12.2.7. Degrading Rivers When the sediment load entering a river reach is less than that leaving the river reach, the river in that reach becomes a degrading river due to erosion of the bed and bank material.
12.2.8. Mountainous Rivers Rivers in mountainous reaches are further divided into incised rivers and boulder rivers. Incised rivers have a steep bed slope and high velocity of flow. The bed and the banks of these rivers are made up of rocks and very large boulders which are, usually, highly resistant to erosion. The sediment transported by an incised river is often different from that of the river bed and comes from the catchment due to soil erosion.
The bed and sides of a boulder river consist of a mixture of boulder, gravel, shingle, and sand. The bed slope and the velocity of flow are smaller than those of incised rivers. The river cross-section is usually well-defined. There is, however, considerable subsoil flow due to high permeability of the bed material.
