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Chapter 4 -- Watershed Processes and Aquatic Ecosystems (continued)

Aggradation

Within any river network, some areas may be aggrading while others are degrading. Grading of the stream channel is influenced by stream flow, sediment load, and many other factors occurring over many years. Aggradation of the channel can occur either slowly or rapidly while degradation is slow and occurs over decades.

Aggradation of the channel increases the tendency for high stream temperatures. As sediment fills in the stream channel, the stream widens and develops a larger surface area. The greater surface area captures a larger amount of solar energy and increases the tendency of the stream to warm up on sunny days. High stream temperatures are most likely during low flows.

Peak flows have greater impacts in aggraded channels. As a channel becomes filled with sediments, it will overflow its banks at a lower discharge. As the channel meanders on the flood plain which is wider due to aggradation, it may undercut slopes and create landslides. Riparian vegetation may also be removed as the channel is reconfigured.

Although increased sediment loads have negative impacts, periodic sediment input is necessary over the long term to maintain certain types of stream habitat. Lack of adequate sediment is rarely a problem except below dams. In highly aggraded areas, gravel mining may be an effective way to remove excessive sediment load and improve ecological conditions.

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