Civil Engineering Professional Engineer Practice Exam

Weirs play a crucial role in river systems primarily for controlling and regulating water levels and flows. By acting as barriers that allow water to flow over them, weirs can help manage the hydrograph of a river, which is essential for various purposes including flood prevention, irrigation, and habitat maintenance for aquatic life.

When a weir is constructed, it creates a controlled environment where the upstream water level can be raised, thus allowing for better management of water resources. This is especially important in times of drought or during periods of excess water due to rainfall. By adjusting the flow over a weir, engineers can ensure that downstream areas receive an adequate supply of water while also mitigating risks associated with flooding, since they can help dissipate excess flow.

While weirs may also have secondary benefits such as contributing to recreational areas or providing some monitoring capabilities, their primary function within river systems is the regulation of water levels and flow dynamics. This function is critical for maintaining the health of the ecosystem and supporting various human activities, making it clear why regulating water levels and flows is the correct answer.