Iowa Code § 468.621
Owners of land may drain the land in the general course of natural drainage by constructing or reconstructing open or covered drains, discharging the drains in any natural watercourse or depression so the water will be carried into some other natural watercourse, and if the drainage is wholly upon the owner's land the owner is not liable in damages for the drainage unless it increases the quantity of water or changes the manner of discharge on the land of another. An owner in constructing a replacement drain, wholly on the owner's land, and in the exercise of due care, is not liable in damages to another if a previously constructed drain on the owner's own land is rendered inoperative or less efficient by the new drain, unless in violation of the terms of a written contract. This section does not affect the rights or liabilities of proprietors in respect to running streams.
Iowa Code § 468.621
87 Acts, ch 225, §306; 89 Acts, ch 126, §2