Erosion and Deposition: Action of Running Water and Groundwater

In the previous articles, we were discussing various types of endogenic and exogenic processes. We have also seen that erosion and deposition are some of the exogenic processes. In this post, we are dealing with the geomorphic agents – running water and groundwater, which causes erosion and deposition. They form various erosional (destructional) and depositional (constructional) landforms.

Even though we are considering the erosional and depositional activities and their landform creation, it should be kept in mind that they are always aided by weathering and mass movements. There are some other independent controls like (i) stability of sea level; (ii) tectonic stability of landmass; (iii) climate etc. which influence the evolution of these landforms.

What does Running Water do?

• Running water, which doesn’t need any further explanation, has two components: one is overland flow on the general land surface as a sheet and the other is linear flow as streams and rivers in valleys.
• The overland flow causes sheet erosion and depending upon the irregularities of the land surface, the overland flow may concentrate into narrow to wide paths.
• During the sheet erosion, minor or major quantities of materials from the surface of the land are removed in the direction of flow and gradual small and narrow rills will form.
• These rills will gradually develop into long and wide gullies, the gullies will further deepen, widen and lengthen and unite to give rise to a network of valleys. (Note: A valley can be formed in various ways like faulting, but here we are dealing only with the formation by means of exogenic geomorphic agent).
• Once a valley is formed, it later develops into a stream or river.

Courses of  a river

A river, which is the best example of the linear flow of running water through a valley, can be divided into three, on the basis of its course – upper course, middle course and lower course.

Upper Course / Stage of Youth (Erosion dominates):

• It starts from the source of the river in hilly or mountainous areas.
• The river flows down the steep slope and, as a result, its velocity and eroding power are at their maximum.
• Streams are few, with poor integration.
• As the river flows down with high velocity, vertical erosion or downward cutting will be high which results in the formation of V-Shaped Valleys.
• Waterfalls, rapids, and gorges exist where the local hard rock bodies are exposed.

Middle Course/ Stage of Maturity (Transportation dominates):

• In this stage, vertical erosion slowly starts to replace with lateral erosion or erosion from both sides of the channel.
• Thus, the river channel causes the gradual disappearance of its V-shaped valley (not completely).
• Streams are plenty at this stage with good integration.
• Wider flood plains start to visible in this course and the volume of water increases with the confluence of many tributaries.
• The work of river predominantly becomes transportation of the eroded materials from the upper course (little deposition too).
• Landforms like alluvial fans, piedmont alluvial plains, meanders etc. can be seen at this stage.

Lower Course/ Stage of Old (Deposition dominates):

• The river starts to flow through a broad, level plain with heavy debris brought down from upper and middle courses.
• Vertical erosion has almost stopped and lateral erosion still goes on.
• The work of the river is mainly deposition, building up its bed and forming an extensive flood plain.
• Landforms like braided channels, floodplains, levees, meanders, oxbow lakes, deltas etc. can be seen at this stage.

Running water: erosion, transportation, and deposition

• Erosion occurs when overland flow moves soil particles downslope.
• The rock materials carried by erosion is the load of the river.
• This load acts as a grinding tool helping in cutting the bottom and sides of the river bed, resulting in deepening and widening of the river channel.

Erosion Types

The work of river erosion is accomplished in different ways, all of which may operate together. They are corrasion, corrosion, hydraulic action etc.

1. Corrasion or Abration: As the rock particles bounce, scrape and drag along the bottom and sides of the river, they break off additional rock fragments. This form of erosion is called corrasion or abration. They are two types: vertical corrosion which acts downward and lateral corrosion which acts on both sides.
2. Corrosion or Solution: This is the chemical or solvent action of water on soluble or partly soluble rocks with which the river water comes in contact.
3. Hydraulic Action: This is the mechanical loosening and sweeping away of material by the sheer force or river water itself. No load or material is involved in this process.

Transportation types

After erosion, the eroded materials get transported with the running water. This transportation of eroded materials is carried in four ways:

1. Traction: The heavier and larger rock fragments like gravels, pebbles etc are forced by the flow of the river to roll along its bed. These fragments can be seen rolling, slipping, bumping and being dragged. This process is called as traction and the load transported in this way are called traction load.
2. Saltation: Some of the fragments of the rocks move along the bed of a stream by jumping or bouncing continuously. This process is called as saltation.
3. Suspension: The holding up of small particles of sand, silt and mud by the water as the stream flows is called suspension.
4. Solution: Some parts of the rock fragments dissolved in the river water and transported. This type of transportation is called solution transportation.

• When the stream comes down from the hills to plain areas with the eroded and transported materials, the absence of slope/gradient causes the river to lose it energy to further carry those transported materials.
• As a result, the load of the river starts to settle down which is termed as deposition.
• Erosion, transportation, and deposition continue until the slopes are almost completely flattened leaving finally a lowland of faint relief called peneplains with some low resistant remnants called monadnocks.

Erosional Landforms due to Running Water

1. Valleys, Gorges, Canyon

• As we discussed above, valleys are formed as a result of running water.
• The rills which are formed by the overland flow of water later develop into gullies.
• These gullies gradually deepen and widen to form valleys.
• A gorge is a deep valley with very steep to straight sides.
• A canyon is characterized by steep step-like side slopes and may be as deep as a gorge.
• A gorge is almost equal in width at its top as well as bottom and is formed in hard rocks while a canyon is wider at its top than at its bottom and is formed in horizontal bedded sedimentary rocks.

2. Potholes, Plunge pools

• Potholes are more or less circular depressions over the rocky beds of hills streams.
• Once a small and shallow depression forms, pebbles and boulders get collected in those depressions and get rotated by flowing water. Consequently, the depressions grow in dimensions to form potholes.
• Plunge pools are nothing but large, deep potholes commonly found at the foot of a waterfall.
• They are formed because of the sheer impact of water and rotation of boulders.

3. Incised or Entrenched Meanders

• They are very deep wide meanders (loop-like channels) found cut in hard rocks.
• In the course of time, they deepen and widen to form gorges or canyons in hard rock.
• The difference between a normal meander and an incised/entrenched meander is that the latter found on hard rocks.

4. River Terraces

• They are surfaces marking old valley floor or flood plains.
• They are basically the result of vertical erosion by the stream.
• When the terraces are of the same elevation on either side of the river, they are called as paired terraces.
• When the terraces are seen only on one side with none on the other or one at quite a different elevation on the other side, they are called as unpaired terraces.

Depositional Landforms due to Running Water

1. Alluvial Fans

• They are found in the middle course of a river at the foot of slope/ mountains.
• When the stream moves from the higher level break into foot slope plain of low gradient, it loses its energy needed to transport much of its load.
• Thus, they get dumped and spread as a broad low to the high cone-shaped deposits called an alluvial fan.
• The deposits are not roughly very well sorted.

2. Deltas

• Deltas are like an alluvial fan but develop at a different location.
• They are found in the mouth of the river, which is the final location of depositional activity of a river.
• Unlike alluvial fans, the deposits making up deltas are very well sorted with clear stratification.
• The coarser material settle out first and the finer materials like silt and clay are carried out into the sea.

3. Flood Plains, Natural Levees

• Deposition develops a flood plain just as erosion makes valleys.
• A riverbed made of river deposits is the active flood plain and the flood plain above the bank of the river is the inactive flood plain.
• Natural levees are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits along the banks of a river.
• The levee deposits are coarser than the deposits spread by flood water away from the river.

4. Meanders and oxbow lakes

• Meanders are loop-like channel patterns develop over the flood and delta plains.
• They are actually not a landform but only a type of channel pattern formed as a result of deposition.
• They are formed basically because of three reasons: (i) propensity of water flowing over very gentle gradient to work laterally on the banks; (ii) unconsolidated nature of alluvial deposits making up the bank with many irregularities; (iii) Coriolis force acting on fluid water deflecting it like deflecting the wind.
• The concave bank of a meander is known as cut-off bank and the convex bank is known as a slip-off
• As meanders grow into deep loops, the same may get cut-off due to erosion at the inflection point and are left as oxbow lakes.
• For large rivers, the sediments deposited in a linear fashion at the depositional side of a meander are called as Point Bars or Meander Bars.

5. Braided Channels

• When selective deposition of coarser materials causes the formation of a central bar, it diverts the flow of river towards the banks, which increases lateral erosion.
• Similarly, when more and more such central bars are formed, braided channels are formed.
• Riverine Islands are the result of braided channels.

What does Groundwater do?

• The part of rain or snow-melt water which accumulates in the rocks after seeping through the surface is called underground water or simply groundwater.
• The rocks through which water can pass easily are called as permeable rocks while the rocks which do not allow water to pass are called as impermeable rocks.
• After vertically going down to some depth, the water under the ground flows horizontally through the bedding planes, joints or through the materials themselves.
• Although the amount of groundwater varies from place to place, its role in shaping the surface features of the earth is quite important.
• The works of groundwater are mainly seen in rocks like limestone, gypsum or dolomite which are rich in calcium carbonate.
• Any limestone, dolomite or gypsum region showing typical landforms produced by the action of groundwater through the process of solution and deposition is called as Karst Topography (Karst region in the Balkans)
• The zones or horizons of permeable and porous rocks which are fully filled with water are called as the Zones of Saturation.
• The marks which show the upper surface of these saturated zones of the groundwater are called as the Water Tables.
• And these rocks, which are filled with underground water, are called as aquifers.
• The water table is generally higher in the areas of high precipitation and also in areas bordering rivers and lakes.
• They also vary according to seasons. On the basis of variability, water tables are of two types: (i) Permanent water table, in which the water will never fall below a certain level and wells dug up to this depth provide water in all seasons; (ii) Temporary water tables, which are seasonal water tables.
• Springs: They are the surface outflow of groundwater through an opening in a rock under hydraulic pressure.
• When such springs emit hot water, they are called as Hot Springs. They generally occur in areas of active or recent volcanism.
• When a spring emits hot water and steam in the form of fountains or jets at regular intervals, they are called as geysers.
• In a geyser, the period between two emissions is sometimes regular (Yellowstone National Park of USA is the best example).

Erosional Landforms due to Groundwater

Sinkholes and caves are erosional landforms formed due to the action of ground water.

1. Sinkholes

• Small to medium sized rounded to sub-rounded shallow depressions called swallow holes forms on the surface of rocks like limestone by the action of the solution.
• A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom.
• When as sinkhole is formed solely through the process of solution, it is called as a solution sink.
• Some sinkhole starts its formation through the solution process but later collapse due to the presence of some caves or hollow beneath it and becomes a bigger sinkhole. These types are called as collapse sinks.
• The term Doline is sometimes used to refer collapse sinks.
• Solution sinks are more common than collapse sinks.
• When several sink holes join together to form valley of sinks, they are called as valley sinks or Uvalas.
• Lapies are the irregular grooves and ridges formed when most of the surfaces of limestone are eaten by solution process.

2. Caves

• In the areas where there are alternative beds of rocks (non-soluble) with limestone or dolomite in between or in areas where limestone are dense, massive and occurring as thick beds, cave formation is prominent.
• Caves normally have an opening through which cave streams are discharged
• Caves having an opening at both the ends are called tunnels.

Depositional Landforms of Groundwater

1. Stalactites and stalagmites

• They are formed when the calcium carbonates dissolved in groundwater get deposited once the water evaporates.
• These structures are commonly found in limestone caves.
• Stalactites are calcium carbonate deposits hanging as icicles while Stalagmites are calcium carbonate deposits which rise up from the floor.
• When a stalactite and stalagmite happened to join together, it gives rise to pillars or columns of different diameters.

Next in the series: Evolution of landforms due to glaciers.