Which of the following best describes an obstruction placed in flowing water for measurement?

A weir is a barrier placed across a flowing body of water, specifically designed to measure the flow of water. It plays a crucial role in hydrology and water resource management by providing accurate data on water flow rates. As water flows over the weir, it creates a measurable drop in water level, allowing for the calculation of the flow based on established relationships between water height and flow rate. This makes weirs an essential tool for monitoring surface water, assessing water quality, and managing aquatic ecosystems.

In contrast, a drain is a structure designed to remove excess water or other liquids, a dam is a barrier constructed to obstruct and often store water, and a channel refers to the physical pathway through which water flows. While these structures may interact with flowing water, they do not specifically serve the purpose of measuring water flow in the same manner as a weir does.