Cause and effect relationships in degradational processes. In general, the cause of degradation is any event the increased the capacity of a stream relative to its sediment load. The effect is the erosion of alluvium, which reduces the down valley stream gradient and lowers the stream's capacity, so that its capacity is adjusted to the sediment load. Any occurrence that reduces sediment delivery to the channel can trigger degradation. E.g., construction of a dam on a stream with high sediment load.
Reservoirs gradually fill with sediment delivered from higher in the watershed.
Below the dam, water is discharged into the stream without a sediment load. This triggers degradation below the dam, as the stream's excess energy incises into material downstream. This is an especially serious problem along the Colorado River in the American Southwest, thanks to the sparse vegetation cover. Reservoirs like Lake Powell and Lake Mead are filling rapidly with sediment. Downstream, the Colorado runs clear and erodes into its channel.