River

I

Introduction

River, any body of fresh water flowing from an upland source to a large lake or to the sea, fed by such sources as springs and tributary streams. The main parts of a river include a channel, in which the water flows, and a floodplain—a flat region of a valley on either side of the channel. Through the channel and floodplain, water and sediment—material transported by the river, such as sand and silt—are transferred from ridges and mountains to the sea or to a lake. A river starts on hillsides as small channels, or rills. The rills combine to make larger channels or tributaries that eventually come together, forming distinct streams. The largest channels formed by this convergence of tributaries are rivers, and they can carry large quantities of fresh water and sediment across continents.

Large rivers are located on every continent. The longest river on Earth is the Nile River in Africa, with a length of 6,695 km (4,160 mi) from its headwaters in Burundi to its mouth at the Mediterranean Sea. The Nile River basin covers an area of 3,349,000 sq km (1,293,000 sq mi). The Amazon River in South America carries the largest amount of water and runs for a length of 6,400 km (4,000 mi). This single river contributes 20 percent of the river water that flows into the world’s oceans. The Yellow River (or Huang He) in China gets its name from the yellow sediments of the soils of central China, and it carries the largest amount of sediment to the ocean. The Yellow River is the second longest river in China, at 5,500 km (3,400 mi), after the Yangtze, which is 6,300 km (3,900 mi) long.

Since the continents formed millions of years ago, rivers have been important geologic forces as conveyors of water and sediment. The rise of human civilization is intimately linked to rivers for access to drinking water, irrigation, transportation, and fisheries. People have irrevocably altered the landscape by maintaining rivers for navigation, constructing irrigation works, and building dams for hydroelectric power generation. Scientists study river systems as they are important to the flow of fresh water over wide areas of land (and eventually into our homes) and across continents. Rivers are also an important part of sensitive habitats, especially wetlands. The study of rivers is necessary to ensure the protection of ecologically important habitats.

II

Formation

A river forms in a watershed, a large area of land from which water contributes to only one stream or river. A watershed is bounded by the ridges or hilltops that divide it from adjacent watersheds, or drainage basins. When rain falls onto hillsides or when snow melts, the water runs downhill and accumulates in streams. A tributary stream eventually joins the main river channel at a confluence. The amount of rain or snow that falls in different parts of a watershed controls the size of a river. For example, the watershed of the Amazon River is 6,000,000 sq km (2,300,000 sq mi), the average rainfall is 2,000 mm (80 in) per year, and the average flow rate is 200,000 cubic meters/second (7,100,000 cubic feet/second, or cfs). In comparison, the watershed of the Mississippi River is 3,000,000 sq km (1,200,000 sq mi), half the size of the Amazon’s. However, the flow rate of the Mississippi River—16,800 cu m/second (593,000 cfs)—is only about one-tenth the flow rate of the Amazon, because less rain falls in the Mississippi watershed. The amount of rainwater that falls and the geology of the watershed also control the drainage pattern of the watershed. The most common type of drainage pattern is called dendritic. This pattern looks like the veins of a leaf. Drainage networks connect all of the areas of the watershed of the river.

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In very large rivers, the water comes from rain that may have fallen as far as 6,000 km (4,000 mi) away. During the journey through rills and streams, the water’s flow may erode and deposit sediment in the river’s channel and on its floodplain (see Erosion; Deposit). The biggest rivers usually carry the largest amount of sediment. Yet some of the largest rivers may carry very little sediment because the watershed may not have a lot of sediment. A river carries the most sediment when the flow is the highest. When a river experiences high flows, it fills in (floods) the floodplain, a flat region of a valley surrounding the river channel. As the water first reaches the floodplain, it may erode the sediment on the floodplain. As the flood drains from the floodplain, slower-moving water may deposit sediment onto the floodplain, replacing some of the sediment lost.

III

Factors That Shape a River

The combination of erosion and sedimentation in a river’s channel and on its floodplain works to produce the characteristic features of that river. The three major influences on patterns of erosion and deposition are geology, the type of sediment that is present, and the amount of water available. From the perspective of geology, generally a river travels through three zones from its headwaters at the top of the watershed to its mouth. The headwater zone in the mountains or hills is where sediment is supplied from hillsides and transported down steep channels with narrow floodplains. In these narrow, steep canyons, the bed of the river may be covered with large boulders as the river passes through many rapids. When the mountains give way to the plains, the steepness of the river channel will decrease from as high as tens of meters per kilometer, a grade of 1 to 10 percent, to less than 1 meter per kilometer. In this middle zone, although the amount of water may increase, the ability of the river to carve into rock and carry sediment decreases because the river channel is less steep. As the flow decreases, so does the power of the river, and the river loses its ability to transport large material. Gradually, the sediment in the river decreases in size from boulders (larger than 256 mm/10 in in diameter) to cobbles (between 64 and 256 mm/ 2.5 and 10 in) to gravel (between 2 and 64 mm/0.08 and 2.5 in). Eventually, as the steepness continues to decrease, the sediment becomes very fine, consisting mostly of sand, silt, and clay. As the river changes in this middle section, the floodplain widens. The third zone of a river is the zone influenced by the ocean or lake where the river ends. The steepness of the river channel in this zone is usually less than 10 centimeters per kilometer, and the sediment is very fine. If enough sediment settles out of the water in the lowest section of the flow, a river may form a delta. A delta differs from a floodplain because in a delta the river splits into many new channels called distributaries. If not enough sediment settles out to form a delta, the river may meet the sea in an estuary. An estuary is usually a wide channel where the fresh water from the river mixes with the salty seawater.

River features are also affected by the flow rate and the size and duration of floods. Some rivers receive rainfall almost every day in at least part of their watershed. Other rivers, such as those in desert regions, receive water only during brief, intense storms that may cause a flash flood (see Flood Control). The melting of snow and glaciers in the spring is a source of water for many rivers. If a river flows year-round, the river is called a perennial river. Usually a slow, steady inflow from groundwater, or water found underground, provides some of the water of a perennial river. If a river flows only during part of the year, the river is called an ephemeral river. An ephemeral river channel may have lots of water flowing though it during the rainy season but be dry as a bone in the late summer.

In the headwater zone of rivers, floods typically last a short time (less than one day) and are very powerful. In the middle zone the duration of floods increases, but the intensity decreases because the area of the floodplain is larger. At the mouth, or delta section of a river, floods can last for several months.

The water flowing in a perennial river may do a great deal of work, eroding and depositing sediment in the channel and floodplain. A perennial flow has enough time and energy to separate the sediment by size. The water moves coarser particles together in areas of the river where the water flows very fast. It deposits these particles sooner than finer particles, which are lighter and can stay suspended in the slower, less powerful flows. In perennial streams, slower flows that occur within the floodplain area (they are slower because the land is not steep here) deposit the finer particles on the floodplain.

In contrast to a perennial river, an ephemeral river may flow for only a few days. Therefore, for most of the year, additional processes may affect the features of the channel and floodplain. These processes include the action of the wind, the burrowing of animals, the growth of vegetation, and the activity of humans. When flow occurs only for short periods, the water may not sort the sediment and may deposit the particles in a mixture ranging in size from coarse to very fine.

IV

River Patterns

River patterns, or general shapes, depend on the geologic zone and the climate of the location. There are four river patterns: meandering, braided, anastomosing, and straight. A meandering pattern follows a winding, turning course. A braided pattern has connected channels that resemble a hair braid. An anastomosing river pattern combines features of the meandering and braided patterns. Some river patterns are simply straight channels. Meandering and braided are the most common patterns. Braided and straight patterns are usually located in the mountains or hills below the headwater zone of rivers, while meandering and anastomosing patterns are located in the middle and mouth zones of most rivers.

The Mississippi River is a classic example of a meandering river that has looping bends of different sizes along its valley. Each bend is the result of sediment depositing on the inside of the bend. As sediment deposits gradually build up, a point bar forms on the inside of the bend. The point bar pushes the river flow against the outside bank of the bend, eroding the bank opposite the point bar. Eventually the bend becomes so sharp that the river bypasses it, cutting a straighter path. The arc of the bend is left behind as the river moves past. The arc may form an oxbow lake (also called a billabong), a pool of water enclosed by the arc and riverbank. A meandering river’s bed is usually covered with sand, while the floodplain is filled with silt and clay.

Braided rivers look completely different from meandering rivers. They have many channels that are constantly changing position because of frequent changes in flow rate and sediment supply. The channels of a braided river—such as portions of the Platte River that flow through Nebraska—change course frequently, so the river’s water may cover the entire floodplain on a regular basis. The sediments of braided rivers are usually gravel and cobbles. Sometimes a meandering river may change into a braided river in the middle zone if the supply of sediment increases as a result of farming or grazing activities in the watershed.

Anastomosing rivers combine the bends of meandering rivers with the multiple channels of braided rivers. The sediments are typically sand, silt, and clay. Oxbow lakes may be rare. The Amazon River is an example of an anastomosing river.

Straight rivers are not common. They are typically located in canyons in mountainous areas or exist as the result of engineering structures that force a river into a straight course. Portions of both the Columbia River (between Washington and Oregon) and the Colorado River (in the southwestern United States) flow straight through canyons.

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