Structures may vary from low gabion walls to very large earth dams lined with mattresses. They are classified according to the shape of the downstream face at the center of the flow.The most common weirs are vertical structures. The downstream face of a vertical weir is flush. These structures are often used on small streams, usually in a system of weirs. High vertical weirs require a stilling basin which may be created by constructing a scour apron and counter weir from gabion mattresses.