Mountain Rivers and Incised Channels

Abstract

An incised river is defined as a river that is experiencing bed-level lowering. From the viewpoint of geomorphological process, mountain rivers either were or are incised rivers. Large rivers may be incised rivers in the upper reaches, but fluvial rivers in the lower reaches. The development of channel incision in mountainous areas depends on the rainfall, watershed vegetation, and soil and rock compositions. Incision may cause landslides, debris flows, and riverbed scour and in conjunction with bank erosion, provides sediment to the flow. A step-pool system is a geomorphologic phenomenon occurring in highgradient mountain streams with alternating steps and pools. Cobbles and boulders generally compose the steps, which alternate with finer sediments in pools to produce a repetitive, staircase like longitudinal profile in the stream channel. The tight interlocking of particles in steps gives them an inherent stability that only extreme floods are likely to disturb. Step-pool system maximizes the resistance, and, thus, controls riverbed incision. A bedrock channel has bedrock exposed along the channel bed or walls for at least approximately half its length, or has bedrock limits to the magnitude and location of bed scour and bank erosion during floods. Bedrock channels most commonly occur in regions of high topographic relief. Relief may be a product of recent tectonic uplift, as in the Himalayan Mountains of central Asia. The exposed bedrock implies that the channels may be particularly sediment-starved during floods and subjects to long term incision. The causes and evolution process of incised rivers and control strategies of channel bed incision are also discussed in this chapter.