These form where the meltwater from a glacier follows a pre-existing river channel. The large volume of water released from the glacier has high levels of energy together with considerable load in the form of rock fragments released from the ice mass which results in rapid erosion taking place. The channels are over deepened, particularly by abrasion, to form meltwater channels. These may also take the form of glacial overflow channels. These are mainly formed when the original channel of a river is blocked by ice. Meltwater builds up behind the blockage and, once the pressure is released, the energy causes the river to erode vertically, creating rapidly-eroded meltwater channels.