Geological agent and features of running water

A geological agent is a natural force or medium (running water, wind, glacier, groundwater, sea waves and gravity) that erodes, transports and deposits earth material and so changes the surface.

Geomorphic features produced by running water

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: formed by vertical (downward) cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: form where hard rock lies over soft rock; a plunge pool forms at the base.
• Potholes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: formed by lateral (side) cutting in the old stage.
• Interlocking spurs: ridges of the valley sides that project alternately into the winding valley of a young river.

Depositional work occurs when the velocity and carrying capacity fall (on gentle slopes, at the river mouth, and on the floodplain during floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad, flat valley floor built by flood deposits.
• Natural levees: low ridges of sand and coarse silt built along the banks during floods.
• Braid bars and point bars: sand and gravel bars in the channel (braid bars) and on the inner bank of bends (point bars).
• Delta: fan-shaped deposit at the river mouth, e.g. the Ganges-Brahmaputra delta.
• Ox-bow lake and meander scars: cut-off bends.