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Life in Freshwater

Stages along a River

The section on Stream Ordering helps explain how a river may be classified but it is important to understand what is the background to the change that occurs as you follow the river from the source to estuary. Essentially, there are three main sections to a river. In the diagram below the three regions can be described as

A the Erosion Zone, where sediment is produced (found in the headwaters)

B the Transfer Zone, where sediment is moved along the river

C the Deposition Zone, where the sediment collects on the bottom

The proportions of these zones vary with the topography and it may be seen that this is rather a simplification of what happens along the river. However, this will fit with the physical and chemical features (abiota) that change along the river. Where the speed of current is fast erosion occurs and when the current slows sediment is deposited. This is why the oxygen content will be at its height in the erosion zone.

Three stages of a river

Water will enter from either groundwater or from local catchment. As well as showing the typical zones A-C the diagram indicates the level of water entering or leaving the river along its length. The size of the arrow suggests the degree of water present. For example in region A the amount of groundwater entering is slight, possibly due to shallow soil. By contrast the run-off from the local terrain (catchment) will be the main source of water. Region A could be steeper and with water rapidly being removed by the faster current the arrows are showing water input. By region B catchment is declining and groundwater could move either way.

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