Streams and rivers

Rivers, streams, creeks, brooks, and rills are long, narrow bodies of water that flow downslope in a channel under the influence of gravity. Although the list is arranged in general order of size, the term “stream” is accepted as the scientific term for any amount of surface water flow. Streams are an integral part of the hydrologic cycle, as they transport water and sediment from the land to the sea.

Background

The vast majority of all the water on Earth is contained in its oceans (97.2 percent) and its ice sheets and glaciers (2.15 percent). This means that 99.35 percent of the world’s water is either salty or frozen. Of the remaining 0.65 percent, groundwater accounts for 0.63 percent, followed by freshwater lakes (0.009 percent), saline lakes and inland seas (0.008 percent), soil water (0.005 percent), atmospheric water (0.001 percent), and stream channels (0.0001 percent). It is obvious that streamflow makes up the smallest amount of water on Earth, yet it has been critical for the development of human societies.

Streams have been associated with the development of civilization from the beginning of human history. The elaborate irrigation systems that developed in Egypt, China, and the Tigris-Euphrates valley in Iraq and portions of surrounding countries started thousands of years ago. Streams have also been used for navigation, municipal water supplies, wastewater disposal, and, in more modern times, power-plant cooling, hydropower, and firefighting. Major cities developed on or near streams: London on the Thames; Albany and New York on the Hudson; Shanghai on the Huangpu, a tributary of the Chang (also known as the Yangtze); New Orleans and Saint Louis on the Mississippi; Montreal on the Saint Lawrence; Vienna and Budapest on the Danube; and Cairo on the Nile. This brief list could include many other cities, but the important fact is that most of the world’s major settlements have developed on or very close to a source of water, and most of these sources are streams.

The Development of Streams

Precipitation events result in several types of natural response. The first response is simply that the precipitation can evaporate before it even reaches Earth’s surface. This generally occurs in drier climatic regimes. The second response occurs when some of the precipitation infiltrates into the soil and forms the groundwater component of the hydrologic cycle. The third response is usually associated with storm events that could easily exceed the infiltration capacity of an area and thereby result in overland flow. This type of flow would travel downslope and eventually reach an area where this surficial downward movement of water coalesces and starts to erode the land surface to a point where a permanent channel starts forming. Over time, streams become perennial (meaning they flow continuously throughout the year) when they have cut deeply enough so that the surface of the stream is below the adjoining water table, thereby allowing groundwater to supply the stream. This movement of groundwater is called “base flow,” and can easily account for one-half or more of the total water flowing in a stream, particularly in coastal plain formations such as those along the eastern coast of the United States. Thus, surface runoff consists of both overland flow and groundwater, even though they move at very different rates (groundwater is much slower).

The ultimate sink for most of the streams in the world as they move from higher to lower elevations is the oceans. Steep gradients, rapids, and waterfalls are generally associated with streams in the uppermost portions of their watersheds. Over time, these streams develop a progressively gentler slope as the irregularities in the channel are removed and the profile becomes smooth. With time, the stream widens its bed and valley as downward cutting of the channel is replaced by lateral cutting. Stream sinuosity increases as meanders develop. A depositional feature of a stream, described as a floodplain, widens with time and downstream progression. Streams often build deltas when they empty into the sea when circumstances permit. Deltas form when the velocity of the water moving in the channel slows down as the stream enters a much larger body of water. The particles settle out and form a variety of deltaic types. Well-known deltas include the Nile and the Mississippi. Where conditions are not favorable for deltaic formation, as in the case of the two largest streams in the world (the Amazon and Congo Rivers), there is no delta.

Some drier areas of the world have streams that are located in interior basins without any outlet to empty into the oceans. These endoreic watersheds represent internal drainage basins. Examples include the Great Basin in Nevada and portions of surrounding states, Lake Chad in sub-Saharan north-central Africa, the Dead Sea in Israel and Jordan, and the Aral and Caspian Sea basins in central Asia.

Runoff Measurement

The amount of precipitation that falls on Earth’s surface, minus the amount that evapotranspires from vegetation back to the atmosphere, either enters the soil to become a component of groundwater or runs over Earth’s surface as overland flow to eventually wind up in streams. The measurement of all of the myriad components of surface and groundwater runoff would be a daunting task at the least. However, it is not necessary to determine the flow rates of all of these upslope contingents of runoff. The simpler procedure is to measure the flow at various points along the stream in order to determine the runoff from part of or the entire watershed.

The earliest attempts to at least partly measure streamflow began about five thousand years ago on the Nile in Egypt. The height of the river could be measured by reference to staff gauges called Nilometers that were fixed to the banks of the Nile. These permanent structures provided a measure of river stage that was then used to alert downstream farmers of the rise or fall of flood waves coming down the Nile. Staff gauges are still used to measure river, lake, and canal levels. In order to determine the volume of water that goes past a given point on a stream during a certain period of time, it is necessary to measure the volume and velocity of water at that point. This flow is calculated by multiplying the area of the cross section in square meters by average velocity in meters per second to yield discharge in cubic meters per second. The first permanent streamgauging station in the United States was established in 1889 on the upper Rio Grande in New Mexico by the US Geological Survey (USGS), an agency of the federal government that, in conjunction with states and municipalities, is responsible for maintaining gauging stations. The total number of gauging stations in the USGS network was once about seven thousand for the whole country, but funding cuts have been reducing the overall total over the years. Regrettably, some stations that were dropped had records of continuous daily measurement in excess of thirty years, thereby representing a loss of valuable hydrologic data.

Watershed Area

The total amount of land that collects all the surface and groundwater within an area is called a watershed (or drainage basin). Watersheds vary enormously in size, from a tiny lot that drains into a pond to the Amazon River basin, with a drainage area of 6.16 million square kilometers. The second and third largest watersheds in area in the world are the Congo in central Africa (3.83 million square kilometers) and the Mississippi (3.26 million square kilometers).

Another characteristic of watersheds is their variation in shape. Although many of them are pear-shaped (reflecting a preponderance of horizontal strata), others are elongated, indicating a landscape of ridges (resistant formations composed of rocks such as schist or gneiss) and valleys (weaker formations composed of shale). This diversity in the underlying rock formations has an important bearing on slopes, infiltration, and runoff characteristics of all watersheds in addition to the obvious effects of climatic variations.

Stream Discharge and Length

Given the huge variation in precipitation, temperature, and landscapes in different regions on Earth, it is obvious that average stream discharge would also vary enormously. Given its large size and favorable location in the wet equatorial belt, with average annual precipitation in excess of two hundred centimeters, the Amazon leads the list with an average discharge of 175,100 cubic meters per second. This astounding amount of flow accounts for about 20 percent of the total discharge of all of the world’s rivers, even though the area of the Amazon watershed is only about 2 percent of the total land area in the world. The second and third largest average discharges in the world are the Congo, with 40,000 cubic meters per second, and the Mississippi, with 18,400 cubic meters per second. Note that the average discharge of the Amazon is nearly ten times that of the Mississippi, even though the area of the Amazon is somewhat less than twice the size of the Mississippi. The difference is attributed to climatic factors in the watersheds.

The four longest streams in the world are the Nile (6,648 kilometers), the Amazon (6,436 kilometers), the Chang (6,300 kilometers), and the Mississippi (5,970 kilometers). Although the Nile is the longest river in the world, it is ranked thirty-third in terms of average discharge, as it flows for a considerable distance through the deserts of northern Sudan and Egypt before it reaches the Mediterranean.

Drainage Density and Topographic Texture

The variation in rock formation and soil type within each watershed governs the number of streams per unit area that can form in different landscapes. Drainage density is determined simply by dividing the total length of streams within a selected region by the area of the region. For example, an area underlain by very resistant beds of hard sandstone or and covered by heavy tree cover in a humid environment would experience minimal development of streams, resulting in a very low drainage density and consequent coarse texture. Another example, also in a humid environment with tree cover, would be an area underlain by weaker and therefore less resistant shale rock that would have more streams in the same size area and therefore have a somewhat higher drainage density number that would be considered to be medium texture. High drainage density (fine texture) would be expected to develop in landscapes underlain by weak rocks in conjunction with limited coverage of vegetation. Instances of very high drainage density and resulting ultrafine texture can be found in the badlands of South Dakota, where the underlying earth materials are very weak clays and shales in conjunction with limited vegetative cover.

Those areas that have coarse texture (low drainage density) have much fewer streams than those watersheds with finer texture that are more easily eroded with many more streams per unit area. The difference in drainage density can therefore range up to two orders of magnitude.

Stream Ecosystems

Free-flowing streams have natural pulses of seasonal floods that maintain a dynamic equilibrium between the biological and physical aspects of aquatic ecosystems. The flora and fauna of stream ecosystems are well adapted to the natural variations in streamflow. For example, water levels and flooding events in the eastern United States are generally highest during the late winter-early spring period when evapotranspiration is low and soil moisture is at or close to its field capacity (the amount of water that is retained in the soil or rock against gravitational forces). This does not mean that flooding events cannot occur during other parts of the year, such as the summer and fall. For example, although long-term records clearly indicate that the usual time for floods to occur in the Mississippi is in the spring, major floods have also occurred in midsummer. Another example is provided by the long-term records of the Connecticut River at Hartford, which show expected floods during March and April but also a second period during the fall, when heavy rainstorms and hurricanes can occur.

The Geologic Work of Streams

Erosion, transportation, and deposition represent the three closely related activities of streams. Stream erosion refers to the removal of earth material from the channel. The same process will occur in both alluvial and bedrock channels, although different removal rates will apply based on resistance to (alluvium is much easier to erode).

As the particles are eroded, they are transported by the stream in one of the following modes: in solution, in suspension, or as bed load. Salts from alteration constitute a very common form of material in solution in all streams. The dissolved matter (salt) cannot be seen, as it is mostly in the form of chemical ions. Stream turbulence allows and silt particles to be carried as suspended load. Highly visible soil particles that have been eroded from the extensively cultivated fields in the Midwest and wind up in the Mississippi have given the river the nickname “the Big Muddy.” Sand, gravel, and cobbles are much larger than clay and silt particles and therefore move as bed load close to the channel floor. The suspended load is usually the greatest of the three forms of stream transport. For example, it is estimated that the Mississippi transports about 90 percent of its total load in suspended form. This high proportion is attributed to the semiarid area of the Missouri River basin, a major tributary of the Mississippi, which also includes the easily eroded Badlands of South Dakota.

Deposition, the third activity of streams, can occur on the streambed, in the floodplain, or on the bottom of a water body—such as an ocean—into which the stream empties. For example, the mouth of the Amazon is 161 kilometers wide as it flows into the Atlantic. The enormous amount of sediment that it carries forms large islands at the mouth or is simply deposited on the adjoining continental shelf.

Stream “capacity” is defined as the maximum amount of suspended load and bed load that can be transported in a stream at a specified discharge. An increase in discharge results in a substantial increase in suspended load, as the faster the water is moving, the greater the turbulence and consequent ability to keep material in suspension. The same effect occurs with bed load: As the stream velocity increases, the ability to move material along the bottom of the stream increases by three to four orders of magnitude as compared to the velocity. Thus, if the velocity of a stream is doubled in a flood situation, bed-load movement can increase by a factor of eight to sixteen times.

There is an enormous range in sediment load transport among the major rivers in the world. For example, the average annual sediment yield varies from a low of 4 metric tons per square kilometer for the Yenisey River in Siberia to a high of 2,600 metric tons per square kilometer for the Huang (also known as the Yellow) River in China. This enormous difference is caused by the large soil-erosion rate in the cultivated and easily eroded silt soils of the Huang River basin as compared to the mostly forested and uncultivated Yenisey River watershed. Another major example is the Mississippi River, where most of the annual sediment yield is attributed to the Missouri River, which flows through subhumid and semiarid grasslands, large portions of which have been cultivated.

Proper design of reservoirs should include estimates of the sediment load carried by an incoming stream. The sediment that gets trapped behind the dam of a will eventually eliminate its storage capacity. For example, several inland reservoirs that were used to supply water to Santa Barbara on the coast in California filled with sediment in only a few decades and had to be abandoned. Even major reservoirs in semiarid to arid regions, where vegetative cover on watershed slopes is limited, will experience reduced lifetimes as a consequence of continuing sediment input.

Exotic Streams

Ancient civilizations that developed in arid regions, such as Egypt or Mesopotamia in what is now Iraq, based their elaborate irrigation systems on streams that originated in upstream mountainous areas that have a water surplus. The Blue Nile originates in Lake Tana in the Ethiopian highlands, which receive heavy summer rains. The White Nile begins in Lake Victoria in east-central Africa, which experiences heavy precipitation throughout the year. The Blue and White Niles join at Khartoum in Sudan and flow north to Egypt through an arid region. The Tigris and Euphrates rivers start in the highlands of east-central Turkey, flow through Syria and Iraq, and empty into the Persian Gulf. Another classic example of an exotic stream is the Colorado River, which begins in the Rocky Mountains of Colorado and Wyoming and flows through Utah, New Mexico, Arizona, Nevada, California, and Mexico before emptying into the Gulf of California. Large portions of this basin, especially the lower part, receive limited precipitation in an area that has experienced considerable population growth and commensurate demand for water.

Anthropogenic Effects on River Systems

It is well known that natural factors occur that may have substantial effects on streamflow. Some obvious examples are wet and dry periods that can occur during any year and other climatological events such as El Niño that may have return cycles of a decade or more. However, these are natural events that may easily be superseded by human intervention, as exemplified by such activities as extensive diversion for agricultural irrigation, interbasin transfers of large volumes of water for public water supply, and the development of a large system of dams.

Dams can have a major impact on streamflow, as the water can be released below a dam without regard for natural cycles; instead, the release may be ruled by a particular need for water or electricity on cycles that may change on an hourly basis. As a result, stream channels below a dam can experience sharp changes based on annual flow, flood peaks, and the sediment load in the stream. These changes can range from sand accumulation in one channel to vegetation moving into another section of the channel to heavy bank erosion in still another part of the stream channel.

Dam building has a long history. For example, dams were built upstream of Cairo on the Nile by the Egyptians some five thousand years ago. The Chinese built a dam with a height of 27.4 meters on the Abang Xi River some twelve centuries ago that is still used for some irrigation water. Dams were built in Western Europe during the late Middle Ages to provide power for waterwheels. The Anasazi constructed small dams on the Mesa Verde in Colorado for irrigation some eight hundred years ago. The use of heavy machinery in the twentieth century led to an enormous expansion of dam construction, especially in the United States and Russia.

In recent years, a growing recognition has developed in the United States that certain dams should be considered for demolition because of problems with salmon runs; scouring of beds and banks below the dam, as the released waters are relatively clear because the sediment load has been left behind; and changes in the temperature of the water that may be undesirable for aquatic life. In stark contrast to the previous discussion of selected dam removal in the United States, China began construction of the Three Gorges Dam on the Chang in 1994. This enormous structure is close to two hundred meters high, has a width of more than two kilometers, and creates a reservoir with a length of six hundred kilometers. The purported benefits include electricity generation without burning coal, navigation improvements, and a potential decrease in flooding events. On the negative side are the large-scale movement of several million people who were living in the river valley; sediment reduction that could affect downstream areas, including the East China Sea; and a growth in the risk of new landslides and the possibility of increased geological instability in an area that is already seismically active.

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