Full opinion text
Opinion BAXTER, J. Is a public entity liable in tort or inverse condemnation for damage to downstream riparian property caused by the discharge of surface waters into a natural watercourse abutting its property? The Court of Appeal held that there could be no liability. We granted the petition of plaintiffs, owners of damaged properties, to consider whether the “natural watercourse rule” stated in Archer v. City of Los Angeles (1941) 19 Cal.2d 19 [119 P.2d 1] (Archer), by which the Court of Appeal believed itself bound, was properly applied in this case, and to decide whether article I, section 19 of the California Constitution compels compensation for damage caused by an increased flow of streamwater that is traceable to surface water runoff from improvements on public property. We conclude that Archer does not correctly state the principles presently applicable to the liability of riparian landowners. To the extent that Archer also held that article I, section 19 of the California Constitution did not create liability, it has been overruled by subsequent decisions of this court. When alterations or improvements on upstream property discharge an increased volume of surface water into a natural watercourse, and the increased volume and/or velocity of the stream waters or the method of discharge into the watercourse causes downstream property damage, a public entity, as a property owner, may be liable for that damage. The test is whether, under all the circumstances, the upper landowner’s conduct was reasonable. This rule of reasonableness applies to both private and public landowners, but it requires reasonable conduct on the part of downstream owners as well. This test requires consideration of the purpose for which the improvements were undertaken, the amount of surface water runoff added to the streamflow by the defendant’s improvements in relation to that from development of other parts of the watershed, and the cost of mitigating measures available to both upper and downstream owners. Those costs must be balanced against the magnitude of the potential for downstream damage. If both plaintiff and defendant have acted reasonably, the natural watercourse rule imposes the burden of stream-caused damage on the downstream property. We also conclude that a governmental entity may be liable under the principles of inverse condemnation for downstream damage caused by an increased volume or velocity of surface waters discharged into a natural watercourse from public works or improvements on publicly owned land. It will be liable if it fails to use reasonably available, less injurious alternatives, or if it has incorporated the watercourse into a public drainage system or otherwise converted the watercourse itself into a public work. Compensation is compelled by the same constitutional principles which mandate compensation in inverse condemnation actions generally. The downstream owner may not be compelled to accept a disproportionate share of the burden of improvements undertaken for the benefit of the public at large. Because downstream riparian property is burdened by the servitude created by the natural watercourse rule, however, consistent with that rule the downstream owner must take reasonable measures to protect his property. Liability on an inverse condemnation theory will not be imposed if the owner has not done so. Finally, because the development of any property in the watershed of a natural watercourse may add additional runoff to the stream, all of which may contribute to downstream damage, it would be unjust to impose liability on an owner for the damage attributable in part to runoff from property owned by others. Therefore, an owner who is found to have acted unreasonably, and to have thereby caused damage to downstream property, is liable only for the proportion of the damage attributable to his conduct. Although we conclude that the Court of Appeal erred in holding that the natural watercourse rule insulated defendants from both tort and inverse condemnation liability, we shall affirm the judgment. After a review of the record we are satisfied that the court properly held that Reliez Creek, the watercourse which is the focus of this litigation, had not itself become a public improvement at the time the damage of which plaintiffs complain occurred and that no public improvements in the creekbed contributed to the damage suffered by plaintiffs. That review also satisfies us that the evidence does not support a conclusion that the damage to any plaintiff’s property was the result of unreasonable conduct by any defendant in the manner in which it discharged surface water runoff into Reliez Creek, or establish that there was damage to plaintiffs’ properties that could not have been prevented had they undertaken reasonable measures to protect their properties. I Underlying Facts Plaintiffs are the owners of property abutting Reliez Creek in Contra Costa County. The ownership interest of each plaintiff extends to the center of the creek and includes the creekbed and banks along the frontage of his or her property. Reliez Creek is a natural watercourse which drains a watershed of approximately 2,291 acres. It is several miles long, and runs from the hills to a confluence with Las Trampas Creek. Plaintiffs’ properties lie on the final 1,500 feet before Reliez Creek joins Las Trampas Creek. Over the last 50 years development in the watershed has transformed an essentially rural environment into one in which 1,294 acres are developed. Public and private improvements in the watershed have prevented or lessened absorption of surface waters. Paving and other treatment has made some ground impervious to water, and the manner in which surface water runoff reaches Reliez Creek has been altered. The result has been an increase in the volume of surface waters discharged into Reliez Creek and in the velocity of the waters in the creek, particularly during times of heavy rains. In recent years the flow has caused scouring, undercutting, and erosion of the banks of the creek on plaintiffs’ properties. The area of improvement is not limited to that owned by defendants, however. Development in the City of Walnut Creek, part of which is in the Reliez Creek watershed, and improvement on the grounds of Acalanes High School adjacent to Reliez Creek, as well as private development of other nonriparian property within the watershed, have added surface water runoff to Reliez Creek. Plaintiffs purchased their respective properties at various times between 1965 and 1978. Many inspected the creek bank at the time they purchased their property. None observed any erosion. Although some erosion of the creek banks occurred subsequently, damage to the creek banks during the winter of 1981-1982, a period of unusually heavy rainfall, was more significant. There was evidence that the increased flow of waters led to failure of the creek banks adjacent to plaintiffs’ properties, widening the creek in some locations from a width of 40 feet to a width of 110 feet. There was also evidence that the city and county were aware that the increased flow of surface waters caused by development was causing and would cause damage to the creek banks. The damage to the creekside property might have been prevented by check dams and dikes, upstream diversion structures, and retention basins. That evidence did not relate the need for such structures to the increased runoff from defendants’ properties or demonstrate that it would be reasonable to impose the cost of such structures on the named defendants, whose property comprised a small percentage of the watershed. In 1983, plaintiffs filed this action to recover for “extensive landslide” damage to their properties adjacent to the Reliez Creek, damage allegedly caused by defendants’ storm drainage system. The failure to maintain the drainage system was alleged to have “extensively eroded Plaintiffs’ real property, triggering landslides which have damaged Plaintiffs’ real property and which now threaten the stability of the remaining portions of Plaintiffs’ real property.” They sought to recover damages on theories of inverse condemnation, nuisance, dangerous condition of public property, and trespass to real property, and, in addition, they sought injunctive relief. Named as defendants were the City of Lafayette (City), the County of Contra Costa (County), the Contra Costa County Flood Control District (District), the California Department of Transportation (CalTrans), the Bay Area Rapid Transit District (BART), and private parties whose involvement is not in issue here. Each public entity was sued on the basis that actions it took or improvements it made or owned contributed to the increased volume and velocity of water in Reliez Creek over that which would have been carried by the creek but for the actions of those defendants, and were a substantial factor in causing the damage to plaintiffs’ properties. The time within which property damage occurred for which recovery was sought was the three-year period prior to the filing of the complaint on September 13, 1983. CalTrans and County were alleged to be developers, designers, builders, owners, and maintained of Highway 24, Old Tunnel Road, and Pleasant Hill Road. BART was sued as the owner and developer of the rapid transit right-of-way through County. All defendants allegedly created and maintained a storm drainage system which included those portions of Reliez Creek that adjoined plaintiffs’ real property. Plaintiffs’ inverse condemnation cause of action, brought under the authority of article I, section 19 of the California Constitution, alleged that plaintiffs had been singled out to suffer a direct and substantial burden of the storm drainage system which was a public use. Their theories were: (1) that City, County, CalTrans, District, and BART were liable because dedicated roads, rights-of-way, culverts, storm drains, other public improvements in the watershed, and the discharge of surface waters collected by private owners pursuant to a county ordinance and discharged into the creek increased the volume and flow of water into Reliez Creek, which increase was a substantial factor in causing the downstream damage; and (2) that as a result of this use and public improvements constructed in the creekbed, Reliez Creek had itself become a work of public improvement. They also alleged in support of that theory that as a condition of development permits City and County had required that irrevocable dedication of storm drainage easements on creekside properties be set out in subdivision maps. The third cause of action, styled as one for a dangerous condition of public property, alleged that unchecked surface water runoff from land and improvements, including roadways and rights-of-way, was channeled into the storm drainage system. In this cause of action plaintiffs claimed that defendants breached their duty of care by permitting the surface waters to be channeled through an inadequate storm drainage system, including Reliez Creek, without remedial action to protect adjacent properties, with the result that erosion, undercutting, destabilization, and landslides occurred on plaintiffs’ property. Similar allegations underlay the nuisance and trespass causes of action. Trial of the liability and damages issues was bifurcated. At the close of plaintiffs’ case-in-chief on liability, the trial court granted defendants’ motions for judgment on the inverse condemnation cause of action (Code Civ. Proc., § 631.8) and nonsuit on the tort causes of action (Code Civ. Proc., § 581c), except insofar as plaintiffs claimed City’s liability arose from the maintenance of two structures, the Sizeler outfall and the sheet pile structure within Reliez Creek. Judgment for City was then granted at the close of defendants’ evidence when the trial court found that plaintiffs had not proved that either structure was a substantial concurring cause of the damage to any plaintiff’s property. In granting defendants’ motions the trial court ruled: (1) there was insufficient evidence to establish that Reliez Creek was a storm drainage public improvement; (2) the “natural watercourse rule” shielded defendants from liability for damage caused by their collection of natural surface water drainage and discharge of that water into a natural channel even if the volume and velocity of the water caused the damage; (3) there was insufficient evidence that BART improvements substantially contributed to plaintiffs’ damages; (4) there was insufficient evidence that water CalTrans diverted from another watershed to the Reliez Creek watershed substantially contributed to the damage; (5) County had no liability because in 1968 it had relinquished ownership and control over any public improvements that might have contributed to the damage; and (6) there was no evidence that District owned or controlled any public improvement within the watershed that might have contributed to the damage. II Plaintiffs’ Appeal Plaintiffs’ appeal was directed principally to the disposition of their inverse condemnation claim. They argued, however, that the trial court erred in applying the Archer natural watercourse rule to give defendants absolute immunity from liability on both the inverse condemnation and the tort claims. With respect to the inverse condemnation claims, plaintiffs also argued that they were entitled to relief under Belair v. Riverside County Flood Control Dist. (1988) 47 Cal.3d 550 [253 Cal.Rptr. 693, 764 P.2d 1070], because defendants’ conduct was not reasonable; and that the trial court erred in holding that, because plaintiffs had not established the damage caused by each defendant individually, they failed to demonstrate that defendants were jointly and severally liable for the combined damage to their property. They argued that the evidence established, as a matter of law, that Reliez Creek had been used in such a way that it had become a work of public improvement; that, as a matter of law, City, County, BART, and CalTrans had acted unreasonably; and that City, County, BART, and Cal-Trans were jointly liable for the damages caused by the water runoff from their roads and paved areas. The Court of Appeal affirmed the judgment of the trial court in all respects, holding that it was bound by Auto Equity Sales, Inc. v. Superior Court (1962) 57 Cal.2d 450, 456 [20 Cal.Rptr. 321, 369 P.2d 937] to apply the Archer natural watercourse rule, which on the facts of this case, immunized defendants from liability for any damage caused by their use of Reliez Creek as a natural drainage channel. Ill The Archer Decision The Court of Appeal based its ruling on this court’s statement in Archer that “there is no diversion [for which liability would exist] if surface waters, flowing in no defined channel, are for a reasonable purpose gathered together and discharged into the stream that is their natural means of drainage even though the stream channel is inadequate to accommodate the increased flow.” (Archer, supra, 19 Cal.2d 19, 26.) The Court of Appeal also concluded that decisions subsequent to Archer had reaffirmed, not repudiated, the holding in Archer that governmental entities are immune from liability under article I, section 19 of the California Constitution insofar as that holding was applicable to damages for which a private owner would be shielded from liability by the natural watercourse rule. That “Archer exception” to inverse condemnation liability held that “[i]f the property owner would have no cause of action were a private person to inflict the damage, he can have no claim for compensation from the state.” (Archer, supra, 19 Cal.2d 19, 24.) To put in perspective our examination of public and private landowner responsibility for damage to downstream property caused by discharge of “surface waters” into a “natural watercourse,” it is therefore appropriate to define these terms and to describe in more detail the Archer decision. A. Surface Waters. , In the arcane area of water law under consideration in this case, the rights and liabilities of private property owners for property damage or personal injury are in large part dependent upon classification of the water as “surface waters,” “flood waters,” or “stream waters.” “Water diffused over the surface of land, or contained in depressions therein, and resulting from rain, snow, or which rises to the surface in springs, is known as ‘surface water.’ It is thus distinguishable from water flowing in a fixed channel, so as to constitute a watercourse, or water collected in an identifiable body, such as a river or lake. The extraordinary overflow of rivers and streams is known as ‘flood water.’ (Tiffany on Real Property (3d ed.) § 740; 8 Cal.L.Rev. 197.)” (Keys v. Romley (1966) 64 Cal.2d 396, 400 [50 Cal.Rptr. 273, 412 P.2d 529].) B. Natural Watercourse. A natural watercourse “is a channel with defined bed and banks made and habitually used by water passing down as a collected body or stream in those seasons of the year and at those times when the streams in the region are accustomed to flow. It is wholly different from a swale, hollow, or depression through which may pass surface waters in time of storm not collected into a defined stream.” (San Gabriel V.C. Club v. Los Angeles (1920) 182 Cal. 392, 397 [188 P. 554, 9 A.L.R. 1200].) A canyon or ravine through which surface water runoff customarily flows in rainy seasons is a natural watercourse. Alterations to a natural watercourse, such as the construction of conduits or other improvements in the bed of the stream, do not affect its status as a “natural” watercourse. (LeBrun v. Richards (1930) 210 Cal. 308, 317, 318 [291 P. 825, 72 A.L.R. 336]; Larrabee v. Cloverdale (1900) 131 Cal. 96, 99-100 [63 P. 143].) A natural watercourse includes “all channels through which, in the existing condition of the country, the water naturally flows,” and may include new channels created in the course of urban development through which waters presently flow. (Larrabee v. Cloverdale, supra, 131 Cal. at p. 100.) Once surface waters have become part of a stream in a watercourse, they are no longer recognized as surface waters. (San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. at p. 398.) C. Archer. Archer, supra, 19 Cal.2d 19, was an action brought by nonriparian landowners for damage caused to their property when surface water runoff channeled into a natural watercourse, from which it was discharged into a lagoon, exceeded the capacity of an outlet pipe from the lagoon to the sea, backed up, and caused flooding. Although the injury was to nonriparian landowners, the court applied the rules then governing the liability of upper riparian landowners to lower riparian owners and the decision has since been relied on as establishing immunity for any damage to downstream riparian property caused by discharge of surface water runoff into a natural watercourse. The plaintiffs in Archer were owners of property near La Ballona Lagoon. La Ballona Creek, a natural watercourse, drained surface waters from an area of about 134 square miles into the lagoon, from which the waters emptied into the Pacific Ocean. As residential and commercial development occurred in the hills at the upper reaches of the creek, surface waters that had not followed a defined course were diverted into ditches and channels which emptied into the creek. Defendants, the City of Los Angeles and the Los Angeles County Flood Control District, straightened, widened and deepened the creek. They constructed concrete storm drains to improve drainage. As a result of these changes less water was absorbed into the ground and the flow of waters from the creek into the lagoon was accelerated. Defendants did not improve the outlet from the lagoon to the ocean, however, and after a heavy rain the water in the lagoon backed up, flooding plaintiffs’ property. They sued on a theory of inverse condemnation under former section 14 (now section 19) of article I of the California Constitution. Nonsuit was granted and plaintiffs appealed. Under what was then this court’s view of article I, section 14, the predecessor to present section 19, the Constitution did not create a cause of action. It did no more than waive the state’s sovereign immunity if a cause of action would otherwise exist. Therefore, plaintiffs could recover only if a private landowner would be responsible for the damage suffered by plaintiffs. If a private party had the right to inflict the damage without incurring liability, the governmental defendants would not be liable. The court therefore analyzed the claim as it would one involving the rights of private landowners to drain surface waters from their property into a natural watercourse. The court stated the applicable rules as: 1. A lower owner may not recover for injury to his land caused by improvements made in the stream for the purpose of draining or protecting the land above, even though the channel is inadequate to accommodate the increased flow of water resulting from the improvements. It is immaterial that the improvements increase the volume and velocity of the water, that the lower owner’s burden of protecting his property is increased, or that his land is damaged. 2. Improvements must follow the natural drainage and may not divert water into a different channel, but straightening, widening, and deepening the channel does not constitute a diversion. 3. There is no diversion if, for a reasonable purpose, diffused surface waters are gathered and discharged into a stream that is their natural means of drainage even if the watercourse is inadequate to accommodate the increased flow. An upper riparian landowner may gather surface waters for a reasonable purpose and discharge them into a natural watercourse without liability to a lower owner for damage caused by the increased flow. Possibly because there was no claim that the defendants had acted unreasonably in their upstream improvements, however, the Archer holding omitted reference to an important qualification on the rights of the upper riparian owner implied in San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, the case on which Archer relied for the rule: i.e., that the purpose for the improvements not only be reasonable, but the improvements must also be constructed in a manner that was no more burdensome to the lower riparian owner than required for that purpose. (182 Cal. at pp. 396, 399-401.) This qualification was implied repeatedly in San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, viz.: “No complaint is made of the manner in which the drains are constructed, or that they are not reasonable improvements for the district they are designed to protect, or that they are unnecessarily injurious to the plaintiff . . . .” (Id., at pp. 399-400.) “[In the related situation of release of water from a mill] no right of action by a land owner below exists because of the increase of volume and consequent acceleration of flow, provided the use is a reasonable one and exercised in a reasonable manner.” (Id., at p. 401.) Certainly the San Gabriel V.C. Club decision did not hold that any surface water drainage into a natural watercourse was immunized in this state. It implied the contrary, and made it clear that this question was not before the court. “[Decisions in other states go further than it is necessary to go in this case, and hold that a riparian owner has no right to complain because the volume of water in the stream is increased by artificially draining surface waters into it above, provided only the stream is the natural drainage channel for the lands so drained.” (182 Cal. at pp. 401-402.) Since there was no issue involving unreasonable conduct in draining surface waters into the stream bed in Archer, supra, 19 Cal.2d 19, that decision also fails to support a conclusion that immunity exists regardless of whether the upstream owner acted reasonably. Moreover, the principal focus of both Archer, supra, 19 Cal.2d 19, and San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, was on alterations of and improvements in the stream bed itself, and on waters that had lost their character as surface waters and had become stream waters before they reached the stream bed improvements (drains) constructed by the defendants. Neither case addressed liability for downstream damage caused by the discharge of surface waters into a natural watercourse. Therefore, we do not assume, as do defendants, that the rule governing surface waters has no application here or that Archer established a rule granting immunity to an upstream riparian owner for damages caused as a result of discharges of surface water into a natural watercourse regardless of whether his conduct was reasonable with regard to downstream owners. IV Rights and Liabilities of Private Property Owners Defendants are both property owners and governmental entities. Their potential liability as governmental entities for damage caused by discharge of surface waters into a natural watercourse is no longer limited, as it was at the time of Archer, supra, 19 Cal.2d 19, to the liability a private party would incur. Additionally, since Archer we have made it clear that private parties do not enjoy the broad immunity recognized at the time of Archer for discharge of surface waters across lower properties. Therefore, we shall first consider the holding of the Court of Appeal that a private property owner has no liability for damage to downstream riparian owners caused by his discharge of surface waters into a natural watercourse. A. The California Civil Law Rule. At common law the “common enemy doctrine” gave an owner of land over which surface water flowed from a higher elevation the right to obstruct the flow of that water, turning it back or diverting it onto the land of another owner, without liability for any damage that might result. (Keys v. Romley, supra, 64 Cal.2d 396, 400-401.) By contrast, the civil law rule adopted for California more than a century ago (see Ogburn v. Connor (1873) 46 Cal. 346) gave the owner of the higher land an easement or servitude over a lower parcel which allowed him to discharge surface waters as they naturally flow from his higher land onto the lower land of the servient owner. (Los Angeles C. Assn. v. Los Angeles (1894) 103 Cal. 461, 466 [37 P. 375]; Gray v. McWilliams (1853) 98 Cal. 157, 165 [32 P. 976]; Ogburn v. Connor, supra, 46 Cal. 347, 352-353.) The lower owner had no right to obstruct that flow. In theory, the owner of the lower parcel accepted it with the burden of natural drainage. (Keys v. Romley, supra, 64 Cal.2d 396, 402.) Nonetheless, the owner of the higher land was not permitted to gather the surface waters “by artificial means and discharge them on to lower lying land in greater volume or in a different manner than they would naturally be discharged.” (San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, 398.) B. The Natural Watercourse Rule.. The rule differed with respect to discharge of surface waters into a natural watercourse. As we noted in Archer (supra, 19 Cal.2d at p. 26), an upper riparian owner had the right, for a reasonable purpose, to discharge surface waters, including those whose volume was increased as a result of development which altered both the absorption, of waters by the soil and the drainage pattern, into a natural watercourse. It was immaterial that the watercourse was inadequate to accommodate the increased flow and flooded downstream property. The riparian owner also had the right to improve the channel even if the accelerated flow caused downstream damage. (Bauer v. County of Ventura (1955) 45 Cal.2d 276, 283 [289 P.2d 1].) Thus, a riparian owner might be the ultimate “beneficiary” of the civil law rule subjecting lower parcels of property to the burden of surface water runoff from parcels at a higher elevation. The owner had the right, in turn, however, to discharge the surface waters into a natural watercourse without liability for damage that the addition of these waters to the stream might do to downstream riparian property. The downstream riparian owner is also deemed to take the property subject to an easement or servitude, one burdening the downstream property with accepting the flow of whatever water is thereby carried onto or through it in a natural watercourse. Moreover, a riparian landowner has had the right to “collect” or “gather” surface waters and discharge them at a location or locations other than those where natural runoff would enter the watercourse. The owner could straighten the stream or improve its bed with paving, drains, or conduits, and, to protect the land, construct dikes or bulkheads even if the result was to increase the volume and velocity of the waters to the injury of lower owners. “[A] riparian owner has no right to complain because the volume of water in the stream is increased by artificially draining surface waters into it above, provided only the stream is the natural drainage channel for the lands so drained. Furthermore, this rule is adopted regardless of whether the so-called common-law rule concerning surface waters prevails in the particular jurisdiction or, as here, the civil-law rule, which forbids the gathering together of surface waters and discharging them as a stream upon adjoining lands. If the surface waters are gathered and discharged into the stream which is their natural means of drainage, so that they come to the land below only as a part of the stream, it is held that no action lies because of their being added.” (San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, 401-402.) “If a riparian owner cannot complain if surface waters be actually added by artificial drainage above to the volume of the stream, it must certainly be that he cannot complain of a drainage improvement which adds no water to the stream but merely protects the adjoining lands against the water already in it.” (Id., at p. 402.) The immunity of the upper riparian owner for downstream damage caused by his discharge of surface water runoff into a natural watercourse through improvements was initially for improvements undertaken to drain and/or protect the upper riparian owner’s land. “[A]n improvement for the purposes of the drainage and protection of lands above does not give a lower riparian owner on the stream a cause of action merely because such improvement increases the volume of water in the stream as it comes to his land, even though the burden he is necessarily under of protecting his land against the stream is thereby increased and his land is injured because of his failure to meet such increased burden; and, further ... the rule is not subject to the limitation that the increased volume must not be such as to make the stream exceed the capacity of its channel.” (San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, 406.) We again recognized that this immunity was for damage to downstream land caused by improvements made in the stream for the purpose of draining or protecting the land above in Archer. (Supra, 19 Cal.2d 19, 24-25.) And, in Archer, since the drainage improvements in the creekbed were permissible, we declined to impose a requirement that the plan minimize downstream damage by provision for improving the outlet from the lagoon into which the watercourse drained. (Id., at pp. 25-26.) The California rules applicable to runoff of surface waters onto adjacent property and into natural watercourses have accommodated progression from a rural, agricultural society to gradual urbanization. Although immunity of upper landowners was limited to “natural” runoff of surface waters, it was broad enough to encompass surface water runoff from fields cultivated in a natural way, even though cultivation altered the runoff from that which occurred from unfilled fields. (Coombs v. Reynolds (1919) 43 Cal.App. 656, 660 [185 P. 877], See also Switzer v. Yunt (1935) 5 Cal.App.2d 71, 78 [41 P.2d 974].) But immunity did not extend to city lots where “changes and alterations in the surface were essential to the enjoyment of such lots . . . .” (Los Angeles C. Assn. v. Los Angeles, supra, 103 Cal. 461, 467.) As suggested above, the natural watercourse rule has two aspects. The first permits the riparian landowner to gather surface waters and discharge them into the watercourse at a location other than that at which natural drainage would occur. The second permits the owner to make improvements in the bed of the stream to improve drainage and to protect the land from erosion by constructing dikes or embankments even though the result may be increased flow and velocity which might damage the property of lower riparian owners. Both aspects of the rule have as their purpose facilitating the development of upstream properties..“Not to permit an upper land owner to protect his land against the stream would be in many instances to destroy the possibility of making the land available for improvement or settlement and condemn it to sterility and vacancy.” (San Gabriel V.C. Club v. Los Angeles, supra, 182 Cal. 392, 401; see also, Archer, supra, 19 Cal.2d at p. 27.) C. The Contemporary Rule of Reasonableness. The modern rule governing landowner liability for surface water runoff and drainage is no longer simply a rule of property law dependent upon the existence of rights, servitudes, or easements. The civil law rule was modified more than a quarter of a century ago by the landmark decision in Keys v. Romley, supra, 64 Cal.2d 396. There we recognized the tendency of the civil law rule limiting immunity for damages caused by surface water runoff onto an adjacent property to inhibit development of land, since any change in the upper property would affect the natural runoff. (Id., at p. 402.) Today a landowner’s conduct in using or altering the property in a manner which affects the discharge of surface waters onto adjacent property is subject to a test of reasonableness. “It is . . . incumbent upon every person to take reasonable care in using his property to avoid injury to adjacent property through the flow of surface waters. Failure to exercise reasonable care may result in liability by an upper to a lower landowner. It is equally the duty of any person threatened with injury to his property by the flow of surface waters to take reasonable precautions to avoid or reduce any actual or potential injury. “If the actions of both the upper and lower landowners are reasonable, necessary, and generally in accord with the foregoing, then the injury must necessarily be borne by the upper landowner who changes a natural system of drainage, in accordance with our traditional civil law rule.” (Keys v. Romley, supra, 64 Cal.2d 396, 409.) At least with respect to surface water runoff onto adjacent lands, the California rule is that stated in Keys v. Romley, supra, 64 Cal.2d 396, 409: “No party, whether an upper or a lower landowner, may act arbitrarily and unreasonably in his relations with other landowners and still be immunized from all liability.” It has been suggested that with the adoption of the reasonable use test for surface waters “there is no longer any valid reason for distinguishing between surface waters and those that flow through a natural watercourse with respect to the rights and obligations of the respective property owners.” (5 Miller & Starr, Cal. Real Estate (2d ed. 1989) § 14:24, p. 357; see also Hall v. Wood (Miss. 1983) 443 So.2d 834, 838 [“As recently as 1978 this Court intimated that there might be a difference in principle between the two types of cases. [Citation.] Upon reflection, that difference escapes us.”].) Defendants argue that the natural watercourse rule does not include a reasonableness element and should remain the law. They contend that the rationale on which the rule is based remains valid, and that application of the Keys v. Romley rule of reasonableness would create a virtual strict liability for public entities owning streets, storm drains, or other water-impervious improvements. Public entities, they claim, would bear extraordinary liability solely because storm waters falling on their thoroughfares ultimately reach a natural watercourse. The natural watercourse rule, they argue, properly permits the natural and intended usage of the creeks and waterways as a means of discharging the waters which would normally be conveyed therein. The argument both misstates the issue and exaggerates the potential liability. Draining surface waters from impermeable surfaces and channeling the flow into a waterway in culverts and storm drains is not the manner in which surface water would naturally be discharged into a waterway. Both the volume and the velocity of the discharge are abnormal, and it is the damage which may be caused by that unnatural method of drainage that is in issue. Our past decisions do not hold that immunity exists under the natural watercourse rule or its analogs for conduct which alters natural drainage and thereby creates a danger to downstream property owners if that danger is unreasonable in light of the purpose of the upstream action, the manner in which it is carried out, and the alternatives that might avoid or mitigate the potential damage. Nor is the reasonable use rule one of strict liability. It requires consideration of all of the relevant circumstances, and anticipates that both the upstream riparian owner and the downstream owners will act reasonably. It does not, however, give defendants what they ask—an unqualified right to discharge surface water runoff in a manner that will cause downstream damage, and even destroy downstream property, without attempting reasonable measures to prevent or minimize downstream damage. Defendants offer no justification for a rule that would distinguish between the discharge of surface waters directly onto another owner’s property and the discharge into a natural waterway that ultimately has the same injurious effect. They seek instead absolute immunity for the discharge of surface water runoff into a natural watercourse whether or not they have reduced or eliminated the capacity of the ground to absorb normal rainfall, channeled the runoff into a single destructive outlet, or otherwise altered the volume and velocity of the waters discharged into the watercourse, and regardless of whether the watercourse is capable of carrying the increased flow of waters. In short, they seek to avoid the conclusion of Keys v. Romley (supra, 64 Cal.2d 396) that both upper and lower landowners must act reasonably with respect to one another. The upper riparian owner, they argue, has a right to engage in unreasonable conduct without regard to the impact of the action on downstream property. Defendants, who assume that Keys v. Romley, supra, 64 Cal.2d 396, does not apply to discharge of surface water into a natural watercourse or to improvements in a watercourse, offer no justification, other than the fact that they might incur liability, for recognizing a distinction between the duty of a riparian property owner to avoid injury to downstream property owners, and the duty of an uphill owner to downhill owners. We do not share the assumption that either a private or public property owner may disregard the impact of its conduct on other properties whether those properties are downstream or downslope. This court has restated the natural watercourse rule in several cases since Keys v. Romley, each of which involved an action against a municipal corporation or other governmental entity. In those cases, however, we have not considered whether that rule, as applied in this state, does include an element of reasonableness, or whether the rule of Keys v. Romley, supra, 64 Cal.2d 396, which expressly holds that the upper owner’s conduct be reasonable, applies to the manner in which a riparian owner discharges surface waters into a natural watercourse. Although this court has not considered the latter question, the Court of Appeal has done so in a series of decisions in which,the court either assumed that the rule of reasonableness is applicable or expressly held it to be applicable to discharges into natural watercourses or flood control improvements in a watercourse. In Ektelon v. City of San Diego (1988) 200 Cal.App.3d 804 [246 Cal.Rptr. 483], nonsuit for a private developer was reversed. The Court of Appeal held that Keys v. Romley, supra, 64 Cal.2d 396, created a “broad rule of reasonableness to be applied to all factual situations, . . .” (200 Cal.App.3d at p. 808.) Therefore, ordinary negligence principles governed the manner in which flood control structures were constructed because “[a]n upstream landowner has no absolute right to protect his land from floodwaters by constructing structures which increase the downstream flow of water into its natural watercourse, but is instead governed by the ordinary principles of negligence.” (Id., at p. 810.) In Martinson v. Hughey (1988) 199 Cal.App.3d 318 [244 Cal.Rptr. 795], the court assumed that the rule of reasonableness applied to the discharge of irrigation waters and surface waters into a natural watercourse. There a lower owner had blocked a natural watercourse with debris which backed water up onto the upper land. Applying the rule to irrigation waters, the court concluded: “The rule we deduce ... is that the upper owner has the right to discharge reasonable and noninjurious amounts of irrigation water through natural areas of flow onto the lower owner’s property. The lower owner has a co-equal burden to receive reasonable and noninjurious amounts of irrigation water through natural flowage channels.” (199 Cal.App.3d 318, 328.) In Weaver v. Bishop (1988) 206 Cal.App.3d 1351 [254 Cal.Rptr. 425], the question was liability for damages for improvements in a natural watercourse constructed for the purpose of protecting the defendants’ property. The court held that the reasonable use doctrine articulated in Keys v. Romley, supra, 64 Cal.2d 396, 408-409, was properly applied in an action predicated on damage caused by the riparian owner’s installation of riprap (boulders) along the stream bank to protect the land from erosion. The riprap altered the flow of the stream sufficiently that erosion occurred on the opposite bank, the owners of which sued. Given an instruction that liability depended on the reasonableness of each party’s conduct, the jury found by special verdict that defendants’ conduct was reasonable, while that of plaintiffs was not. The Court of Appeal reasoned in Weaver v. Bishop that neither the rule which gave a riparian owner absolute immunity for alteration in stream flow to protect his property, nor the “common enemy doctrine” which permits an owner to protect himself against floodwaters, even by turning them onto another’s land, should apply. “The common enemy doctrine is one form of the ‘right to inflict damage,’ which was traditionally referred to under the [rubric] ‘damnum absque injuria’ (harm without legal injury). This notion, peculiar to water law, rested on the ‘generally perceived reasonableness’ of actions taken to protect one’s property and on a policy of encouraging the preservation of land resources. [Citation.] However the nearly unanimous trend has been away from per se rules based on categorical judgments of ‘generally perceived reasonableness,’ and toward fact-based determinations of reasonableness in the particular circumstances of each case.” (Weaver v. Bishop, supra, 206 Cal.App.3d 1351, 1357, fn. omitted.) “[A]s Keys acknowledges and illustrates, the general trend in water-damage cases is to replace the rigidities of property law with the more flexible, conduct-oriented principles of tort. (See 64 Cal.2d at p. 408.) Under the latter as expressed in the Second Restatement of Torts, defendants’ liability would depend on a balancing of reasonableness, either by analogy to the rules concerning interference with water use, or under the rules of nuisance and trespass.” (206 Cal.App.3d at p. 1358.) The Court of Appeal in this case reasoned that important policy reasons had initially supported the immunity doctrine and saw no compelling reason to: reject it, noting that it was bound in any case to follow the precedent established in Archer. The court observed that the Archer rule had been followed in Deckert v. County of Riverside (1981) 115 Cal.App.3d 885, 895, 896 [171 Cal.Rptr. 865], and elected to join the Deckert court, rather than the courts which had concluded that the rule of absolute immunity had been replaced by a rule of reasonableness. The Deckert opinion followed what that court believed to be the rule established in Archer and our subsequent decision in Bauer v. County of Ventura, supra, 45 Cal.2d 276, without mention of Keys v. Romley, supra, 64 Cal.2d 396, however. It does not support a conclusion that the Archer rule does not include a requirement of reasonableness or that, if the Archer rule does not, it survived Keys v. Romley. By contrast, Weaver v. Bishop, supra, 206 Cal.App. 3d 1351, Ektelon v. City of San Diego, supra, 200 Cal.App.3d 804, and Martinson v. Hughey, supra, 199 Cal.App.3d 318, and, of course, Keys v. Romley, supra, 64 Cal.2d 396, reflect the nationwide trend toward merger of the rules governing diffused surface water and those governing watercourses. (See Tarlock, Law of Water Rights and Resources (1993) § 305[1], pp. 3-14 to 3-15.) We need not decide whether the natural watercourse rule applicable at the time of Archer, supra, 19 Cal.2d 19, included an element of reasonableness because we agree with those courts which have held that Keys v. Romley states a rule that is applicable to all conduct by landowners in their disposition of surface water runoff whether the waters are discharged onto the land of an adjoining owner or into a natural watercourse,,as well as to the conduct of upper and lower riparian owners who construct improvements in the creek itself. Although Keys v. Romley was decided in the context of damage caused to adjacent land by the discharge of surface waters, the reasoning of the court has broader applicability. The decision rests on the broad principle that a landowner may not act “arbitrarily and unreasonably in his relations with other landowners and still be immunized from all liability, [f] It is therefore incumbent upon every person to take reasonable care in using his property to avoid injury to [other] property . . . .” (Keys v. Romley, supra, 64 Cal.2d at p. 409.) While the court spoke in terms of the responsibilities of adjacent landowners with respect to surface waters, we did not intend thereby to imply that the obligation to take reasonable care was not one imposed also on upper and lower riparian owners. There is no exception from the rule of reasonableness for riparians. No logic would support such a distinction and we decline to recognize one. Defendants’ argument that Keys v. Romley is not and should not be applicable to discharge of surface waters into a natural watercourse overlooks the authority on which Keys v. Romley relied for the rule of reasonableness that it enunciated. That rule is derived from Armstrong v. Francis Corp. (1956) 20 N.J. 320 [120 A.2d 4, 59 A.L.R.2d 413]. (Keys v. Romley, supra, 64 Cal.2d at p. 410.) Armstrong is a natural watercourse case. The facts which gave rise to the New Jersey Supreme Court’s decision to abandon the common enemy/immunity rule in Armstrong v. Francis Corp., supra, 20 N.J. 320 [120 A.2d 4], are not unlike the facts in the case before this court. A housing developer stripped his tract, covered a stream that was a natural watercourse, and built a drainage system which conveyed not only the surface water runoff, but also percolating waters under the surface, into a pipe which discharged these waters into the stream above plaintiffs’ property. As a result, the increased volume of water and its accelerated speed tore into the banks of the stream and, at the time of the lawsuit had carried away 10 feet of the plaintiffs creekside property with no end to the erosion in sight. Adopting the rule of reasonableness for that state, the New Jersey Supreme Court concluded: “Social progress and the common wellbeing are in actuality better served by a just and right balancing of the competing interests according to the general principles of fairness and common sense which attend the application of the rule of reason.” (120 A.2d at p. 10.) New Jersey is not alone in abandoning the common enemy/immunity doctrine. A majority of jurisdictions now apply a rule of reasonableness to the discharge of surface waters into a natural watercourse when the discharge will overtax the capacity of the watercourse or drainage channel and cause damage to downstream property. (5 Waters and Water Rights, supra, § 59.02(b)(1), p. 503; see, e.g., Heins Implement Co. v. Hwy. & Transp. Com’n (Mo. 1993) 859 S.W.2d 681; Hansen v. Gary Naugle Const. Co. (Mo. 1990) 801 S.W.2d 71; Johnson v. NM Farms Bartlett, Inc. (1987) 226 Neb. 680 [414 N.W.2d 256]; Martin v. Weckerly (N.D. 1985) 364 N.W.2d 93; Peterson v. Town of Oxford (1983) 189 Conn. 740 [459 A.2d 100]; County of Clark v. Powers (1980) 96 Nev. 497 [611 P.2d 1072].) The suggestion that the court would find the reasoning of Armstrong v. Francis Corp., supra, 120 A.2d 4, persuasive, but only insofar as surface water runoff onto adjacent property is concerned and not in the context in which that case was decided, is further undermined by recognition that other cases on which the court relied in Keys v. Romley, supra, 64 Cal.2d 396, also stated rules of reasonableness applicable to both runoff onto adjacent property and into a natural watercourse. In Bassett v. Salisbury Manufacturing Company (1862) 43 N.H. 569, 576, the court discussed both types of drainage and stated: “[S]o far as a similarity of benefits and injuries exists, there should be a similarity in the rules of law applied.” “The rights of each land-owner being similar, and his enjoyment dependant [sic] upon the action of the other land-owners, these rights must be valueless unless exercised with reference to each other, and are correlative. The maxim, 'Sic utere,’&c., [Sic utere tuo ut alienum non laedas—use your own property in such a manner as not to injure that of another] therefore applies, and, as in many other cases, restricts each to a reasonable exercise of his own right, a reasonable use of his own property, in view of the similar rights of others. Instances of its similar application in cases of water-courses ... are too numerous and familiar to need more special mention. As in these cases of the water-course, so in the drainage, a man may exercise his own right on his own land as he pleases, provided he does not interfere with the rights of others.” (Id., at p. 577.) In Swett v. Cutts (1870) 50 N.H. 439, 446, also relied on by the court in Keys v. Romley (supra, 64 Cal.2d at p. 404), the same theme of reasonable conduct regardless of the nature of the water right being exercised was again expressed: “The doctrine which we maintain adapts itself to the ever varying circumstances of each particular case,—from that which makes a near approach to a natural water-course, down by imperceptible gradations to the case of mere percolation, giving to each land owner, while in the reasonable use and improvement of his land, the right to make reasonable modifications of the flow of such water in and upon his land, [f] In determining this question all the circumstances of the case would of course be considered; and among them the nature and importance of the improvements sought to be made, the extent of the interference with the water, and the amount of injury done to the other land owners as compared with the value of such improvements, and also whether such injury could or could not have been reasonably foreseen.” (50 N.H. at p. 446.) Most recently, the Missouri Supreme Court has brought all types of water within the rule of reasonableness. In Heins Implement Co. v. Hwy. & Transp. Com’n, supra, 859 S.W.2d 681, 691, the court observed: “The standard we sanction today is in harmony with the most basic tenets of our law. ‘Reasonableness is the vital principle of the common law.’ [Citation.] Reasonable use concepts already govern the rights of users of our watercourses, subterranean streams, and subterranean percolating waters. [Citations.] To some extent, they have also applied to upper land owners through the modified common enemy doctrine. Their extension to the management of all diffuse surface waters finally ‘bring[s] into one classification all waters over the use of which controversy may arise.’ ” Defendants have offered no persuasive reason to limit the requirement that landowners act reasonably with regard to one another to their treatment of surface water discharge onto adjacent property while permitting unreasonable conduct when the waters are discharged into a natural watercourse. Indeed, defendants appear to overlook the impact on their own interests of a rule which would afford them no recourse if a private developer discharged surface water runoff into a natural watercourse adjoining publicly owned property in a manner which undercut and washed away a portion of that property. We agree with plaintiffs, therefore, that the rule of Keys v. Romley applies to this dispute. Under that rule: “The issue of reasonableness becomes a question of fact to be determined in each case upon a consideration of all the relevant circumstances, including such factors as the amount of harm caused, the foreseeability of the harm which results, the purpose or motive with which the possessor acted, and all other relevant matter. (Armstrong v. Francis Corp. (1956) supra, 20 N.J. 320.) It is properly a consideration in land development problems whether the utility of the possessor’s use of his land outweighs the gravity of the harm which results from his alteration of the flow of surface waters. [Citation.] The gravity of harm is its seriousness from an objective viewpoint, while the utility of conduct is meritoriousness from the same viewpoint. (Rest., Torts, § 826.) If the weight is on the side of him who alters the natural watercourse, then he has acted reasonably and without liability; if the harm to the lower landowner is unreasonably severe, then the economic costs incident to the expulsion of surface waters must be borne by the upper owner whose development caused the damage. If the facts should indicate both parties conducted themselves reasonably, then courts are bound by our well-settled civil law rule.” (64 Cal.2d at p. 410.) As we have shown above, however, the “well-settled civil law rule” dictates a different result for riparian owners than that applicable to upland owners. Under the Keys v. Romley rule, if both parties act reasonably with respect to draining surface waters onto adjacent property the upper owner will be liable for damage caused by the alteration of the natural flow of the water. The result will differ in disputes between riparian owners, each of whom acts reasonably. The civil law rule with respect to natural watercourses, unlike that applicable to draining surface waters onto adjacent property, immunizes the upper riparian owner for damage caused by the alteration of the natural discharge of surface water into a watercourse and by improvements in the stream bed. Therefore, if the upper owner acts reasonably, or if the lower owner has not acted reasonably to protect the property, the lower riparian owner must continue to accept the burden of damage caused by the stream water. As we noted earlier, however, the reasonableness of a landowner’s action in discharging surface water runoff into a natural watercourse or in altering the watercourse itself cannot be determined in isolation. An owner in the lower reaches of a natural watercourse whose conduct has a relatively minor impact on the stream flow in comparison with the combined effect of actions by owners in the upper reaches of the watercourse may not be held liable for any damage caused by the stream flow beyond the proportion attributable to such conduct. If the rule were otherwise, owners at the lowest reaches of a watercourse could preclude development of upstream property by imposing on a single upstream owner the cost of all damage caused by the addition of surface water runoff if that addition combined with the existing stream flow damaged the lowest properties. The purpose of both the civil law rule creating immunity for damage caused by surface water runoff onto adjacent property and the natural watercourse rule which imposed the burden of damage caused by upstream development on the downstream owner was to ensure that development of property would not be foreclosed by imposition of liability for damage caused by changes in the treatment of surface water occasioned by that development. Keys v. Romley and the application of the rule of reasonableness to natural watercourses further that purpose. The rules applicable to surface water runoff onto adjacent property or into a natural watercourse have been modified only by limiting the immunity created by the civil and common law rules to conduct that is reasonable. The trial court and the Court of Appeal thus erred in concluding that the natural watercourse rule immunized defendants from tort liability as landowners for damage caused by their discharge of surface water runoff into Reliez Creek regardless of the reasonableness of their conduct. We shall nonetheless affirm the judgment of the Court of Appeal. Even were we to assume that the evidence would support a finding that increased surface water runoff or altered discharge of surface water runoff caused by improvements on any defendant’s property was a substantial cause of the damage to plaintiffs’ properties, plaintiffs have not established that defendants acted unreasonably in the construction of improvements or alteration of the method of discharge of the runoff; nor have plaintiffs established that they acted reasonably to protect their properties from stream-caused damage. V Inverse Condemnation The Court of Appeal held that substantial evidence supported the trial court’s conclusion that Reliez Creek was not a public improvement because respondents had not exercised control over the creek, had not erected structures other than the Sizeler outfall and sheet pile structure in it, and had not accepted the dedicated storm drainage easements. Therefore, the court held, notwithstanding the increased runoff into Reliez Creek caused by defendants’ streets, roads, and other public works, the natural watercourse rule also immunized respondents from liability in inverse condemnation for the damage to plaintiffs’ property. It did so because at common law a governmental entity, like a private party, had a right to collect surface waters on its land and discharge them into a natural watercourse. Plaintiffs dispute both the conclusion that defendants could not be liable in inverse condemnation even if defendants’ use of Reliez Creek to drain their roads and other public works was unreasonable, and the conclusion that defendants’ actions with respect to the creek did not cause it to become a public work. Moreover, they argue, the question of public use is one to be decided as a matter of law by the appellate court, and the Court of Appeal erred in applying a substantial evidence standard of review. Finally, they argue that the Court of Appeal erred in holding that there can be no recovery against a public entity whose conduct contributed to the damage unless the plaintiff establishes the proportionate share of damage caused by that entity. We agree with the Court of Appeal that plaintiffs’ evidence did not establish that Reliez Creek had become a public work. We also agree