If the amount of sediment coming into a given reach of a stream is less than the capacity of the reach, the sediment quantity required to satisfy the capacity of the stream will come from crosion of the bed and there will be lowering of the bed level If the banks are erodible, material can be picked up from the banks and widening of the stream will also result. This process is known as degradation or retrogression.
Since large capacity reservoirs store behind them a large percentage of the sediment load carried by the stream, the flow over the dam and through the sluices is fairly free of sediment. This results in degradation downstream of the dam reservoir.
Mention of the phenomenon of degradation is found in Irrigation Manual by Mullins published as early as 1889.
Buckley also mentioned about it in 1905 in his book on Irrigation Works in India.
The phenomenon was probably first described in U.S.A. by Gilbert in 1917, when he clearly differentiated between scouring and degradation.
Degradation has been observed on several streams of the world.
Most of the degradation observations in India and Pakistan are from the Indo-Gangetic plain, where a thick stratum of alluvial material is available.
The degradation on the Colorado river downstream of the Hoover dam deserves special mention because of the tremendous quantity of material removed. Measurements have shown that about 110 million m³ of material have been removed from the channel in a distance of 147 km downstream of the dam.
Degradation occurs very commonly downstream of large capacity reservoirs when a large percentage of the incoming sediment is trapped in the reservoir and practically clear water flows downstream.
However, degradation can occur in other situations also as illustrated by the example of the Cherry creek in U.S.A. A very large pool was formed in the bed of this stream due to removal of large quantities of sand for construction purposes.
Most of the sediment load coming from the upstream was dropped in the pool because of the reduced velocity in the pool; the water passing downstream of the pool was thus carrying sediment much less than its capacity. As a result, degradation started downstream of the pool and it resulted in lowering of the bed by as much as 49 m.
Degradation is retarded when rock outcrops or erosion-resistant materials exist in the stream section downstream of the dam. Then the tendency the stream is to cut the banks or, if the stream is meandering, to cut away the inside of the bends and thus attain a new equilibrium condition.
Degradation is also likely to occur near the confluence of two rivers. Tributaries. are, in general, steeper than the main stream and carry less run-off. After regulation of the flows in the main stream (e.g. by construction of a dam), degradation starts and this results in a chain reaction.
Because of degradation in the main stream, the slope of the tributary increases. Therefore, the tributary will acquire sediment transport capacity in excess of the incoming sediment load.
Hence the tributary goes on degrading until the sediment load is equal to the sediment transport capacity. Under this condition the tributary will have a slope flatter than its initial slope.
Sometimes the lowering of the bed level does not stop at the main tributaries but extends to subtributaries too. The lowering of the bed often causes problems at existing bridges, road culverts, etc.
Degradation also takes place if the water discharge in a stream is increased, sediment load and sediment size remaining the same.
Another possible way by which equilibrium can be disturbed so as to cause degradation is by increase of water surface slope, though this happens rarely.