Running Water

The Big Picture

In humid regions that receive heavy rainfall, running water is the most important geomorphic agent degrading the land surface. It works through two components: overland flow, which moves across the general land surface as a sheet, and linear flow, which moves as streams and rivers confined to valleys.

Most erosional landforms made by running water are associated with vigorous, youthful rivers flowing over steep gradients. As erosion continues, these steep channels gradually become gentler, lose velocity, and begin depositing material actively. Depositional forms can occur along steep-slope streams too, but these are small-scale compared to those on medium or gentle slopes. The gentler the gradient, the greater the deposition.

As stream beds flatten through continued erosion, down-cutting becomes less dominant and lateral erosion of banks increases. Hills and valleys are progressively reduced to plains. This raises a question: is complete reduction of relief of a high landmass possible?

Stages of Landscape Evolution

Overland flow causes sheet erosion. Depending on irregularities of the land surface, this flow concentrates into narrow or wide paths. The friction of the flowing water column removes material from the surface, forming small, narrow rills. These rills develop into long, wide gullies, which further deepen, widen, lengthen, and unite to form a network of valleys.

In early stages, down-cutting dominates and irregularities like waterfalls and cascades are removed. In middle stages, streams cut their beds more slowly and lateral erosion of valley sides becomes severe. Valley sides are reduced to lower and lower slopes. Divides between drainage basins are likewise lowered until almost completely flattened, leaving a lowland of faint relief with some low resistant remnants called monadnocks standing out here and there. This type of plain formed by stream erosion is called a peneplain — an almost plain.

The characteristics of each stage can be summarised:

Erosional Landforms

Valleys

Valleys start as small, narrow rills that develop into gullies, which then deepen, widen, and lengthen to form valleys. Based on dimensions and shape, several types are recognised.

A gorge is a deep valley with very steep to straight sides, almost equal in width at top and bottom. A canyon has steep, step-like side slopes and may be as deep as a gorge, but is wider at the top than at the bottom. A canyon is essentially a variant of gorge. Valley type depends on the type and structure of rocks: canyons commonly form in horizontal bedded sedimentary rocks, while gorges form in hard rocks.

Potholes and Plunge Pools

On rocky beds of hill-streams, more or less circular depressions called potholes form through stream erosion aided by abrasion of rock fragments. Once a small, shallow depression forms, pebbles and boulders collect in it and get rotated by flowing water, causing the depression to grow. A series of such depressions eventually joins, deepening the stream valley.

At the foot of waterfalls, large potholes form — quite deep and wide — due to the sheer impact of water and rotation of boulders. These large, deep holes at the base of waterfalls are called plunge pools.

Incised or Entrenched Meanders

Streams flowing rapidly over steep gradients concentrate erosion on the channel bottom; lateral erosion is minimal compared to streams on gentle slopes. Streams on gentle slopes develop sinuous or meandering courses due to active lateral erosion. Meandering courses are common over floodplains and delta plains where gradients are very gentle. But very deep and wide meanders can also be found cut into hard rocks — these are called incised or entrenched meanders.

River Terraces

River terraces are surfaces marking old valley floor or floodplain levels. They may be bedrock surfaces without alluvial cover, or alluvial terraces consisting of stream deposits. Terraces are basically products of erosion — they result from vertical erosion by the stream into its own depositional floodplain. Several terraces can occur at different heights, indicating former river bed levels. When terraces occur at the same elevation on either side of a river, they are called paired terraces.

Depositional Landforms

Alluvial Fans

Alluvial fans form when streams flowing from higher levels break onto foot slope plains of low gradient. Streams flowing over mountain slopes carry very coarse load. This load becomes too heavy to carry over gentler gradients, so it gets dumped and spread as a broad, low to high cone-shaped deposit. Streams flowing over fans are not confined to their original channels for long — they shift position across the fan, forming many channels called distributaries. In humid areas, alluvial fans show low cones with gentle slope from head to toe; in arid and semi-arid climates, they appear as high cones with steep slopes.

Deltas

Deltas resemble alluvial fans but develop at a different location. The load carried by rivers is dumped and spread into the sea. If this load is not carried far into the sea or distributed along the coast, it spreads and accumulates as a low cone. Unlike alluvial fans, delta deposits are very well sorted with clear stratification. The coarsest materials settle out first; finer fractions like silts and clays are carried out into the sea. As the delta grows, river distributaries increase in length and the delta continues building into the sea.

Floodplains, Natural Levees and Point Bars

Deposition develops a floodplain just as erosion makes valleys. The floodplain is a major landform of river deposition. Large materials are deposited first when the stream channel breaks onto a gentle slope. Fine materials like sand, silt and clay are carried by relatively slow-moving waters in gentler channels and deposited over the bed; when waters spill over the banks during flooding, they deposit above the bed. …