Does the natural watercourse rule apply to the discharge of surface water into natural waterways?

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, 50 Cal.Rptr. 273, 412 P.2d 529) 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.

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[867 P.2d 740] Defendants, who assume that Keys v. Romley, supra, 64 Cal.2d 396, 50 Cal.Rptr. 273, 412 P.2d 529, 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 [7 Cal.4th 354] 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, 50 Cal.Rptr. 273, 412 P.2d 529, 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. ¹⁶