RIVER ACTION

River action

Flowing water is more influential in shaping the surface form of our planet than any other gradational process, primarily because of the sheer number of streams on Earth. Through both erosion and deposition, water flowing down slope over the land surface, particularly when concentrated in channels, modifies existing landforms and creates others.

The study of flowing water as a gradational process, together with the study of the resulting landforms, is termed fluvial geomorphology (from Latin: fluvius, river).

Surface Runoff

Liquid water flowing over the surface of Earth—that is, surface runoff—can originate as ice and snow melt or as outflow from springs, but most runoff originates from direct precipitation. When precipitation strikes the ground, several factors interact to determine whether surface runoff will occur. Basically, runoff is generated when the amount, duration, and/or rate of precipitation exceed the ability of the ground to soak up the moisture

The process of water soaking into the ground is called infiltration, and the amount of water the soil and surface sediments can hold is the infiltration capacity. A portion of the infiltrated water will seep down to lower positions and reach the zone of saturation beneath the water table, while much of the rest will eventually return to the atmosphere by evaporation from the soil or by transpiration from plants

Channels that are empty of water much of the time like this are described as having ephemeral flow.

Perennial streams flow all year, though not always with the same volume or at the same velocity. Most arid region streams flow on an ephemeral basis although some may have intermittent flow, which lasts for a couple of months in response to an annual rainy season or spring snowmelt.

Stream system

Smaller streams that contribute their water and sediment load to a larger one in this way are tributaries of the larger channel, which is called the trunk stream

Drainage Basins

Each individual stream occupies its own drainage basin (also known as watershed, or catchment), the expanse of land from which it receives runoff. Drainage area refers to the measured extent of a drainage basin, and is typically expressed in square kilometers or square miles. Because the runoff from a tributary’s drainage basin is delivered by the tributary to the trunk stream, the tributary’s drainage basin also constitutes part of the drainage basin of the trunk stream.

The drainage divide represents the outside perimeter of a drainage basin and thus also the boundary between it and adjacent basins. The drainage divide follows the crest of the interfluve between two adjacent drainage basins.

Erosion by Streams

Fluvial erosion is the removal of rock material by flowing water.

Fluvial erosion may take the form of the chemical removal of ions from rocks or the physical removal of rock fragments (clasts). Physical removal of rock fragments includes breaking off new pieces of bedrock from the channel bed or sides and moving them as well as picking up and removing preexisting clasts that were temporarily resting on the channel bottom.

Breaking off new pieces of bedrock proceeds very slowly where highly resistant rock types are found.

One way that streams erode occurs when stream water chemically dissolves rock material and then transports the ions away in the flow. This fluvial erosion process, called corrosion (or solution, or dissolution), has a limited effect on many rocks but can be significant in certain rock types, such as limestone.

Hydraulic action refers to the physical, as opposed to chemical, process of stream water alone removing pieces of rock. As stream water flows down slope by the force of gravity, it exerts stress on the streambed

Abrasion, a process even more powerful than hydraulic action. As rock particles bounce, scrape, and drag along the bottom and sides of a stream channel, they break off additional rock fragments. Because solid rock particles are denser than water, the impact of having

clastic load thrown against the channel bottom and sides by the current is much more effective than the impact of water alone.

This wear and tear experienced by sediments as they tumble and bounce against one another and against the stream channel is called attrition.

Types of river erosion

Head ward erosion

Lateral erosion

Vertical erosion

Stream Transportation

A stream directly erodes some of the sediment that it transports, and most chemical sediments are delivered to the channel in base flow, but a far greater proportion of its load is delivered to the stream channel by surface runoff and mass movement

Some minerals are dissolved in the water and are thus carried in solution. The finest solid particles are carried in suspension, buoyed by vertical turbulence.

Some grains too large and heavy to be carried in suspension bounce along the channel bottom in a process known as saltation. Particles that are too large and heavy to move by saltation may slide and roll along the channel bottom in the transportation process of traction.

There are three main types of stream load. Ions of rock material held in solution constitute the dissolved load. Suspended load consists of the small clastic particles being moved in suspension. Larger particles that saltate or move in traction along the streambed comprise the bed load. The total amount of load that a stream carries is expressed in terms of the weight of the transported material per unit time.

Stream Deposition

Because the capacity and competence of a stream to carry material depend primarily on flow velocity, a decrease in velocity will cause a stream to reduce its load through deposition.

Alluvium is the general name given to fluvial deposits, regardless of the type or size of material. Alluvium is usually recognized by the characteristic sorting and/or rounding of sediments that streams perform.

Stages of the river

Upper: Characteristically youthful stages are found in higher elevation, in mountainous area where the slope of the land is steeper. Water that flow over such landscape will flow very fast

·         The river flowing down a steep slope

·         The channel is deeper than is wide and V-shaped valley due to downcutting rather than lateral erosion

·         It velocity is strong and fast

·         It is capable of moving all sediments sizes from ions in solution, to silt and clay, also cobbles and boulders

·         Steep sided cliff flanks the river

·         Rapids may be present due to water velocity and the presence of boulders in the river channel

·         Water fall may feature in the young stage of the river

·         Erosion is prominent in this stage

·         Plunge pool

Middle/mature

Is an in-between stage, the river still downcuts though to a much laser degree than a youthful river does but it also erodes laterally though not as extensively when compared to old age river. The landscape over which it passes is steep the river’s slope enable a velocity capable of moving not only the finer sediments but also the lager pebbles and cobbles by ways of rolling, bouncing and saltation along the river bed. The area through which the river flow may be mountainous but they may not be as in youthful river locale.

·         The channel of the mature river is u shaped but deeper than and not as wide as the old river channel. In general the channel is broader with gentle slope

·         The river flow down a moderate slope/gradient

·         The velocity is greater than an old age river but less than a youthful one and capable of moving many sediment sizes from ions in solutions to silt, clay and cobbles but normally not boulders unless peak flooding occurs

·         Meanders may be present though they will not be as curvy as those found in old stage rivers

·         A flood plain exists with grassy areas besides the river

·         Erosion is present though deposition may occur

·         Cliff may flank the river at a distance (bluff)

Lower

Old Age Rivers actually have more distinguishing features than other stages

·         The river flow down in a very shallow gradient because the general landscape surrounding the river is flatter

·         The channel is wider than it is deep with a very broad and U-shaped valley primarily by extensive lateral erosion

·         Its velocity is quite slow and that means the river is capable of moving small sized sediments silt and clay

·         A wide flood plain which is usually marshy and swampy due to flooding of the river valley characterizes this stage

·         Curvy S-shaped meanders are abundant and prominent feature

·         Yazoo stream run parallel to the main river but do not join it

·         Ox-bow Lake exists within the flood plain where meanders are cut off from the main stream due to extensive erosion and deposition. Such cut off meanders do not receive water to replenish is supply and will eventually dry up and become meander scar

Or

Oxbow lake A portion of an abandoned stream channel that is cut off from the rest of the stream by the meandering process and filled with stagnant water.

·         Natural levees

·         Point bars, are areas of deposition on the inside curves of a meander when water velocity decreases on the inside curves, deposition of sediments occurs filling inside curves over time

·         Braided stream A network of converging and diverging stream channels within an individual stream system that are separated from each other by deposits of sand and gravel.

·         Delta A low, level plain that develops where a stream flows into a relatively still body of water so that its velocity decreases and alluvial deposition occurs.

Deltas develop only at those river mouths where the fluvial sediment supply is high, the underwater topography does not drop too sharply, and waves, currents, and tides cannot transport away all the sediments delivered by the river. These circumstances exist at the mouths of many, but not all, rivers.

Types of delta

Bird’s-foot deltas are constructed in settings where the influence of the fluvial system far exceeds the ability of waves, currents, and tides of the standing water body to rework the deltaic sediment into coastal landforms or to transport it away. In this type of delta, numerous distributaries slightly above sea level extend far out into the receiving water body.

Arcuate

cuspate

River rejuvenation

Rejuvenation occurs when a channel being in a state of equilibrium or progressive sedimentation changes its predominant process to erosion. The main causes of rejuvenation are dynamic change eustatic change and static change. Both dynamic and eustatic changes affect the base level of a river. The base level is the lowest point to which a river can flow and erode. Various landforms are formed by reactivated channel process.

Rejuvenation involves a lowering of the base level. This can be either a relative change in base level, ie sea level stays the same but land levels rise (isostatic change), or an absolute change in base level where the sea level itself falls (eustatic change).

1. Dynamic changes

Dynamic changes involve an upwards movement of the land which raises the height of the river above sea level, its base level. This increases the gravitational potential of the river and so increases the energy available to the river to erode and transport material. Two mechanisms which cause land levels to change are

·         Orogenesis

·         Isostatic rebound.

Orogenesis

Plate movement is responsible for compressing and thickening the crust at convergent margins leading to mountain building. The uplift accompanying this mountain-building effectively lowers base level, and rivers begin active downward erosion. Movement of the land upwards, usually along faults, involves a steepening of the river gradient and an increase of energy.

Isostatic rebound

During Ice age the massive weight of the ice was enough to depress the crust underneath. In the centre of these sheets, the crust would have been forced down by almost a kilometre. This crustal depression was accommodated by the flow of the mantle asthenosphere away from the centre of the depression. When the ice sheets melted, about 10,000 years BP, this removed a huge weight from the crust, and the mantle started to flow back, pushing up the crust. This uplift of land increases the height of the land above sea level and rejuvenates rivers in the area.

2. Eustatic changes

This means an actual fall in sea level or base level. Changes in sea level brought about by the growth or decay of the ice sheets are called glacio-eustatic changes, to distinguish them from other eustatic changes which may result from variations in the capacity of the ocean basins.

The growth of the ice sheets at the beginning of the last glaciations about 25,000 years BP interrupted the global water cycle by locking up precipitation as ice in glaciers and ice sheets. Although some water did return to the oceans as meltwater, the amounts removed through evaporation far exceeded this and sea levels fell. It is thought that sea levels at the last glacial maximum were about 125m lower than today’s levels. This represents a substantial drop in base level and subsequent rejuvenation as rivers cut down to this level.

These do not involve changes in base level, but are the result of the river’s ability to erode more, due to:

·         changes in the load transported by the river

·         increase in discharge due to increase rainfall

·         increase discharge through river capture.

Landforms of rejuvenation

Rivers erode, transport and deposit material throughout their course.

There are three types of landform:

·         terraces (paired, unpaired and types of terraces)

·         knick points (waterfalls, rapids)

·         incised meanders (entrenched, ingrown).

Terraces

Terraces are the level ‘steps’ that are seen on the sides of river valleys which mark the level of the old floodplain. Terraces can be continuous; they are seen as long benches stretching along the valley. Terraces can be formed through either dynamic or climatic causes.

In either case, the river will cut down, eroding the old floodplain and leaving it above the new floodplain, and beyond the effects of flooding.

Terraces are described as paired or unpaired.

Paired terraces

Paired terraces are levels on either side of the floodplain which are the same height. There may be several pairs of terraces on the floodplain, like a flight of steps with their corresponding terrace on the other side

Unpaired terraces

Unpaired terraces form when lateral migration is dominant compared to incision. This is particularly clear in wide valleys where meander migration has taken place. Meandering rivers may cut down quite slowly, in a case, that meander migrates across the floodplain, eroding one side and then the other.

Knick points

As the river cuts down and readjusts its profile to its new base level, it will come to a point upstream where the new ‘readjusted’ profile meets the older profile. This forms a sharp increase in gradient called a knick point, which can show itself as a waterfall or rapids.

Incised meanders

If meanders are already established on the floodplain, then downcutting will result in incised meanders, which can cut down several metres below the old floodplain. There are two types of incised meander:

·         entrenched meanders

·         ingrown meanders.

Entrenched meanders

Entrenched meanders are those which cut down vigorously. Uplift is more rapid than downcutting, and as a result the river produces a symmetrical valley cross-section and the river is found at the bottom of a winding gorge

Ingrown meanders

Ingrown meanders have both components of down cutting and lateral erosion. They form when uplift or incision is gradual and this gives the meander time to shift sideways and so produce an asymmetric cross-section. This has a gentle slip-off slope and a steep river cliff, an enlarged form of the meander.

Qn Make a simple line sketch of the area. Label the following: river cliff, slip-off slope, ingrown meander, floodplain, river flow direction (indicate with arrow), paired terrace (left and right), earlier terrace.

Graded river

As erosion proceeds, the gradient of a river bed becomes progressively gentler, and the energy of running water decreases. Eventually the capacity of running water for transporting sediment load will come to match the supply. In this condition, neither erosion nor sedimentation occurs on the river bed and its gradient does not vary.

Aggradation The progressive accumulation of sediment along or within a stream.

Degradation The topographic lowering of a stream channel by stream erosion.

River capture

When rapid head erosion proceeds into an adjacent drainage basin, the valley head eventually works its way towards another channel, and it becomes connected with the upper reaches of the formerly separate basin. The point where river capture takes place is called an elbow of capture. The lower reaches of the captured river are deprived of the headwaters and a dry valley named a beheaded river or a wind gap remains. On the other hand, there is an increase in the discharge of the river with the enlarged drainage basin and under-cutting is accelerated to form a gorge.

Drainage pattern

Ways of characterizing drainage basin

One method is to look at the drainage pattern, which is the way that the various streams within a drainage basin are spatially organized.

Another way to characterize a drainage basin is by calculating the drainage density of a watershed. Drainage density refers to the relative density of natural drainage channels in a given area. This value is calculated by dividing the total length of all streams in the basin by the area of the basin:

A third way to characterize a drainage network is through the process of stream ordering. Streams within any drainage network can be arranged into a hierarchy based on their size. Stream ordering is useful because it provides a good relative measure of a stream’s place in a basin hierarchy, which is typically a function of how many tributaries occur in any given watershed.

Hortons law

Dendritic-A branching, tree-like drainage pattern evolves in areas of uniform rock resistance and structure, with little distortion by folding or faulting.

Radial-In a radial drainage pattern, streams radiate outward from a central point, forming a spoke-like pattern of rivers. This kind of pattern tends to evolve where streams flow away from rounded upland areas such as a volcano.

Trellis-A trellis drainage pattern is a system of streams that develops in areas such as the Ridge and Valley province in the Appalachian Mountains where rocks are folded. In this area, major streams tend to flow parallel to one another in adjacent valleys within the folded mountain belts. Minor tributaries flow into larger streams at right angles.

Rectangular-A rectangular drainage pattern occurs when joints and faults steer streams at right angles to one another. This pattern occurs because water flows preferentially to these zones of weakened bedrock where the water more freely erodes.

Deranged—A distinctively chaotic drainage pattern characterized by irregular direction of stream flow, few tributaries, swamps, and many lakes, that develops in recently glaciated terrain.

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