A weir is a small dam built across a river or stream to control water flow. It helps manage water levels, allowing for irrigation, flood control, and maintaining aquatic habitats. Weirs can also create a calm area for fish and other wildlife. Weirs are often made from materials like concrete, stone, or wood. They can vary in size and design, depending on their purpose. Some weirs are equipped with spillways to allow excess water to flow over, while others may have gates to regulate water release. Overall, weirs play a crucial role in water management systems.