Drainage Basins

A drainage basin collects water from a large area and delivers it to a channel or lake. Drainage basins reflect the operation of physical laws affecting water flow over the ground surface and through rocks. These basins concentrate groundwater flow from a broad area into rivers, which, in turn, carry away both water and sediment.

Characteristics of Drainage Basins

A drainage basin is an area that collects water, which accumulates on the surface from rain or snow. Its slopes deliver the water to either a channel or a lake. Normally, the channel that collects the water leads to the ocean. In this case, the drainage basin is defined as the entire area upstream whose slopes deliver water to that channel or other channels tributary to it. Thus, strictly speaking, drainage basins are defined as natural units only when streams enter bodies of water such as lakes or the ocean or when two streams join.

Less often, there is no exit to the ocean. This type of basin is called a basin of inland or interior drainage. Notable examples are the basins containing the Great Salt Lake in Utah, the Dead Sea in Israel and Jordan, and the Caspian Sea in Asia. The Basin and Range province in the Rockies (an area of about 1 million square kilometers extending from southern Idaho and Oregon through most of Nevada, western Utah, eastern California, western and southern Arizona, southwestern New Mexico, and northern Mexico) has at least 141 basins of inland drainage. The center of these basins is usually marked by a “playa,” a level area of fine-grained sediments, often rich in salts left behind as inflowing waters evaporate. At certain times in the geological past, when the annual rainfall was heavier, some of these basins filled with water to the point of overflowing. At this point, the drainage system may have connected to another interior basin or a river system that drained to the sea. The basin now containing the Great Salt Lake (known to geologists as Lake Bonneville) overflowed at Red Rock Pass about 15,000 years ago. The overflowing waters discharged into the Snake River system and thus to the Columbia River and the Pacific. Drainage basins may change in character over relatively short periods of geological time. There is some evidence that the entire Mediterranean Sea was a basin of inland drainage about 3 to 5 million years ago. Substantial salt deposits are found on its bed, and traces of meandering rivers have been seen in certain geological sections.

Groundwater and Erosion

Although the term “drainage basin” is generally thought of as applying to the surface, the rock beneath the surface is an important component of the basin as a hydrological unit. Much of the water at the surface sinks into the soil and the underlying rocks, where it is stored as soil water in the “unsaturated zone” and as groundwater. Soil water either sinks farther to become groundwater or flows through soil and back out onto the surface downslope when and where the soil is saturated. Groundwater moves slowly through the rock (millimeters to centimeters per day). Still, it eventually seeps into stream channels and provides the base flow of water in rivers long after rain falls.

This characteristic of groundwater can lead to a circumstance that alters the definition of a drainage basin when the rocks are primarily composed of limestone or any rocks susceptible to solution. Because limestone is soluble in acidic water (natural rain is slightly acid, and contact with various airborne pollutants increases the acidity), over thousands of years, percolating groundwater dissolves substantial volumes of rock. It causes a system of underground channels to develop, which may eventually become enlarged into caverns. The Former Yugoslavia is famous for its underground cave systems; the United States (US) states of Kentucky, Florida, and New Mexico are well known for their limestone terrain and underground drainage systems. Essentially, every karst landscape on Earth is home to a system of caverns formed by chemical erosion over long periods. In these cases, the route of the water underground may bear little or no relation to the pattern of channels and slopes seen on the surface, some or all of which may have become completely inactive. Determining the drainage basin is difficult as various types of tracers (colored dyes or other readily detectable chemicals) have to be placed in the water to identify the points of egress so that an interpretation of the underground channels may be made. The pattern of water flow to any given site will also depend on the location of the storm waters causing the flow. The prolonged solution of limestone to form underground river systems and caves emphasizes that water moving through the basin removes solid rock. As the rivers dissolve their way downward, they leave some caves “high and dry” above the general level of the underground water (the water table), pointing out that rivers work down through the rock with time.

This aspect of drainage basins is harder to observe in areas of less soluble rock, even though water flowing out of the drainage basin carries sediment (small particles of soil and rock) and has been doing so for long periods of geological time. Thus, in the long run, the surface of the Earth is gradually lowered, and even in the short run, enormous amounts of sediment may be removed from a basin every year. The Mississippi removes about 296 million metric tons per year, or 91 tons per square kilometer, though this is small compared to the Ganges, which takes out 1,450 million metric tons per year, or 1,520 tons per square kilometer. Both of these are dwarfed by the average annual sediment load carried by the Yellow River (Huang He) in China, of 1.6 billion tons. If there were no corresponding uplift of the drainage basins or other interference, this removal would lead to the leveling of entire basins within 10 to 50 million years, depending on the lowering rate and the mean altitude of the basin.

The process of erosion proceeding at different rates in adjacent basins may cause the drainage divide (the line separating different flow directions for surface waters) to migrate toward the basin with the lower erosion rate. This is most common in geologically “new” terrain when stream systems are not deeply incised into the rocks. Drainage diversions may be simulated, as in the Snowy Mountain diversion in Australia, where waters are diverted across a divide by major engineering works to provide irrigation water for the Murray Darling River basin and hydroelectric power generation.

Floods

When winter snow melts, or severe storms bring heavy rain to large areas, the water that falls to the surface flows into channels and floods the rivers. Floods are not abnormal; they are an expected occurrence in drainage basins. It is easy to understand that when basin relief is high, and slopes are steep, as in the Rockies or the Appalachians, floods tend to generate higher flood peaks than when slopes are gentle. A basin that is round tends to concentrate floodwater quickly because the streams tend to converge in the middle. In contrast, long, narrow stream has the effect of attenuating the flow peak, even when the total amount of water falling on the basin may be the same. Similarly, a forest tends to attenuate flood peaks and promote higher river flows between flood peaks than does open farmland. With the latter, there is a tendency for water to flow rapidly off the surface into channels, whereas in a forest, the leaves of trees intercept much water, and the impact of rain on the surface is weaker, partly because leaf cover protects the soil. Because the soil is not so well protected, sediment loss from the surface into streams is greater from farmland than from forests and is higher again from land disturbed by major building projects.

Classification and Measurement of Drainage Basins

Various methods have been devised to classify basins according to size. The most common method depends on a numbering system applied to the streams that drain them. All the “fingertip” streams are labeled with 1. When two of these tributaries meet the channel, it is termed a second-order channel and is labeled with 2. Subsequently, the order of a stream increases by one only when two streams of equal order join. Otherwise, if two streams of unequal order join, the order given to the downstream segment is that of the larger two orders. The order of the drainage basin is then the order of the stream in the basin. In this type of numbering system (called Horton/Strahler ordering), the Mississippi drainage basin is an eleventh- or twelfth-order basin. The exact number depends on the detail (map scale) with which the fingertip streams are defined. The larger orders are rare because of the requirement for another river of roughly similar magnitude to join in to make the next higher-order basin.

Basin order may be used as a relatively natural basis for collecting other data about the basin. The simplest measure is the area in square kilometers. In addition, the basin relief, or the height difference between the lowest and the highest points, and the mean relief, or the average height of the basin above the outlet, may be recorded. The most precise method of recording basin relief is by computing the hypsometric (height) curve for the basin, which requires an accurate topographic map. When constructed, it shows, for any altitude, the proportion of the basin area above that particular altitude and, for comparative purposes, it may be produced in a dimensionless form by dividing both the height and the area measures by their maximum values or by the difference between the maximum and minimum heights if zero is not the minimum height.

Basin shape and basin dimensions (length and width) may also be recorded. However, the notion of basin shape suffers from the problem that no completely unambiguous numerical measure exists that can be used to define the shape of an area in the plane (that is, on a map). The problem is especially intractable if there are indentations in the basin's edge. All measures are dependent to a considerable degree on the accuracy of source maps. In mountainous terrain, such maps may often be much less than perfect, if they exist. Even with automated drafting aids and digitizers (which automatically record positions on maps and save them as a data file), measuring basin properties is tedious and time-consuming. Unless there are pressing reasons for a new analysis, it is common to rely on data tabulations made by hydrological or environmental agencies whenever possible.

Measurements are made of drainage basin properties because they are often used in statistical analyses with the known flow of the gauged rivers to predict flow characteristics for rivers that have not been metered. The direct measurement of stream flow, while straightforward in principle, is time-consuming, especially in the early stages, and a flow record is not very useful for predictive purposes until it has recorded at least twenty years of flow (preferably much more). Because of the high capital and maintenance costs involved in collecting river records, there has been an understandable emphasis on records for large rivers; the economic benefits from the prediction (and eventual control) of the flow are more obvious, and measured flow records can sometimes be supplemented by anecdotal evidence of historic large floods, those flows that are often of most interest in land-use planning (for example, zoning of land for residential use). It has been acknowledged that the hydrological behavior of low-order basins is less well understood. More information has been collected on them, especially for urban areas where the routing of the large quantities of water that run off from impermeable surfaces in the city (roofs and roadways) has been recognized as a serious planning problem, especially regarding groundwater recharge and the flow systems that connect with urban sewage systems.

Significance

Control of water outflow from drainage basins is necessary in some regions to promote irrigation, supply domestic and industrial water, generate power, and implement flood control. The Hoover Dam on the Colorado River was originally conceived as a control dam, but hydroelectrical generators were also included to help defray its costs by selling power. There are fifteen major dams in the Colorado basin. Aside from the legal technicalities of water ownership and redistribution, difficulties arise from the fact that large areas of the basin have to be regulated to control substantial amounts of water. In addition, there are economies of scale in large projects, particularly in constructing large dams and reservoirs. A single control dam strategically placed may regulate flow downstream for hundreds of kilometers, whereas it would require hundreds of small dams on first- and second-order streams to achieve the same effect.

Large control dams do generate problems. The reservoirs trap sediment coming from upstream, eventually filling them, at which point they will become useless. Small reservoirs may fill within a few years. An original estimate for the Hoover Dam suggested that it would take four hundred years to fill Lake Mead; after only fourteen years, surveys revealed that the water capacity had been reduced by 5 percent and that sediment in the lake bottom reached a maximum of 82 meters where the upstream river entered the still waters of the lake. Downstream of a dam, the reduced sediment content and the regulated water flow often seriously affect riparian environments. There may be a variety of channel responses, often unpredictable, to the interference in the river regime caused by the dam. The stream may cut into its bed, change the dimensions of its channel, or even aggrade its bed. In the case of the Hoover Dam, the water downstream, deprived of its sediment by the dam, had an increased ability to remove fine sediment from the river bed but left coarser rocks behind because the flood peaks that would normally have removed them were now controlled and much reduced. The result is an “armoring” of the stream bed with coarse rocks, an effect that extends 100 kilometers downstream in the case of the Colorado River below the Hoover Dam. In the Colorado system, the net effect of controlling flood peaks has been for rapids to stabilize and increase in size as sediment becomes trapped in them. A corollary of the “winnowing” of fine material has been the disappearance of river beaches and an increased propensity to pollution as sediment becomes much less mobile and more concentrated in space. In 2024, Lake Mead faces many issues related to global climate change, drought conditions, and increased demand for water supply. The issues facing Lake Mead are intrinsically tied to its role as a drainage basin for the Colorado River. They shine a light on the complexities of watershed dynamics, including snowpack dependence, runoff reduction, and the overall interconnected nature of water systems.

Principal Terms

basin order: an approximate measure of the size of a stream basin based on a numbering scheme applied to river channels as they join together in their progress downstream

channel: a horizontal depression in the ground surface caused and enlarged by the concentrated flow of water

erosion: the displacement of sediment from one location to another on the ground surface

groundwater: water that sinks below the ground surface to join the water table, slowly flowing toward river channels

hydrological: relating to the systematic flow of water in accordance with physical laws

limestone: a rock composed primarily of the mineral calcite or dolomite, which can be dissolved by water that is acidic

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