The law governing liability for property damage caused by surface waters in California derives from the Keys v. Romley (1966) 64 Cal.2d 396, 50 Cal. Rptr. 273, 412 P.2d 529 decision. There, the defendants owned unimproved land abutting the plaintiffs' property. The defendants did some grading and leveling of the land and built an ice rink, causing rain runoff to flow toward the plaintiffs' land. The plaintiffs also erected a commercial structure and, thereafter, removed a dirt pile from the rear of their property. Over the next several years, the plaintiffs' property was flooded by surface waters flowing from the defendants' adjoining land. ( Keys, supra, 64 Cal.2d at p. 399.) On appeal, the dispositive question was under what circumstances could the defendants be found liable for the flooding because of their use and alteration of their property. In analyzing the question, the Supreme Court first reviewed the various rules adopted in different jurisdictions to govern liability for property damage caused by surface water. "Surface water" is defined as: "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 . . . . 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." (Keys, supra, 64 Cal.2d at p. 400.) The court noted that California had always followed the civil law rule, which held that "the owner of an upper, or dominant, estate is entitled to discharge surface water from his land as the water naturally flows. As a corollary to this, the upper owner is liable for any damage he causes to adjacent property by the discharge of water in an unnatural manner. In essence, each property owner's duty is to leave the natural flow of surface water undisturbed." ( Id. at pp. 405-406.) Called upon to determine whether this rule should be applied in developed urban areas, the court characterized the traditional civil law rule as "unnecessarily rigid and occasionally unjust, particularly in heavily developed areas," because it placed "the entire liability for damages on one owner on the basis of the unvarying formula that he who changes conditions is liable." ( Id. at p. 407.) The court also determined that the liability for interfering with surface waters is best resolved under tort principles, rather than under property law principles. ( Id. at pp. 407-408.) The Keys rule "attempts to determine the rights of the parties by an assessment of all the relevant factors. The key point of departure from the earlier rules is that the reasonable use rule treats the issue of surface waters as a question of tort law instead of property law." (6 Miller & Starr, California Real Estate (3d. ed. 2000) 14.21, com., p. 52.) The court determined that "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." (Keys, supra, 64 Cal.2d at p. 410, citing Armstrong v. Francis Corporation (1956) 20 N.J. 320 120 A.2d 4, 10.) "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." (Keys, supra, at p. 410.)