Describe and explain the changes in river velocity along its long profile and the channel characteristics associated with these changes.

At it's source a river has high gravitational potential energy (GPE) because of it's height above it's base level (generally the sea or a lake). This GPE translates into high velocity and therefore high erosivity as the river attempts to reach it's base level. Sources will generally be found in resistant rock types because of their height. The combination of high erosivity and resistant rock creates features such as steep sided gullies, waterfalls and pools and riffles in the upper section of the river. As it reaches more level ground closer to it's base level the river loses GPE and flows more slowly. The channel will be wider than in the upper course because it will have combined with other tributaries meaning a greater volume of water. This means velocity is highly differentiated along the cross profile of the river at this stage of the long profile. The deepest parts of the channel - in the middle and outer edge of meanders - flows faster because less of the river is in contact with the friction causing sediment of the river bed. As the river nears its base level, its GPE and therefore it's velocity reduces even further. The sediment it has accumulated through earlier periods of high erosivity gets deposited and can form braided channels or deltas. The channel here becomes even wider than in the middle course due to the volume of water now in the river system.