Terminology in open-channel structures:\nA structure that dams up a stream or river and over which water flows is called what?

Introduction / Context:
Open-channel measurement and control structures include notches, weirs, flumes, and dams. The terms are sometimes used loosely, but in hydraulics a weir has a specific function and geometry: a barrier or crest across a channel over which water flows, enabling head–discharge relationships and level regulation.

Given Data / Assumptions:

• Flow passes over a crest with a defined head upstream.
• Free overfall or submerged (drowned) conditions may occur.
• Permanent structure across the full channel width (usually).

Concept / Approach:
A weir is a low dam or barrier designed so that flow passes over its crest. It is used to measure discharge (via stage–discharge calibration) and to control upstream water levels. Notches are smaller openings used in laboratory tanks or small channels. An “orifice” is a submerged opening through a wall; a “dam” generally impounds water but is not primarily defined by overflow measurement functions.

Step-by-Step Solution:

Verification / Alternative check:
Standard weir types include sharp-crested, broad-crested, ogee, and Cipolletti (trapezoidal). Each has known head–discharge formulas, confirming the measurement role that distinguishes weirs from general dams.

Why Other Options Are Wrong:

• Orifice: Submerged opening through which water issues.
• Notch: Laboratory-scale opening in a tank wall.
• Dam: Generic impounding structure; may not be designed for calibrated overflow.

Common Pitfalls:
Using “dam” and “weir” interchangeably; overlooking that notches are essentially small weirs used for measurement at smaller scales.

Final Answer:
weir