Texas Transportation Code - TRANSP § 254.006. Restrictions on Drainage Authority

(a) The commissioners court of a county may not construct a ditch without an outlet to a natural waterway large enough to carry off all water that may collect in the ditch.

(b) The commissioners court may not change the natural course of any branch, creek, or stream. The public road must cross a branch, creek, or stream at the water's natural crossing.

(c) A culvert must be of sufficient size to allow water to flow, at its natural rate at its normal peak level, from the side of the road where the road intersects the natural channel to the natural channel at the other side of the road.