Running Water Landforms — Erosional & Depositional

6.1 What Are Landforms and Why Do They Evolve?

Once weathering has worked on the rocks and soils that make up the Earth's outer surface, the geomorphic agents — running water, groundwater, wind, glaciers and waves — take charge of erosion. Erosion changes the surface; the eroded material is transported and eventually dumped, and the resulting deposition changes the surface again. The chapter that begins here is about the small to medium tracts that emerge from this give-and-take: landforms.

Most geomorphic processes act slowly, so the results take a long time to appear. Every landform has a beginning. Once formed, it may slowly or rapidly change in shape, size and character because the same processes keep working on it. When climate shifts, or when the land mass moves vertically or horizontally, the intensity of the processes — or even the processes themselves — may change, leading to fresh modifications. The word evolution, in this chapter, refers to two things: (i) the transformation of a piece of the Earth's surface from one landform into another, and (ii) the transformation of an individual landform after it has once been formed. In other words, every landform has a history, and a landmass passes through stages of development that geographers compare to the stages of human life — youth, mature and old age.

The Five Geomorphic Agents

6.2 Running Water — The Master Sculptor of Humid Lands

In humid regions that receive heavy rainfall, running water is regarded as the most important of all geomorphic agents in the wearing down of the land surface. Running water has two clear components. The first is overland flow, in which water moves as a thin sheet over the general land surface. The second is linear flow, in which water concentrates into streams and rivers that occupy valleys.

Most of the erosional landforms produced by running water are associated with vigorous, youthful rivers flowing over steep gradients. With the passage of time, those steep stream channels are worn down and turn gentler. Slope reduction takes velocity away from the river, which now starts depositing actively. There can certainly be small depositional forms even on steep slopes, but these are minor compared with the huge depositional features built up by rivers flowing over medium and gentle slopes. The gentler the channel, the greater the deposition. When the bed becomes very gentle, downward cutting (vertical erosion) loses its dominance and lateral erosion of banks takes over — and the hills and valleys finally get reduced to plains.

From Rills → Gullies → Valleys → Peneplain

Overland flow causes sheet erosion. Depending on the irregularities of the land surface, the sheet may concentrate into narrow or wide paths. The friction of the column of flowing water removes minor or major quantities of material in the direction of flow. Small narrow rills form first; rills enlarge into long, wide gullies; gullies deepen, widen, lengthen and unite into a network of valleys.

In the early stages, down-cutting (vertical erosion) dominates: irregularities such as waterfalls and cascades are removed. In the middle stages, the streams cut their beds more slowly, and lateral erosion of valley sides becomes severe. Gradually the valley sides are reduced to lower and lower slopes. The divides between drainage basins are likewise lowered until they are almost completely flattened, leaving finally a lowland of faint relief with some low resistant remnants — monadnocks — standing out here and there. This type of plain produced by stream erosion is called a peneplain (an "almost plain").

Stage 1 — Youth

Streams are few during this stage with poor integration. They flow over original slopes, showing shallow V-shaped valleys with no floodplains, or with very narrow floodplains along trunk streams. Stream divides are broad and flat with marshes, swamps and lakes. Meanders, if present, develop over these broad upland surfaces and may eventually entrench themselves into the uplands. Waterfalls and rapids may exist where local hard rock bodies are exposed.

Stage 2 — Mature

During the mature stage, streams are plenty with good integration. The valleys are still V-shaped but now deep; trunk streams are broad enough to have wider floodplains within which streams may flow in meanders^? confined within the valley. The flat broad inter-stream areas, swamps and marshes of youth disappear and the divides turn sharp. Waterfalls and rapids disappear.

Stage 3 — Old

Smaller tributaries during old age are few with gentle gradients. Streams meander freely over vast floodplains that show natural levees, oxbow lakes^? and similar features. Divides are broad and flat with lakes, swamps and marshes. Most of the landscape is at — or only slightly above — sea level.

6.3 Erosional Landforms of Running Water

Valleys — V-Shape, Gorge & Canyon

Valleys begin life as small narrow rills, enlarge into long wide gullies, and gullies further deepen, widen and lengthen into valleys. Depending on dimensions and shape, several types are recognised: V-shaped valleys, gorges and canyons. A gorge is a deep valley with very steep to almost straight sides, almost equal in width at its top and at its bottom — typically formed in hard, resistant rocks. A canyon is characterised by steep step-like side slopes and may be just as deep as a gorge; it is wider at the top than at the bottom and is in fact a variant of the gorge. Canyons commonly form in horizontal bedded sedimentary rocks, while gorges form in hard rocks.

Potholes & Plunge Pools

Over the rocky beds of hill-streams, more or less circular depressions called potholes form because of stream erosion aided by the abrasion of rock fragments. Once a small, shallow depression forms, pebbles and boulders collect in it and are rotated by flowing water; the depression grows in dimensions. A series of such depressions eventually joins and the stream valley deepens. At the foot of waterfalls, large potholes — quite deep and wide — form because of the sheer impact of falling water and the rotation of boulders. Such large, deep holes at the base of a waterfall are called plunge pools.

Incised or Entrenched Meanders

In streams that flow rapidly over steep gradients, erosion is concentrated on the bottom of the channel. Lateral erosion on the valley sides is not much, compared with streams flowing on low and gentle slopes. Streams flowing over gentle slopes develop sinuous or meandering courses — meandering is common on floodplains and delta plains. But very deep and wide meanders can also be found cut into hard rocks — these are called incised or entrenched meanders. They are interpreted as evidence that a meandering river was rejuvenated by uplift and forced to cut down vertically, preserving its old curvy course in solid rock.

River Terraces

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

6.4 Depositional Landforms of Running Water

Alluvial Fans

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

Deltas — The Cousins of Alluvial Fans

Deltas are like alluvial fans, but they develop at a different location — the meeting point of a river and the sea (or a lake). The load carried by the river is dumped and spread in the sea. If this load is not carried far away or distributed along the coast by waves and currents, it spreads and accumulates as a low cone. Unlike alluvial fans, the deposits making up deltas are very well sorted with clear stratification — the coarsest materials settle out first and the finer fractions like silt and clay are carried out into the sea. As the delta grows, the river's distributaries continue to increase in length and the delta extends further into the sea. NCERT Figure 6.4 shows a satellite view of the Krishna river delta in Andhra Pradesh.

Floodplains, Natural Levees and Point Bars

Just as erosion makes valleys, deposition develops a floodplain. A floodplain is a major landform of river deposition. Large-sized materials are deposited first when a stream channel breaks onto a gentle slope; the finer sand, silt and clay are carried by the slower-moving waters of gentle channels in the plains and laid down on the bed and — when the waters spill over the banks during a flood — above the bed.

The river bed made of recent river deposits is called the active floodplain. The floodplain above the bank is called the inactive floodplain. The inactive floodplain contains two kinds of deposits — flood deposits (fine silt and clay carried by spilled waters) and channel deposits (coarser material left by abandoned or cut-off channels which gradually fill up). The flood plains in a delta are called delta plains.

Natural levees are low, linear and parallel ridges of coarse deposits along the banks of large rivers, often cut into individual mounds. They form because the river drops its coarsest load right at its banks the moment it spills over. Point bars — also known as meander bars — are sediments deposited in a linear fashion by flowing waters along the convex (inside) bank of meanders of large rivers. They are almost uniform in profile and width and contain mixed sizes of sediments.

Meanders

In large flood and delta plains, rivers rarely flow in straight courses. The loop-like channel patterns called meanders develop over flood and delta plains (NCERT Figure 6.6 shows a satellite scene of the meandering Burhi Gandak river near Muzaffarpur, Bihar with several oxbow lakes). A meander is not a landform but only a type of channel pattern.

Meanders develop because of three reasons: (i) the propensity of water flowing over very gentle gradients to work laterally on the banks; (ii) the unconsolidated nature of alluvial deposits making up the banks, with many irregularities that water can attack; and (iii) the Coriolis force acting on flowing water, deflecting it just as it deflects winds.

When the gradient becomes extremely low, water flows leisurely and starts working laterally. Slight irregularities along the banks slowly turn into a small curvature; the curvature deepens because of deposition on the inside of the curve and erosion along the outside bank. Where there is no deposition and no erosion, meandering tendency is reduced. In meanders of large rivers, active deposition occurs along the convex bank and undercutting along the concave bank. The concave bank is called the cut-off bank, showing up as a steep scarp; the convex bank presents a long, gentle profile. As meanders grow into deep loops, the same may get cut off due to erosion at the inflection points and are left as oxbow lakes.