The shape of river valleys changes as rivers flow downstream – River long and cross profiles
The river drainage basin
Rivers change immensely on their journey from Source areas (where they start) to their finishing point at their mouths. The drainage basin, as shown above, is the area of land drained by a river system (a river and its tributaries). It includes the surface run-off in the water cycle, as well as the water table. Drainage basins are separated by watersheds. A drainage basin is an example of an open system because it is open to inputs from outside, such as precipitation, and is responsible for outputs out of the system, such as output of water into the sea and evaporation of water into the atmosphere. The water cycle is a series of processes by which water is evaporated from the sea and eventually condenses and precipitates over the land. You can see how water travels through the drainage basin system on the diagram opposite.
The long profile of a river is a way of displaying the channel slope of a river along its entire course. Therefore, it shows how a river loses height with increasing distance towards the sea.
The long profile of a river
The diagram shows how the long profile changes downstream. In the upper reaches (also known as the source) the gradient is at its steepest because of vertical erosion. This changes further downstream as lateral erosion (side to side) from the river and deposition start to replace vertical erosion as the dominant process. Finally, the river really flattens out as it approaches the mouth as deposition become dominant. In addition, along the upper part of the long profile or Thalweg there is more turbulence, lots of bed load in comparison to discharge and lots of roughness and fiction. As more streams and tributaries join the river, roughness decreases, discharge and velocity increases and the erosive power of bed load will decrease. As a result the gradient of the river will generally decrease creating a concave long profile with distance downstream, and deposition serves to enhance this phenomenon further. The cross profile diagrams show that in the source area the drainage basin (an area of land drained by a river and its tributaries) contains V shaped valleys and waterfalls, and the dominant processes are erosion. Erosion tends to be vertical (straight down into the land).
In the middle section of the drainage basin the river starts to erode laterally. This section contains meanders and Ox bow lakes, and the river creates a flood plain often with Levees. Here, Material is deposited and erosion can also occur.
In the lower drainage basin deposition dominates as a river enters a sea or lake, the valley is at its widest and deltas and estuaries are major landforms and habitat.