Landforms and running water

A landform is a small to medium part of the earth's surface, with its own shape, size and materials, made by particular processes and agents. Several related landforms make up a landscape. Every landform has a beginning and a history of change; a landmass passes through stages rather like a life: youth, maturity and old age.

Running water is the most important agent in humid regions. It works in two ways:

• Overland flow as a sheet over the land, causing sheet erosion.
• Linear flow in streams and rivers in valleys.
• Erosional landforms go with vigorous, youthful rivers on steep slopes. As channels become gentler, rivers slow down and deposit more: the gentler the slope, the greater the deposition.

How valleys grow: overland flow gathers into paths and cuts small, narrow rills; rills grow into gullies; gullies deepen, widen, lengthen and join into a network of valleys.

• In the early stages, down-cutting dominates and removes waterfalls and cascades.
• In the middle stages, down-cutting slows and lateral erosion of valley sides becomes severe; slopes get lower.
• Finally, divides are almost flattened, leaving a lowland of faint relief, a peneplain ('almost a plain'), with a few low, resistant remnants called monadnocks.

The three stages:

• Youth: few streams, poorly joined, with shallow V-shaped valleys and little or no floodplain. Divides are broad and flat, with marshes, swamps and lakes. Waterfalls and rapids occur where hard rock is exposed.
• Mature: many well-joined streams; valleys still V-shaped but deep; trunk streams have wider floodplains with meanders inside the valley. The swamps of youth disappear, divides become sharp, and waterfalls and rapids disappear.
• Old: few small tributaries with gentle gradients; streams meander freely over vast floodplains with natural levees and oxbow lakes. Divides are broad and flat again, with lakes and swamps; most land is at or just above sea level.

Example (new): after heavy monsoon rain on a bare, ploughed slope, you can see tiny channels a few centimetres wide: these rills, left unchecked over many years, can join into gullies deep enough to swallow a tractor, as in the Chambal ravines.