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Channel Processes: Aggradation

Aggradation involves the raising of the streambed elevation, an increase in width/depth ratio, and a corresponding decrease in channel capacity. Over-bank flows occur more frequently with less-than-high-water events. Excess sediment deposition in the channel and on floodplains is characteristic of the aggrading river. Often, the cause of aggradation is an increase in upstream sediment load and/or size of sediment exceeding the transport capacity of the channel. Aggradation can be a result of instability caused by over-widening of the channel with a resultant decrease in stream power and shear stress. In other instances, aggradation can be caused by tectonic process downstream of an affected reach, inducing deposition from a channel slope decrease. Increases in the width/depth ratio causes aggradation by decreasing depth, decreasing velocity due to increased frictional resistance, changing slope, reducing shear stress and unit stream power, decreasing competence and reducing sediment transport capacity. Based on the relations in Figure 10, a decrease in unit stream power indicates a corresponding dramatic reduction in both size and transport rate of bedload.

Adverse consequences associated with aggradation include channel avulsion (complete abandonment and initiation of a new channel) and major changes in the evolution of stream types. The sediment supply and adverse effects on beneficial uses can be very high due to the corresponding adjustments of the channel. Examples of aggradation processes are shown in Figures 33 to 35. The obvious decline of fish habitat, elevated stream temperatures and loss of biological function in these aggrading environments are common adverse consequences.

Figure 33

Figure 33. Aggradation of coarse gravel and cobble on an over-wide C3 stream type on lower West Fork- Southwestern Colorado.

Figure 34

Figure 34. Aggradation of sand and fine gravel in a C4 stream type on Blue Joe Creek, Idaho.

Figure 35

Figure 35. Aggradation on Willow Creek, Colo. due to excess sediment supply from upstream sources.

Bedload models and entrainment calculations can be useful in predicting aggradation potential, as long as the models are locally calibrated. The bedload models using stream power equations are preferred as they accommodate velocity and shear stress changes often associated with channel disturbance.

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