While pumped storage plants use small water flows with a high head, low-head run-of-river plants need the large water flows found in major rivers. Unlike the power plants in the Alps, the difference in level between the upper and lower reaches is small. Run-of-river power plants function continuously and cover normal power needs. They are therefore known as base-load power plants or off-peak supply stations. If ever it becomes necessary to turn them off, valuable energy is wasted as the water rushes over the weir.