Tailwater

Part of speech: noun

Definitions

1. The water that flows downstream beneath a dam or weir ; the runoff that collects in a field after irrigation or heavy rain
2. Water running on the downstream side of a dam or spillway ; surplus water that drains across farmland following watering or a downpour
3. The flow found below a barrage or weir ; the excess moisture left standing over cultivated ground after irrigation or storms

Etymology: Tailwater is a plain compound: "tail" in the sense of the back or rear end of something, joined to "water." Its literal sense is the stretch of water lying at the rear, or downstream, end of a structure, which for a dam means the water that has already passed through or over it and flows away below. The image makes the logic clear. A dam holds water back on its upstream face, which is the head of the reservoir, while the water released from it spills out on the downstream side, the tail. The same logic governs the word's counterpart, "headwater," the water at the front of a stream or structure. The pairing of head and tail is a familiar way of naming the two ends of a body of flowing water, and tailwater applies that pattern to the engineered structure.