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Rivers and Landscapes |
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Most streams are formed in mountains and hills from surface run-off, by the emergence of absorbed rainwater from the ground (as springs), or from melting glaciers. Over many years a stream becomes a river by eroding its bed. The course of a river can be divided into three sections:
River Erosion Process
Boulders and pebbles carried by the current scour and excavate the bed by corrasion. The rocks carried by the river are themselves worn down by abrasion as they collide with and rub against each other, so that abrasion of the boulders in the upper course provides the fine particles in the lower course. Fine particles are transported in suspension by the water. Rocks that are too large to be suspended are picked up from the bed of the river by the turbulence, only to be dropped again. This bouncing action is called saltation. Boulders are rolled along the river bed by traction. Solution action is another form of weathering performed by a river. Weak acids in the water, such as carbonic acid, may dissolve the rocks over which the water passes. Most erosion occurs when the river is in spate, when its movement is most turbulent and its speed increases. Valleys, Gorges and Canyons
In the upper part of its course a river erodes chiefly by vertical corrasion, cutting a steep V-shaped valley that winds between interlocking spurs of high land. The level of a river is changed when there is either an isostatic lift in the land or an eustatic fall in the sea level. In both cases the river is forced to regrade its course to a new base level and in so doing cuts a new valley in the original floodplain. This rejuvenated erosion results in the formation of river terraces.
Localized undercutting by lateral erosion on both sides of the narrow neck of an incised meander can produce a natural bridge. When a passage is eventually excavated the river bypasses the meander, leaving an abandoned meander loop beyond the bridge. The Grand Canyon, one of the world's scenic wonders, was first cut in Miocene times (about 26 million years ago) as the Colorado Plateau was slowly uplifted by earth movements. The canyon has a maximum depth of about 2,080m from the plateau top to the Colorado River. Differential erosion of the horizontal strata of sandstone, limestone and shales has formed a spectacular terraced valley up to 24km wide. River capture, which sometimes occurs in the upper course, results in an elbow-bend in the river and an H-shaped gorge. This happens when a stream erodes the land at its source until it breaks into the valley of another stream, and the adjacent stream is diverted into the new gorge. Waterfalls and Rapids
In the torrent stage of a stream, resistant bands of rock sometimes project transversely across the valley. If the hard band of rock dips gently downstream, then a series of rapids develop, as in the River Nile cataracts, where hard crystalline bands of rock cut across the river as it flows through the Nubian desert north of Khartoum. If the resistant layer is horizontal or dips upstream and covers a softer rock, then a waterfall may eventually result.
Potholes are also a feature of the upper course of a river. They are formed when eddies whirl around pebbles, causing them to spin and act as grinding tools on the rock below. River Meanders
In the middle course of a river, most outcrops and formations are worn away and the bed is fairly flat. The current is just strong enough to carry debris from the upper course.
The curves of a meandering river that flows across a wide flood plain slowly migrate downstream as erosion occurs on the outer bank of the bends and as sediments are deposited on the inner banks. The changing shape of the bends is due to the current, which usually follows a helical or corkscrew pattern as it goes downstream, flowing faster on the outer bank and sweeping more slowly towards the inner bank where it deposits a series of point bar sediments. When a river is in spate, silt or alluvium may be spread over the floodplain. The river bed is raised higher than the surrounding land by deposition, while the river itself is contained by embankments, or levees, which are formed from the deposition of silt. Levees may break when the river is swollen and large areas of the floodplain may be inundated. At this time a river may alter its course, as did the Hwang Ho in China in 1852, when it shifted its mouth 500km to the north of the Shantung Peninsula. Perhaps the most famous levee disaster in modern times was that of New Orleans where a massive levee break took place as a result of Hurricane Katrina on August 29 2005, which submerged 80% of the city and killed almost 1500 people. On a smaller scale, individual meanders may be cut off if the river breaks through the narrow neck of land separating a meander loop. The river straightens its course at this point and the abandoned loop is left as an oxbow lake which gradually degenerates into a swamp as it is silted up by later floods. A river is described as braided when it becomes wide and shallow and is split into several streams separated by mid-channel bars of sand and shingle. Braiding often develops where a river emerges from a mountain region onto a bordering plain. The sudden flattening of the slope checks the velocity of the stream and sediment is deposited. River Deltas
As the delta progrades into the water the coarsest sediment is carried through the river channel and laid down on the delta surface to form the top beds. A good example of a lacustrine delta is found where the River Rhone enters Lake Geneva. The river is milky grey in color because it is heavily charged with sediment acquired from its passage through the Bernese Oberland. The river plunges into the clear waters of the lake and slows down immediately, leaving the material it has transported to contribute to the outgrowth of the delta. Ultimately the lake may become completely silted up, although some lakes are initially divided by deltaic outgrowth. Derwent Water and Bassenthwaite in the English Lake District were originally one lake but are now separated by delta flats that were produced by the River Derwent.
Lagoons, marshes and coastal sand spits are also characteristic features of most deltas. The Mississippi River Delta has most of these features including levees, bayous (distributaries) and etangs (lagoons). The delta progrades seawards by way of several major channels which resemble outstretched fingers. Alluvial Fans and Cones
In semi-arid regions where a river emerges from a canyon or a wadi onto a lowland, often most of the water sinks into the porous alluvium of the fan and the whole load of sediment is deposited without spreading, creating a steep-sided structure called an alluvial cone. When closely spaced streams flow down onto a piedmont plain their fan deposits may eventually coalesce to create a bajada, which has a more gently inclined surface than that of the isolated alluvial cone. These features can be seen in the arid intermontane basins of the western United States. They are bordered by fault-line scarps which overlook rock pediments that are covered by spreads of sand and gravel. Death Valley, in California, is a fault-bounded basin of inland drainage with bajadas that merge into a piedmont gravel zone around a central salt-encrusted playa lake which is below sea level. The deposits are formed from material that is washed down from the surrounding mountains by intermittent streams during flash floods. The nature of the sediment varies from coarse fan debris in the piedmont zone to fine sands and salt deposits in the center of the basin.
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The force of flowing water, known as hydraulic action, removes loose material from the surface and forces apart cracks in rocks.
Incised meanders occur with renewed down-cutting so that bends in a river are etched into the bedrock. In some cases an asymmetrical valley is formed where lateral erosion on the outside of a bend produces river cliffs and a more gentle slip-off slope develops on the inside bend. If erosion is mainly vertical, then symmetrical valleys are formed.
In its outlet from Lake Erie the Niagara River plunges 50m over a hard dolomitic limestone ledge. The less resistant shales and sandstone beneath have been eroded by eddying in the plunge pool and by water splashing and dripping back under the ledge, leaving the limestone unsupported. This process of headward erosion has resulted in the formation of a receding gorge 11km long downstream from the falls.
Waterfalls are also produced by glacial action where, due to the gouging of the main valley by ice, the valleys of tributary streams are left hanging high above the main valley floor. These hanging valleys often produce magnificent falls which plunge down the side of the main glacial trough.
However, as a river flows onto flatter slopes it slows down and the coarsest debris is deposited. This debris may form sand and gravel bars around which the river is forced to flow. These deflections in its course develop into bends as the outer edges are eroded and as bars of sediment are deposited on the inner edges. In time, the curves become increasingly exaggerated and the river meanders.
Deposition is concentrated where a river is slowed on entering a lake or the sea. A delta forms at this point as long as no strong currents or tides prevent silt from settling. A typical cross-section through a delta shows a regular succession of beds in which fine particles of material - which are carried out farthest - create the bottom beds, whereas coarser material is deposited in a series of steep, angled wedges known as the foreset beds.
Marine deltas are formed when the ocean currents at the river mouth are negligible, as in partially enclosed seas such as the Mediterranean and the Gulf of Mexico. The classic marine delta is exemplified by the arcuate type of the River Nile. Sediment is deposited in a broad arc surrounding the mouth of the river, which is made up of a series of distributary channels crossing the delta.
When a mountain torrent flows into a main valley its velocity is greatly reduced and some of its load is deposited as an alluvial fan. This occurs, for example, in the upper Rhone valley in Switzerland, where hanging tributary streams cascade down the mountainside to be abruptly checked on reaching the floor of the main glacial trough. The alluvial fans that have been built up by these streams are now sufficiently high to be the sites of villages.