Does the well-settled civil law rule of reasonableness apply to landowner's actions in discharging surface water runoff into a natural watercourse?

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

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