In Locklin v. City of Lafayette (1994) 7 Cal.4th 327, the Supreme Court considered a variant of the facts in Keys. Public entities owned streets, drainage systems and other water-impervious improvements. Surface waters were discharged therefrom into a natural watercourse causing an increased flow of stream water which damaged plaintiffs' downstream properties. 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.' " ( Locklin, supra, 7 Cal.4th at p. 345, quoting San Gabriel V.C. Club v. Los Angeles (1920) 182 Cal. 392, 397, 188 P. 554.) Under the natural watercourse rule, an upstream property owner who altered the natural drainage, channeling surface waters into a natural watercourse, was immune from any resulting damage to property downstream. ( Locklin, supra, 7 Cal.4th at p. 344.) The defendants in Locklin relied on this historic rule and argued that "extraordinary liability" would result if the court set it aside. ( Id. at p. 352.) The Supreme Court, however, held that a rule that distinguishes between the discharge of surface waters directly onto another owner's property and the discharge into a natural waterway that ultimately results in identical damage would be unjustified. It rejected the public entities' request for "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." ( Id. at 353) The court then imposed the Keys reasonable use test when surface waters channeled into a waterway damaged downstream property. One difference in the application of the reasonable use test in Locklin from that in Keys relates to the result required when all relevant parties have acted reasonably. The civil law rule modified in Keys imposed liability whenever the uphill owner altered the natural flow of surface waters and damaged adjacent property. After Keys, if all parties acted reasonably, the uphill owner was liable. The natural watercourse rule changed by Locklin, however, provided immunity when an upstream owner's alteration of the flow of surface water into a natural watercourse damaged downstream property. After Locklin, if all parties acted reasonably, the downstream owner could not recover for its damage. ( Locklin, supra, 7 Cal.4th at pp. 360-361.) In Locklin v. City of Lafayette (1994) the Supreme Court adopted a standard of reasonableness for assessing the rights of riparian property owners: "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 property owner may be liable for that damage. The test is whether, under all the circumstances, the upper landowner's conduct was reasonable. . . . If both plaintiff and defendant have acted reasonably, the natural watercourse rule imposes the burden of stream-caused damage on the downstream property." ( Id. at p. 337.)