Flood and river channel types

A flood is a rise in river discharge and water level so that the water overflows its banks and covers the surrounding floodplain. It is caused by heavy rainfall, snow or glacier melt, glacial lake outburst floods (GLOF), the breach of a landslide dam, or dam failure.

River channels are classified by their plan shape (pattern) as:

 Straight:       ----------------------
                 ======================

 Meandering:      ___      ___
                 /   \    /   \
              __/     \__/     \__

 Braided:         ___   ___   ___
                 /   \_/   \_/   \
                 \___/ \___/ \___/

1. Straight channel: nearly straight, with sinuosity less than 1.05. It is rare and short, found in steep, narrow, bedrock valleys. Flow is fast, but the thalweg (deepest line) still swings from bank to bank.
2. Meandering channel: a single, sinuous channel with sinuosity above 1.5. It forms on gentle slopes in fine, cohesive sediment. Erosion occurs on the outer (concave) bank and deposition on the inner (convex) bank as point bars. Cut-offs form ox-bow lakes.
3. Braided channel: many small channels dividing and rejoining around bars. It forms where the slope is steep, the bed load is heavy, the banks erode easily and the flow is flashy, e.g. the Koshi, Narayani and Karnali rivers below the hills.
4. Anastomosing channel: several stable, interconnected channels separated by vegetated islands, on very low slopes.