The sound of running water appears to stimulate dam-building, and the sound of a leak in a dam triggers them to repair it. To build a dam, beavers use log poles, around long and in diameter, to brace against the banks. They align these in the direction of the water's flow at an angle of around 30 degrees. Heavy rocks weigh down the poles, and grass is stuffed between them. Beavers continue to pile on more material until the dam settles into a compact slope on the enclosed side. Dams can be as low as to as high as tall and can stretch from to several hundred meters long. Beaver dams appear to be more effective than human-made concrete dams in trapping water and slowly releasing it. Lake-dwelling beavers do not need to build dams.