Water flows across land. When land is developed, this can alter the natural flow of water, which can damage other landowners. Under what circumstances is a landowner liable to others for altering the natural flow of water? Under modern California law, this is a complex question. The issue is complicated, because modern California law is in transition between older rules, which were rigid, simple and arguably unjust, and a new approach, which is based on a flexible rule of reason. Water law is also complex, because it makes a series of distinctions between surface waters, flood waters, natural watercourses and tidewater.

The old rule in California regarding surface waters – water that flows across the surface of land, but is not in any regular watercourse – is the so-called civil law rule. Under this rule, landowners cannot interfere with the natural flow of surface water. Under this rule, if an upstream landowner makes any change, which damages some one downstream, the upstream owner is absolutely liable for damages.

This strict rule was modified by the landmark decision of Keys v. Romley (1966) 64 Cal. 2d 396, 412 P. 2d 529. Keys stated the reasonable use doctrine, under which the reasonableness of the behavior of both landowners is examined. To evaluate reasonableness, all factors must be taken into consideration, including the benefits to the upstream landowner of the alterations and the relative costs to the downstream owner. The general rule stated is that all landowners have a duty to behave reasonably, in defense of their own interests and to avoid foreseeable damage to others.

Keys, however, did not totally shift the rule to a free-form evaluation of who was more reasonable. Gdowski v. Louie (2000) 84 Cal. App. 4th 1395. Rather, it modified the civil law rule. Under the new approach, the court looks at the reasonableness of the behavior of both landowners. If the upstream owner was reasonable, but the downstream owner was unreasonable, the upstream owner is not liable for damages. If, however, the downstream owner was reasonable, he or she can collect damages from the upstream owner, even if the upstream owner was reasonable.

Floodwaters traditionally are subject to a different rule. Floodwaters, which have escaped from ordinary channels under circumstances that do not ordinarily exist, are subject to the common enemy rule, under which every landowner can do whatever he or she likes to defend his or her land from the flood, without regard to the damage it does to others. This common enemy rule has now been modified by a reasonability requirement. Weinberg Co. v. Bixby (1921) 185 Cal 87. The current form of the common enemy rule gives general immunity to landowners, but only if their behavior was reasonable.

Natural watercourses are traditionally subject to yet another rule. Under this rule, an upstream landowner may concentrate waters, and discharge them into a natural watercourse, such as a stream or a river, into which the water naturally flows, even if the natural watercourse is too small to handle the increase flow. In Locklin v. City of Lafayette (1994) 7 Cal. 4th 327, the California Supreme Court modified the natural watercourse rule in light of Keys. It held an upstream owner liable, for discharging water into a natural watercourse, if the upstream owner acted unreasonably. Unlike Keys, however, if the upstream owner acted reasonably, he or she was immune from liability, even if the downstream owner also acted reasonably.

This discussion relates only to disputes between private landowners. Public agencies are often involved in water disputes, because publicly created improvements often alter the flow of water. The liability of public agencies for altering water flow is examined as part of inverse condemnation law.