HOW RIVERS ARE FORMED?

Raindrops may fall thousands of meters before they hit the ground. The impact of falling raindrops can move sediments. Large amounts of soil can be moved this way. Precipitation that has neither soaked into the ground nor evaporated may form runoff. Runoff is water that flows across the land. It runs downhill on even the gentlest slopes. It is called the consequent stream. As the water runs downhill, it cuts into the soil forming many tiny grooves called Rills (small streams). If erosion continues, the rills widen and deepen, becoming Gullies. Gullies act as channels for runoff. Gullies are actually tiny stream valleys. When gullies reach a low lying area, runoffs from several gullies merge together. A larger stream is formed as a result. The greater part of the water, which falls as rain, runs off the surface of the earth to form streams. These unite to make rivers, which in turn flow into the sea or lakes. (See GOH 47)

GEOLOGICAL WORK OF RIVERS

The geologic work of streams consists of three interrelated activities: Erosion, Transportation and Deposition. (GOH 48)
Erosion by a river is the progressive removal of mineral material from the floor and sides of the river, whether this be carved in bedrock or in residual or transported overburden.
Transportation of rock particles by rivers can take place in one of four ways. When carried in suspension, small particles remain in the water current without settling to the bottom. This is made possible by turbulence in the flowing water. Some what larger particles may be transported by saltation, a hopping or jumping motion in which the particle occasionally strikes the channel bottom. If larger still, a rock fragment is turned over and over along the channel floor, a process termed rolling. The forth mode of transportation is in chemical solution. Here the particles are individual units of matter of molecular size and are of course invisible. Generally speaking, fine clays and silts are carried in suspension, sands and gravels by saltation, and cobbles and boulders by rolling.
Deposition is the progressive accumulation of transported particles upon the stream bed and flood plain, or on the floor of the standing body of water into which the river empties.
Obviously, erosion cannot occur without some transportation taking place, and the transported particles must eventually come to rest. Therefore, Erosion, Transportation and Deposition are simply three phases of a single activity. Perhaps as much as 95% of all landforms are sculptured by rivers.

LANDFORMS ASSOCIATED WITH RIVER EROSION

WAYS IN WHICH RIVER ERODES

Rivers erode in various ways, depending upon the nature of the river materials, and the tools with which the current is armed.

a. Hydraulic Action
The force of the flowing water, alone, exerting impact and a dragging action upon the bed can erode poorly consolidated alluvial materials such as gravel, sand, silt and clay, a process termed Hydraulic Action. (GOH 50)

b. Abrasion (Corrasion)
Where rock particles carried by the swift current strike against the bedrock or river walls, chips of rock are detached. The rolling of cobbles (pebbles) and boulders over the river bed will further crush and grind smaller grains to produce an assortment (variety) of grain sizes. These processes of mechanical wear and combined under the general term Abrasion, which is the principal means of erosion in bedrock too strong to be affected by simple hydraulic action.

c. Corrosion
The chemical processes of rock weathering – acid reactions and solutions – are effective in removal of rock from the rivers and may be designated as corrosion. Effects of corrosion are mostly marked in limestone, which is a hard rack not easily carved by abrasion, but yielding readily to the action of carbonic acid in solution in the river water.

d. Attrition
This is the wear and tear of the transported materials themselves when they roll and collide with one another. The coarser boulders are broken into smaller stones; the angular edges are smoothed and rounded to form pebbles. The finer materials are carried further downstream to be deposited.

LANDFORMS

a. Pothole
One interesting form produced by river abrasion is the pothole, a cylindrical hole carved into the hard bedrock of a swiftly moving river. Potholes range in diameters from a few inches to several feet; the larger ones may be many feet deep.

b. Waterfall
A waterfall is formed when a river flowed first over hard bedrock and then over soft bedrock. Abrasion wore away the soft bedrock more quickly than the hard bedrock. In time, the level of the river flowing over the soft bedrock becomes lower than the level of river flowing over the hard bedrock. Thus a waterfall is formed.

c. Canyon
A deepening Gorge or Canyon is perhaps the most striking landforms associated with erosion. The gorge is steep-walled and has a v-shaped cross section. The river occupies all the bottom of the gorge. Form the steep walls much weathered rock material is shed into the river. Landslides occur frequently, large fallen masses sometimes temporarily damage the stream. A young gorge may afford the only passage through a mountain range. The Royal Gorge of the Arkansas River, in the Rocky Mountain Front Range of South Colorado, is a striking example.

d. Rapids
A river in its youth stage, flows down a steep slope along a fairly straight path. As a result, the water in the river flows swiftly, often forming rapids. (GOH 51)

e. V-shaped Valley
The fast moving water erodes the land quickly. It can carry much sediments and cut deeply into the bedrock. Such erosion produces steep-sided v-shaped valleys. With the passage of time, as erosion continues, the valley’s v-shape become wider and less steep. The valley floor becomes flat.

LANDFORMS ASSOCIATED WITH RIVER DEPOSITS

Streams deposit sediments when they slow down or decrease in volume. The slower the water is moving, the less carrying power it has. The smaller the volume of the river the less water there is to carry sediment. Many landforms develop a result of stream deposition. These landforms are found in almost all parts of the world.

a. Ox-bow Lakes
As a river develops, meanders become more and more curved. In time, the river cuts through the land between the ends of a meander. Flowing through this shortcut, or cutoff, the river picks up speed. Slower water in the meanders deposits sediments near the cutoff. The sediments seal off the meander. The cut off meander dry up or become an Ox-bow Lake. (Strahler 424, GOH 53)

b. Natural Levees
Rivers that occupy valleys with broad, flat valley floors on occasion build a landform called Natural Levee that parallels its channel. Natural levees are built by successive floods over many years. When a river over flows its banks, its velocity immediately diminishes, leaving coarse sediments deposited in strips bordering the channel. As the water spreads out over the valley, a lesser amount of fine sediments is deposited over the valley floor. This uneven distribution of material produces the very gentle slope of the natural levee. The natural levees of the lower Mississippi rise 6 meters above the valley floor.
The area behind the levee is characteristically poorly drained for the obvious reason that water cannot flow up the levee and into the river. Marshes called Back Swamps result. A tributary stream that attempts to enter a river with natural levees often has to flow parallel to the main stream until it can breach the levee. Such streams are called Yazoo Tributaries after the Yazoo River, which parallels the Mississippi for over 300 kilometers. (Strahler 425)

c. Flood Plains
Rivers in their lower course carry large quantities of sediments. During annual or sporadic(occasional) floods, these materials are spread over the low lying adjacent areas. A layer of sediment is deposited during each flood, gradually building up a fertile floodplain. Flood plains make good farmland, the flood deposits enrich the soil.
The depositing streams generally have either meandering or braided (tangled) courses, depending on the kind of sediment in transit. Both types construct flood plains, and there are similarities and differences in the surface forms associated with each. The flood plains of meandering rivers are more suitable for agriculture use than those of braided streams because the alluvium has a finer texture and hence retains more moisture and plant nutrients.
d. Alluvial Terraces
These are flat benches of alluvium situated on flood plains. They mark former levels of deposition below which the rivers have since cut their present graded plain. Such terraces may result from lowering of base level or from a sudden decrease in load.

e. Deltas
A delta is a landform made from deposits at the mouth of a river. A river slows down and stops at its mouth. The mouth of a river is the point where it flows into an ocean or other large body of water. By the time it reaches the mouth, a river has already deposited much of its load of sand, silt and clay. Such sediments build the Delta.
Delta plains are generally favorable areas for agricultural land use because of their fine textured alluvial soils, but flood danger, poor drainage, and salinity in swells, and depressions can be local problems. The delta plains in Eastern and Southeastern Asia have some of the densest rural populations in the world, based primarily on the cultivation of rice. Other intensively cultivated delta plains include those of the Mississippi, Nile, Po, Rhine, Tigris – Euphrates and Colorado Rivers. The broad delta of Amazon is unique in that it is largely unused and supports only a sparse pastoral (rural) population.
Some coastal plains are produced through the joining of adjacent deltas. The plains along the Eastern side of Sumatra, the Northern part of Java, the Coast of the Gulf of Guinea, in Western Africa, and parts of the Guiana Coast of Northern South America [Atlas 18-D2] are of this type. Many alluvial plains that border the ocean in such Tropical areas as these are almost always bordered by dense tangles of mangroves, and swamp palms that hinder the reclamation of the seaward margins of deltas. (Strahler 429)

f. Alluvial Fan

An alluvial fan is a landform made of deposits from a river flowing onto a level land. A fast mountain river slows down when it reaches flatter land. At this point, much of the stream’s load settles to bottom of the river. The settled load can collect into a landform called Alluvial Fan. Usually, the coarse material is dropped near the base of the slope, while finer material is carried farther out on the plain. An alluvial fan is much like a delta that built on land instead of in a large body of water.