundation on Economic Trends v. Heckler, 756 F.2d 143, 151 (D.C.Cir.1985). Congress, in enacting NEPA, meant “to insure a fully informed and well-considered decision.” Vermont Yankee, 435 U.S. at 558, 98 S.Ct. at 1219. But NEPA “does not mandate particular results”; it “simply prescribes the necessary process.” Robertson v. Methow Valley Citizens Council, 490 U.S. 332, 350, 109 S.Ct. 1835, 1846, 104 L.Ed.2d 351 (1989). “If the adverse environmental effects of the proposed action are adequately identified and evaluated, the agency is not constrained by NEPA from deciding that other values outweigh the environmental costs.” Id.; see also Baltimore Gas, 462 U.S. at 97, 103 S.Ct. at 2252. Thus, “[t]he role of the courts is simply to ensure that thagency has adequately considered and disclosed the environmental impact of its actions and that its decision is not arbitrary or capricious.” Baltimore Gas, 462 U.S. at 97-98, 103 S.Ct. at 2252 (emphasis added). Like NEPA, the CWA does not articulate its own standard of review; therefore the appropriate scope of review for both NEPA claims and CWA claims is the standard set forth in the APA. 5 U.S.C. § 706(2)(A) (1994); see Town of Norfolk v. U.S. Army Corps of Engineers, 968 F.2d 1438, 1445 (1st Cir.1992); Oregon Natural Resources Council v. U.S. Forest Service, 834 F.2d 842, 851-52 (9th Cir.1987). Under the APA, “[t]he reviewing court shall ... hold unlawful and set aside agency action, findings, and conclusions found to be arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law.” 5 U.S.C. § 706(2)(A). Errors of law are reviewed by the court de novo. 5 U.S.C. § 706 (1994) (“the reviewing court shall decide all relevant questions of law”); Howard v. FAA, 17 F.3d 1213, 1215 (9th Cir.1994). On the other hand, the task of a court reviewing agency action under the APA’s “arbitrary and capricious” standard, 5 U.S.C. § 706(2), is “to determine whether the [agency] has considered the relevant factors and articulated a rational connection between the facts found and the choice made.” Baltimore Gas, 462 U.S. at 105, 103 S.Ct. at 2256 (emphasis added) (citations omitted); see also Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43, 103 S.Ct. 2856, 2866-67, 77 L.Ed.2d 443 (1983); Bowman Transp., Inc. v. Arkansas-Best Freight Sys., Inc., 419 U.S. 281, 285-86, 95 S.Ct. 438, 442, 42 L.Ed.2d 447 (1974); Citizens to Preserve Overton Park, Inc. v. Volpe, 401 U.S. 402, 415-17, 91 S.Ct. 814 823-24, 28 L.Ed.2d 136 (1971). If the agency decision was based on a consideration of the relevant factors and there has not been “a clear error of judgment,” then the agency decision was not arbitrary or capricious. Overton Park, 401 U.S. at 416, 91 S.Ct. at 823-24; Marsh v. Oregon Natural Resources Council, 490 U.S. 360, 378, 109 S.Ct. 1851, 1861, 104 L.Ed.2d 377 (1989). In State Farm, the Supreme Court offered several examples of circumstances in which an agency action “normally” would be considered arbitrary and capricious: situations where “the agency has relied on factors which Congress has not intended it to consider, entirely failed to consider an important aspect of the problem, offered an explanation for its decision that runs counter to the evidence before the agency, or is so implausible that it could not be ascribed to a difference in view or the product of agency expertise.” State Farm, 463 U.S. at 43, 103 S.Ct. at 2867. These are merely “examples,” Puerto Rico Sun Oil Co. v. U.S. EPA, 8 F.3d 73, 77 (1st Cir.1993); others could be recited as well. Whether reviewing an EIS or a rulemaking proceeding, the “reviewing court should not attempt itself to make up for such deficiencies; we may not supply a reasoned basis for the agency’s action that the agency itself has not given.” State Farm, 463 U.S. at 43, 103 S.Ct. at 2867 (citing SEC v. Chenery Corp., 332 U.S. 194, 196, 67 S.Ct. 1575, 1577, 91 L.Ed. 1995 (1947)). ‘While this is a highly deferential standard of review, it is not a rubber stamp.” Citizens Awareness Network, Inc. v. U.S. Nuclear Regulatory Comm’n, 59 F.3d 284, 290 (1st Cir.1995). Although “the ultimate standard of review is a narrow one,” the court must undertake a “thorough, probing, indepth review” and a “searching and careful” inquiry into the record. Overton Park, 401 U.S. at 415-16, 91 S.Ct. at 823-24. In order for an agency decision to pass muster under the APA’s “arbitrary and capricious” test, the reviewing court must determine that the decision “makes sense.” Puerto Rico Sun Oil, 8 F.3d at 77. Only by “carefully reviewing the record and satisfying [itself] that the agency has made a reasoned decision” can the court “ensure that agency decisions are founded on a reasoned evaluation of the relevant factors.” Marsh, 490 U.S. at 378, 109 S.Ct. at 1861 (internal quotation omitted). IV. THE NEPAIEIS ISSUE The National Environmental Policy Act (NEPA), 42 U.S.C. § 4321 et seq., declares a broad national commitment to protecting and promoting environmental quality. Robertson, 490 U.S. at 348,109 S.Ct. at 1844-45, 42, U.S.C. § 4331 (1994). The primary mechanism for implementing NEPA is the Environmental Impact Statement (EIS). 42 U.S.C. § 4332 (1994). The EIS is an “action-forcing” procedure, designed “[t]o ensure that this commitment is infused into the ongoing programs and actions of the Federal Government.” Robertson, 490 U.S. at 348, 109 S.Ct. at 1845 (quotation omitted). NEPA requires that an agency considering any action that would have a significant impact on the environment prepare an EIS. The EIS must contain a “detailed statement” including, inter alia, the environmental impacts of the proposed project, and all reasonable alternatives to the project. 42 U.S.C. § 4332(C) (emphasis added). We previously emphasized the word “detailed” because “it connotes the careful, reasoned and fully explained analysis which we think Congress intended.” Silva v. Lynn, 482 F.2d 1282, 1284 n. 2 (1st Cir.1973). Thus, the EIS helps satisfy NEPA’s “twin aims”: to ensure that the agency takes a “hard look” at the environmental consequences of its proposed action, and to make information on the environmental consequences available to the public, which may then offer its insight to assist the agency’s decision-making through the comment process. See Robertson, 490 U.S. at 350, 356, 109 S.Ct. at 1846, 1849; Baltimore Gas, 462 U.S. at 97, 103 S.Ct. at 2252. The EIS thus “helps insure the integrity of the process of decision,” providing a basis for comparing the environmental problems raised by the proposed project with the difficulties involved in the alternatives. Silva v. Lynn, 482 F.2d at 1285. A. Consideration of Environmental Impacts In its EIS, the agency must “consider every significant aspect of the environmental impact of a proposed action,” Baltimore Gas, 462 U.S. at 97, 103 S.Ct. at 2252 (quoting Vermont Yankee, 435 U.S. at 553, 98 S.Ct. at 1216), and “evaluate different courses of action,” Kleppe, 427 U.S. at 410, 96 S.Ct. at 2730. The EIS’s discussion of environmental impacts “forms the scientific and analytic basis for the comparisons” of alternatives, 40 C.F.R. § 1502.16 (1995), which are “the heart” of the EIS, id. at § 1502.14; see Part IV(B), infra. The discussion of impacts must include both “direct and indirect effects (secondary impacts) of a proposed project.” Sierra Club v. Marsh, 976 F.2d 763, 767 (1st Cir.1992); 40 C.F.R. § 1502.16(b). The agency need not speculate about all conceivable impacts, but it must evaluate the reasonably foreseeable significant effects of the proposed action. Sierra Club v. Marsh, 976 F.2d at 767. In this context, reasonable foreseeability means that “the impact is sufficiently likely to occur that a person of ordinary prudence would take it into account in reaching a decision.” Id. An environmental effect would be considered “too speculative” for inclusion in the EIS if it cannot be described at the time the EIS is drafted with sufficient specificity to make its consideration useful to a reasonable decision-maker. Id. at 768. Nevertheless, “[reasonable forecasting ... is ... implicit in NEPA, and we must reject any attempt by agencies to shirk their responsibilities under NEPA by labeling any and all discussion of future environmental effects as ‘crystal ball inquiry.’” Scientists’ Inst. for Pub. Info. v. Atomic Energy Comm’n, 481 F.2d 1079, 1092 (D.C.Cir.1973). Plaintiffs contended in the district court that the Forest Service failed to adequately assess the impact of Loon Corp.’s planned expansion on Loon Pond. Plaintiffs listed a number of specific areas of concern. The district court found the Forest Service’s consideration of environmental impacts to be adequate, and plaintiffs have not appealed this point. Accordingly, we need not pursue this issue here. B. Consideration of Alternatives “[0]ne important ingredient of an EIS is the discussion of steps that can be taken to mitigate adverse environmental consequences” of a proposed action. Robertson, 490 U.S. at 351, 109 S.Ct. at 1846. As one aspect of evaluating a proposed course of action under NEPA, the agency has a duty “to study all alternatives that appear reasonable and appropriate for study ..., as well as significant alternatives suggested by other agencies or the public during the comment period.” Roosevelt Campobello Int’l Park Comm’n v. United States EPA, 684 F.2d 1041, 1047 (1st Cir.1982) (quotations omitted); Valley Citizens for a Safe Env’t v. Aldridge, 886 F.2d 458, 462 (1st Cir.1989); City of Carmel-By-The-Sea v. U.S. Dept. of Transp., 95 F.3d 892, 903 (9th Cir.1996). As stated in the Council on Environmental Quality (“CEQ”) regulations implementing NEPA, the consideration of alternatives is “the heart of the environmental impact statement.” 40 C.F.R. § 1502.14. These implementing regulations are entitled to substantial deference. Robertson, 490 U.S. at 355, 109 S.Ct. at 1848 (citing Andrus v. Sierra Club, 442 U.S. 347, 358, 99 S.Ct. 2335, 2341, 60 L.Ed.2d 943 (1979)). The regulations require that the EIS “Rigorously explore and objectively evaluate all reasonable alternatives, and for alternatives which were eliminated from detailed study, briefly discuss the reasons for their having been eliminated.” 40 C.F.R. § 1502.14(a). It is “absolutely essential to the NEPA process that the decisionmaker be provided with a detailed and careful analysis of the relative environmental merits and demerits of the proposed action and possible alternatives, a requirement that we have characterized as ‘the linchpin of the entire impact statement.’ ” NRDC v. Callaway, 524 F.2d 79, 92 (2d Cir.1975) (citation omitted); see Silva v. Lynn, 482 F.2d at 1285; All Indian Pueblo Council v. United States, 975 F.2d 1437, 1444 (10th Cir.1992) (holding that a thorough discussion of the alternatives is “imperative”). “The ‘existence of a viable but unexamined alternative renders an environmental impact statement inadequate.’ ” Resources Ltd. v. Robertson, 35 F.3d 1300, 1307 (9th Cir.1993) (quoting Idaho Conservation League v. Mumma, 956 F.2d 1508, 1519 (9th Cir.1992)); see Grazing Fields Farm v. Goldschmidt, 626 F.2d 1068, 1072 (1st Cir.1980) (Even the existence of supportive studies and memo-randa contained in the administrative record but not incorporated in the EIS cannot “bring into compliance with NEPA an EIS that by itself is inadequate.”). Because of the importance of NEPA’s procedural and informational aspects, if the agency fails to properly circulate the required issues for review by interested parties, then the EIS is insufficient even if the agency’s actual decision was informed and well-reasoned. Grazing Fields Farm, 626 F.2d at 1072; see Massachusetts v. Watt, 716 F.2d 946, 951 (1st Cir.1983). C. The Requisite Level of Detail One purpose of the EIS requirement is to “provide decisionmakers with sufficiently detailed information to aid in determining whether to proceed with the action in light of its environmental consequences.” Northwest Resource Info. Ctr., Inc. v. National Marine Fisheries Serv., 56 F.3d 1060, 1064 (9th Cir.1995). What level of detail is sufficient depends on the nature and scope of the proposed action. Valley Citizens, 886 F.2d at 463; Mumma, 956 F.2d at 1520. The discussion of environmental effects of alternatives need not be exhaustive. “[W]hat is required is information sufficient to permit a reasoned choice of alternatives as far as environmental aspects are concerned,” All Indian Pueblo Council, 975 F.2d at 1444 (quoting NRDC v. Morton, 458 F.2d 827, 836 (D.C.Cir.1972)); see also Carmelr-By-The-Sea, 95 F.3d at 903, information sufficient for the agency to “Vigorously explore and objectively evaluate” all reasonable alternatives. 40 C.F.R. § 1502.14(a); All Indian Pueblo Council, 975 F.2d at 1444. The courts have applied “a rule of reason in determining whether an EIS contains a reasonably thorough discussion of the significant aspects of the probable environmental consequences.” Carmel-By-The-Sea, 95 F.3d at 899 (quotation omitted); see also Grazing Fields Farm, 626 F.2d at 1074; Massachusetts v. Andrus, 594 F.2d 872, 884 (1st Cir.1979); cf. Marsh, 490 U.S. at 373, 109 S.Ct. at 1859 (supplemental EIS). One aspect of this determination is whether the agency has gone “beyond mere assertions and indicate[d] its basis for them.” Silva v. Lynn, 482 F.2d at 1287. The agency “must ‘explicate fully its course of inquiry, its analysis and its reasoning.’” Massachusetts v. Andrus, 594 F.2d at 883 (quoting Silva v. Lynn, 482 F.2d at 1284-85). The court must determine whether, in the context of the record, the agency’s decision—and the analysis on which it is based—is too unreasonable for the law to permit it to stand. See Sierra Club v. Marsh, 976 F.2d at 769. We apply a rule of reason because courts should not “fly speck” an EIS and hold it insufficient based on inconsequential or technical deficiencies. Swanson v. U.S. Forest Service, 87 F.3d 339, 343 (9th Cir.1996). “The statute must be construed in the light of reason if it is not to demand what is, fairly speaking, not meaningfully possible.... But implicit in this rule of reason is the overriding statutory duty of compliance with impact statement procedures to the fullest extent possible.” Scientists’ Inst., 481 F.2d at 1092 (quotations omitted). The agency must “squarely turn[ ]” all “procedural corners” in its EIS. Citizens Awareness Network, 59 F.3d at 290 (quoting Adams, 38 F.3d at 49). The question whether a particular deficiency or combination is sufficient to warrant holding it legally inadequate, or constitutes merely a “fly speck,” is essentially a legal question, reviewable de novo. Oregon Environmental Council v. Kunzman, 817 F.2d 484, 493 (9th Cir. 1987). Applying these standards to the instant ease, we conclude that the Forest Service has not rigorously explored all reasonable alternatives, in particular the alternative that Loon Corp. be required to build artificial water storage ponds, instead of withdrawing water for snowmaking from, and discharging water into, an “outstanding resource water” like Loon Pond. The adverse environmental impacts of using Loon Pond were before the agency, and more than one eommenter proposed building artificial water storage ponds, a proposal that would, on its face, avoid some of those adverse impacts. One such commenter, Paul Beaudin of the Lincoln Committee of Concerned Citizens (LCCC), enclosed clippings pointing up “the wisdom of [Loon Corp.’s] need to enact the LCCC’s proposal for water containment pond[s] high up on the Boyle Brook.” JA, vol. II, Response to Public Comment on RDEIS at A-12. The LCCC proposal itself, made two months earlier, referred to a letter from the National Ecology Research Center recommending consideration of water storage alternatives other than Loon Pond, and enclosed a map indicating where up to three containment ponds could be installed. LCCC listed some nine advantages, including the cost-saving factor of servicing two-thirds to three-fourths of Loon Corp.’s snowmaking system by gravity feed. Instead of “rigorously explor[ing]” the alternative of using artificial water storage units instead of Loon Pond, the Forest Service’s Final EIS did not respond to these comments at all. The agency did not in any way explain its reasoning or provide a factual basis for its refusal to consider, in general, the possibility of alternatives to using Loon Pond for snowmaking, or LCCC’s reasonably thoughtful proposal in particular. This failure violated the Forest Service’s EIS obligation under NEPA. See 40 C.F.R. § 1502.9(b) (1995); 42 U.S.C. § 4332(C)(iii) (1994). The use of artificial storage ponds is not so facially implausible that it can be dismissed out of hand. The Forest Service, on another occasion, required the Sugarbush Ski Area in Vermont to construct, for its snowmaking operations, three artificial water storage ponds capable of holding 123.5 million gallons of water on 22.9 acres of private land. JA, vol. I, at 457, 465. This is 73% more than the 71 million gallons of water that the ROD estimates would be withdrawn from Loon Pond under the approved Loon Mountain expansion project. Beaudin/LCCC proposed constructing three similar ponds in the Boyle Brook area high up Loon Mountain. In addition, the record contains evidence that Loon Corp. owns 365 acres of private land at the base of the ski area, where similar storage ponds could be constructed, and that such ponds could be filled with water from the East Branch, which is typically high enough in the spring to contribute to flooding in downstream areas. Our conclusion is buttressed by NEPA’s requirement that an agency consider and an EIS discuss “steps that can be taken to mitigate the adverse environmental consequences” of a proposed project. See Robertson, 490 U.S. at 351, 109 S.Ct. at 1846. Even though there is no requirement that the agency reach a particular substantive result, such as actually formulating and adopting a complete mitigation plan, the agency must discuss “the extent to which adverse effects can be avoided,” i.e., by mitigation measures, “in sufficient detail to ensure that environmental consequences have been fairly evaluated.” Id. at 352, 109 S.Ct. at 1847. This duty—coupled with the comments alerting the agency to the environmental consequences of using Loon Pond for snowmaking and suggesting the containment pond solution— required the Forest Service to seriously consider this alternative and to explain its reasoning if it rejected the proposal. Nor can the Forest Service claim that its failure to consider an alternative to using Loon Pond for snowmaking was a de minim-is or “fly speck” issue. The record indicates serious adverse consequences to Loon Pond if it is used “as a cistern,” to use EPA’s words, and at least a reasonable probability that the use of artificial storage ponds could avoid those consequences. The existence of this non-de minimis “viable but unexamined alternative renders [the Loon EIS] inadequate.” See Resources, Inc., 35 F.3d at 1307. After the matter had proceeded to court, counsel for the Forest Service argued that constructing artificial storage ponds large enough to serve as an alternative to using Loon Pond would not be a viable alternative for reasons that were conclusorily stated. The district court accepted this argument. But this “post hoc rationalization of counsel” cannot overcome the agency’s failure to consider and address in its FEIS the alternative proposed by eommenters. State Farm, 463 U.S. at 50, 103 S.Ct. at 2870; see Burlington Truck Lines, Inc. v. United States, 371 U.S. 156, 168, 83 S.Ct. 239, 245-46, 9 L.Ed.2d 207 (1962); NRDC v. U.S. EPA 824 F.2d 1258, 1286 n. 19 (1st Cir. 1987). Such post hoc rationalizations are inherently suspect, and in any event are no substitute for the agency’s following statutorily mandated procedures. As noted supra, even if the agency’s actual decision was a reasoned one, the EIS is insufficient if it does not properly discuss the required issues. Grazing Fields Farm, 626 F.2d at 1072. In sum, how “probing” an investigation NEPA requires of alternatives depends on the circumstances, including the nature of the action at issue. Valley Citizens, 886 F.2d at 463. Thus, the reviewing court must be flexible in evaluating the depth of analysis to require in an EIS, because, while NEPA “does not mandate particular results,” it does require that the agency have adequately identified and evaluated a project’s environmental consequences. Robertson, 490 U.S. at 350, 109 S.Ct. at 1846. “NEPA’s success in large part arises from the use of legal concepts [that are flexible] such as ‘reasonableness’ and ‘adequacy’ that permit courts to adapt it successfully to so many different kinds of circumstances surrounding so many different kinds of governmental decisions.” Valley Citizens, 886 F.2d at 463. Although in Valley Citizens we found the agency’s analysis of alternatives “brief but adequate,” 886 F.2d at 462, the contrast with the instant case is instructive. In Valley Citizens, we found that nothing in the record or in comments on the draft “point[ed] out any inaccuracy” in the agency’s cost “descriptions” or in its “discussions” of other non-environmental considerations. Id. In contrast, in the instant case, the final EIS contains no “description” or “discussion” whatsoever as to why an alternative source of water such as an artificially created storage pond would be impractical. The agency has discretion to balance competing concerns and to choose among alternatives, but it must legitimately assess the relative merits of reasonable alternatives before making its decision. After a searching and careful review of the record in the instant case, we are not convinced that the Forest Service’s decision was founded on a reasoned evaluation of the relevant factors, Marsh, 490 U.S. at 378, 109 S.Ct. at 1861, or that it articulated a rational connection between the facts found and the choice made, Baltimore Gas, 462 U.S. at 105, 103 S.Ct. at 2256. Hence, it acted arbitrarily and capriciously in granting Loon Corp.’s special use permit for the expanded ski resort. Moreover, because the Forest Service did not satisfy the requirement that it “rigorously explore and objectively evaluate” all reasonable alternatives, 40 C.F.R. § 1502.14(a), its decision was not in accordance with law. See 5 U.S.C. § 706(2)(A). D. The Preservation Issue The Forest Service argues that plaintiffs have not preserved their argument that the agency should have more seriously considered, as an alternative to Loon Pond, some other source for water and some other location to discharge the effluent from Loon Corp.’s snowmaking pipes. It contends that plaintiffs faded adequately to raise their contentions during the public comment period, so they waived their right to pursue these challenges on their merits. The Forest Service argues that, “[i]f commenters could require agencies to undertake detailed comparative analyses merely by asserting the superiority of an alternative site, configuration or method, only the imaginations of project opponents would limit the length of EISs and the duration of the NEPA process.” Forest Service Brief at 53. Raising the specter of catastrophe only obfuscates the real issues here: whether the Forest Service adequately considered alternatives to using Loon Pond as a vehicle for Loon Corp.’s snowmaking, with adequacy based on the reasonableness and practicality of the alternatives, and whether the Forest Service adequately explained in its FEIS why it decided against such alternative's. The Forest Service relies on Roosevelt Campobello: “In order to preserve an alternatives issue for review, it is not enough simply to make a facially plausible suggestion; rather, an intervenor must offer tangible evidence that an alternative site might offer a substantial measure of superiority as a site.” 684 F.2d at 1047 (emphasis added) (quotation omitted). The Forest Service’s reliance on Roosevelt Campobello is misplaced. That ease, and the precedents it relied on, dealt with a claim that the agency had not considered all appropriate alternative sites on which to locate a particular project. Obviously, the number of potential locations for any project is infinite, and an agency cannot be expected to consider seriously every possible location before approving a project. In such a context, the agency is only required to consider “all alternatives which were .feasible and reasonably apparent at the time of drafting the EIS.” Id.; see also Seacoast Anti-Pollution League v. Nuclear Regulatory Comm’n, 598 F.2d 1221, 1229 (1st Cir.1979) (Agency need not “ferret out every possible alternative, regardless of how uncommon or unknown.”) (quoting Vermont Yankee, 435 U.S. at 551, 98 S.Ct. at 1216). The situation in the instant case is wholly different. It is one thing to ask whether there are “known,” “feasible,” alternative sites on which to locate a project, and a different matter to ask whether the Forest Service in the instant case should have considered an alternative .means of implementing the expansion of the Loon Mountain Ski Area—a particular means of operation that would do less environmental damage—without changing the site to another state or another mountain. Here, the Forest Service was alerted by commenters to the alternative of using artificial storage ponds instead of Loon Pond for snowmaking; but even without such comments, it should have been “reasonably apparent” to the Forest Service, Roosevelt Campobello, 684 F.2d at 1047, not “unknown,” Seacoast Anti-Pollution League, 598 F.2d at 1229, that such an alternative existed. In the instant case, at least two commen-ters, Paul Beaudin of LCCC and plaintiff Dubois, provided notice to the Forest Service, informing it of the substance of their proposed alternative. Though not detailed, these comments submitted in response to the Forest Service’s RDEIS made clear that the commenters thought the agency should consider some alternative source of water other than Loon Pond and some alternative place to discharge the water after it had gone through the snowmaking pipes. They argued that such an alternative would reduce the negative environmental impact on Loon Pond from depleting the pond’s water and from refilling the pond with polluted water either from the East Branch or from acidic snowmelt. Dubois explicitly and Beaudin by reference suggested the possibility of new man-made storage units to accomplish these goals. These comments provided sufficient notice to “alert[ ] the agency” to the alternative being proposed and the environmental concern the alternative might address. See Seacoast Anti-Pollution League, 598 F.2d at 1229 (quoting Vermont Yankee, 435 U.S. at 558, 98 S.Ct. at 1216). Because the comments to the EIS were sufficient to notify the agency of the potential alternatives, see Adams v. U.S. EPA, 38 F.3d 43, 52 (1st Cir.1994), the district court erred in concluding that plaintiffs were required to “offer[] specifics as to how to implement a suggested alternative water storage system.” Memorandum and Order at 31. Such “specifies” are not required. As we reasoned in Adams, the purpose of public participation regulations is simply “to provide notice” to the agency, not to “present technical or precise scientific or legal challenges to specific provisions” of the document in question. Adams, 38 F.3d at 52. “It would be inconsistent with the general purpose of public participation regulations to construe the regulations strictly.” Id. Moreover, NEPA requires the agency to try on its own to develop alternatives that win “mitigate the adverse environmental consequences” of a proposed project. Robertson, 490 U.S. at 351, 109 S.Ct. at 1846. “In respect to alternatives, an agency must on its own initiative study all alternatives that appear reasonable and appropriate for study at the time, and must also look into other significant alternatives that are called to its attention by other agencies, or by the public during the comment period afforded for that purpose.” Seacoast Anti-Pollution League, 598 F.2d at 1230 (emphasis added). Particularly given this directive, the alert furnished by Beaudin and Dubois required exploration and discussion by the Forest Service of the idea that environmental damage might be reduced, by the use of artificial storage ponds instead of Loon Pond for snowmaking purposes. Therefore, the district court should have rejected the Forest Service’s argument that Dubois failed to adequately preserve the issue of alternatives. V. SUPPLEMENTAL EIS Plaintiffs also appeal the district court’s conclusion that the Forest Service was not required, under NEPA, to prepare a supplemental EIS. The question of a supplemental EIS is premised on the dual purposes of the EIS: to assure that the public who might be affected by the proposed project be fully informed of the proposal, its impacts and all major points of view; and to give the agency the benefit of informed comments and suggestions as it takes a “hard look” at the consequences of proposed actions. See Robertson, 490 U.S. at 349, 356, 109 S.Ct. at 1845, 1849, 40 C.F.R. §§ 1502.1, 1502.9(a) (1995). An agency “shall” prepare a supplemental EIS if, after issuing its latest draft EIS, “[t]he agency makes substantial changes in the proposed action that are relevant to environmental concerns.” 40 C.F.R. § 1502.9(c)(1)(i) (1995). The use of the word “shall” is mandatory, not precatory. It creates a duty on the part of the agency to prepare a supplemental EIS if substantial changes from any of the proposed alternatives are made and the changes are relevant to environmental concerns. See Marsh, 490 U.S. at 372, 109 S.Ct. at 1858. Thus, as explained by CEQ, an additional alternative that has not been disseminated previously in a draft EIS may be adopted in a final EIS, without further public comment, only if it is “qualitatively within the spectrum of alternatives that were discussed” in the prior draft; otherwise a supplemental draft is needed. See Forty Most Asked Questions Concerning CEQ’s NEPA Regulations, 46 Fed.Reg. 18026, # 29b (1981). Plaintiffs argue that the project proposed as Alternative 6, appearing for the first time in the Final EIS, embodies “substantial changes” from any of the alternatives proposed in the prior drafts of the EIS, and that those changes are “relevant to environmental concerns.” See 40 C.F.R. § 1502.9(c)(1)(i). Therefore, plaintiffs assert that, by not describing Alternative 6 in a supplemental EIS-which would give the public an opportunity to comment on it and give the Forest Service the benefit of those comments in its consideration of the environmental impact of Alternative 6—the Forest Service collided with both the public information and the agency guidance objectives of NEPA. In response, defendants argue that plaintiffs’ interpretation of the previously discussed alternatives is incorrect, because Alternative 6 is merely a scaled-down modification of Alternative 2 which, as proposed in two phases in the RDEIS, would have been far larger and far more intrusive on the environment than the new preferred Alternative 6. Plaintiffs reply that only Phase I and not Phase II of Alternative 2 was seriously considered and analyzed prior to the development of Alternative 6 in the final EIS. Defendants deny this assertion. We conclude, based on the record in this case, that a supplemental EIS was required. The scope of review of a reviewing court is the APA’s “arbitrary and capricious” standard. Marsh, 490 U.S. at 375-76, 109 S.Ct. at 1860; see Part III, supra. The Court in Marsh was especially deferential to the “informed discretion of the responsible federal agencies,” due to the “high level of technical expertise” required in that ease to analyze the relevant documents regarding soil composition and a dam’s impact on downstream turbidity. Marsh, 490 U.S. at 377, 379, 109 S.Ct. at 1861, 1862. In the instant case, however, nothing in the FEIS indicates that any such technically complex scientific analysis would be required in order for this court to determine that Alternative 6 involves a “substantial change” from the prior proposals at Loon Mountain. Alternative 6, adopted by the Forest Service as its preferred alternative in the final EIS, does not fall “within the spectrum of alternatives” that were considered in previous drafts, even if Phase II of Alternative 2 had been adequately analyzed prior to the FEIS. Alternative 6 entails a different configuration of, activities and locations, not merely a reduced version of a previously-considered alternative. Phase II of Alternative 2 proposed expanding the ski area primarily on land that is not within the current permit area; in contrast, Alternative 6 squeezes much of its expansion into that current permit area. To accomplish this, Alternative 6 widens existing trails so as to eliminate buffers that currently separate the trails. It also envisions a 28,500-square-foot base lodge facility within the existing permit area. And it develops ski trails, access roads and lifts on land that the p