What does tailwater mean?
Tailwater is water that flows downstream after passing a mill, dam, weir, or similar structure. In the most common technical usage, it is the volume of water moving away from the structure and continuing downstream, rather than water being held back upstream. The word can also refer to the channel or area where that water flows. Tailwater conditions matter in hydraulics because changes in the amount and speed of downstream flow can affect erosion, channel stability, water quality, and nearby infrastructure. Around an old mill, the term may simply describe the river below the millrace. In engineering contexts, “tailwater” is more specific than “downstream water,” while “tailrace” more precisely names the physical passage that carries the flow.
Water flowing downstream from a mill, dam, weir, or other hydraulic structure.
"The tailwater surged through the channel after the gates were opened."
Chiefly a technical term in hydraulics, water management, and the description of mills and dams.
The channel, area, or body of downstream water receiving the flow from a mill, dam, or similar structure.
"The old pumping station stood beside the tailwater at the foot of the weir."
This sense focuses on the physical location or channel rather than only the moving water.
Used when referring to separate downstream flows, reaches of water, or discharges from different structures or locations.
"The report compared the tailwaters below the upstream and downstream dams."
A dam may stop the river's flow in one direction while sending a distinct stream of tailwater rushing onward below it.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does tailwater come from?
“Tailwater” is a compound formed within English from “tail” and “water.” The word “tail” originally referred to the end or rear of an object, and in hydraulic descriptions came to mean the downstream side of a mill or dam. “Tailwater” developed to name the water associated with that downstream end, particularly at mills and waterworks.
How tailwater is actually used
The term is chiefly used in hydraulic engineering, mills, dams, and discussions of water management. It is more technical than the general expression 'downstream water,' and its exact sense can depend on context.
Easily confused with tailwater
A tailrace is the channel or passage that carries water away from a mill or dam, whereas tailwater refers primarily to the water flowing downstream.
Backwater can mean water held up behind an obstruction, while tailwater is water flowing away downstream from a mill or dam.