1. A channel for carrying away excess water; such as, at a reservoir or dam: The city engineers and Mr. Carlton designed a new wasteweir in an attempt to avoid floods that had plagued the area of the city near the river.
2. An overfall that functions for the escape, or overflow, of superfluous liquids from a canal, reservoir, pond, etc.: A wasteweir is a dam that is built across a river to regulate the flow of water, to divert it, or to change its level.

The wasteweir is also a term for a barricade for fish; such as, a fence placed in a stream to catch or to keep them from swimming away from the area.