What is a weir used for in water systems?

A weir is a structure placed across an open channel, primarily used to measure the flow of water. It helps to determine the volume of water passing through a given point, providing essential data for managing water resources. By creating a change in the water level, a weir allows for the accurate measurement of flow rate, which is crucial for various applications, including flood control, irrigation planning, and environmental monitoring.

The other options focus on different functions that water systems may perform but do not accurately describe the primary purpose of a weir. While purifying water and controlling algae growth are critical aspects of managing bodies of water, these tasks are generally accomplished through various treatment processes or biological control methods rather than through the structure of a weir. Similarly, enhancing irrigation is typically achieved through different devices and methods rather than the use of a weir, which serves a more focused role in measurement and flow management.