Chapter 11 - Freshwater: Streams, Lakes, Wetlands, and Groundwater

flood

A relatively high stream discharge that overtops the stream banks, covering land that is not usually underwater.

hydrologic cycle

The continuous circulation of water among the hydrosphere, the atmosphere, the biosphere, and the geosphere; also called the water cycle. Much water seeps into the ground—the geosphere—to become part of a vast, subterranean reservoir of groundwater. Water infiltrating into the subsurface comes from direct precipitation and from surface water sources such as rivers and lakes. Although surface water is more conspicuous, 60 times more water is stored as groundwater than in all streams, lakes, and wetlands combined. Groundwater flows through pore spaces in sediment and bedrock in the subsurface but typically does so much more slowly than water flowing on the surface. A small amount of water is incorporated into the biosphere in the form of plant and animal tissue. The hydrologic cycle refers to the movement of not only water but also energy from one part of the globe to another. Ocean currents transport huge quantities of heat from the equator toward the poles, thus cooling the equator and warming the higher latitudes. Evaporation is a cooling process, whereas condensation releases heat. Water vapor is a greenhouse gas, which warms the atmosphere, but clouds and sparkling glaciers reflect sunlight back out to space and thereby cool Earth. There are so many feedback cycles, many with opposite effects, that one scientist wrote, "Boldly simplified, water acts as the venetian blind of our planet, as its central heating system and as the fridge, all at the same time."

aquifer

A body of rock that can yield economically significant quantities of groundwater; should be both porous and permeable.

aquitards

A body of sediment or rock that has low porosity and permeability and that inhibits the flow of groundwater.

column

A cave deposit formed when a stalactite and a stalagmite meet and grow together.

stalagmite

A cone-shaped deposit of calcite formed over time as drops of water fall to the same spot on the floor of a cavern, release carbon dioxide gas upon impact, and precipitate the mineral as a result of the increase in pH.

oxbow lake

A crescent-shaped lake created where a meander loop is cut off from a stream and the ends of the meander become plugged with sediment. Meandering streams typically form in regions of very low slope gradients. In addition, meandering streams typically carry a large suspended load that includes an abundance of clay. During floods, the mud is carried onto the adjacent floodplains where it settles out and is deposited. Subsequently, this clay-rich mud is exposed along the banks of the river, especially along the outside of meander bends. As anyone who has spent time making ceramics knows well, wet clay is sticky. This sticky nature allows river banks made of clay to become very steep or even overhanging without collapsing. The abundance of vegetation further helps stabilize the banks of meandering rivers. As a result, meandering streams typically flow in a single channel that changes slowly through the process of lateral migration described previously.

Oligotrophic lakes

A deep lake characterized by nearly pure water but with low concentrations of nutrients, thus sustaining relatively few living organisms. As a lake fills with sediment, it becomes shallower and sunlight reaches more and more of the lake bottom. The sunlight allows bottom-rooted plants to grow. As the plants die and rot, their litter adds nutrients to the lake water. Plankton increase in numbers, as do the numbers of fish and other organisms. The lake may become so productive that its surface is covered with a green scum of plankton or a dense mat of rooted plants. The litter from this biomass contributes to sediment filling the lake, and eventually the lake becomes a swamp.

point bar

A deposit of sediment in the slower water on the inside of a meander. Through time, the point bar builds further and further out into the river as the outside bank of the meander is eroded back. In this manner, the meander slowly migrates laterally toward its outside bank. Occasionally, such lateral migration of a stream causes it to cut across the neck of a meander, forming a short-cut and isolating an old meander loop

Sinkhole

A depression on Earth's surface caused by the collapse of a cavern roof or by the dissolution of surface rocks, usually limestone.

alluvial fan

A fan-shaped accumulation of sediment created where a steep mountain stream rapidly slows down as it reaches a relatively flat plain. Alluvial fans are common in many arid and semiarid mountainous regions, where they commonly occur where a fault has caused a mountain range to be uplifted relative to the adjacent valley. In areas characterized by active faulting, it is common for the upper portion of an alluvial fan to be cut-off, or beheaded, from the lower portion of the fan as the fault continues to move through time. As the fault accumulates more offset through time, the lower portion of the fan slowly is buried as the valley fills with sediment, whereas the upper portion—the head—of the fan is uplifted as part of the mountain front where it is usually eroded. Most alluvial fans are relatively small, covering areas ranging from less than a square kilometer to a few square kilometers

delta

A fan-shaped accumulation of sediment formed where a stream enters a lake or ocean; includes a nearly flat delta top that is partly onshore and partly offshore, a more steeply dipping delta front located offshore, and a muddy prodelta located further offshore at the base of the delta front. Both deltas and alluvial fans are commonly fan shaped and resemble the Greek letter delta (Δ). A large delta may cover thousands of square kilometers. Even though deltas cover only a small fraction of Earth's total land surface, they are environments that involve each of Earth's four spheres. The delta is composed of solid sediment, but it is a watery zone where branched distributaries meet open water and much groundwater is contained between the individual sediment grains in the delta. Because floods regularly deliver nutrient-rich sediment to deltas, they typically are fertile, and delta ecosystems are rich in natural plant and animal life. The fertile soils and easy access to transportation systems make deltas desirable places for human habitation, but the elevation of delta land along the coast is barely above sea level and the river elevation, so it is certain to be flooded during atmospheric disturbances such as heavy rains and hurricanes.

well

A hole dug or drilled into Earth, generally for the production of water, petroleum, natural gas, brine, or sulfur, or for exploration.

kettle lake

A lake that forms in a depression created by a receding glacier, filled with the water from the melting glacier.

karst topography

A landscape that forms over limestone or other soluble rock and is characterized by abundant sinkholes, disappearing streams, and caverns.

lake

A large, inland body of standing water that occupies a depression in the land surface. Streams flowing into the lake carry sediment, which can fill the depression in a relatively short time, geologically speaking. Soon the lake becomes a swamp, and with time the swamp fills with more sediment and vegetation and becomes a meadow or forest with a stream flowing through it. Most lakes exist in places that were covered by glaciers during the latest ice age. The glaciers created lakes in several ways. Flowing ice eroded numerous depressions in the land surface, which then filled with water. The Great Lakes of North America and the Finger Lakes of upper New York State are examples of large lakes occupying glacially scoured depressions. Many modern lakes occupy glacially dammed valleys. Many of the smallest such lakes have already become swamps. In the next few hundred to few thousand years, many of the remaining lakes will fill with mud. Lakes also form by nonglacial means. A volcanic eruption can create a crater that fills with water to form a lake, such as Crater Lake in Oregon. Oxbow lakes form in abandoned river channels. Other lakes, such as Lake Okeechobee in the Florida Everglades, form in flatlands with shallow groundwater. These types of lakes, too, fill with sediment and, as a result, exist for a limited time.

perched aquifer

A local aquifer formed where a layer of impermeable rock or sediment exists above the regional water table and creates a locally saturated zone.

competence

A measure of the largest particles that a stream can transport. A fast-flowing stream can transport cobbles and even boulders in addition to small particles. A slow stream carries only silt and clay.

stream

A moving body of water confined in a channel and flowing downslope; a river is a large stream fed by smaller ones.

turnover

A process, occurring in fall and spring in temperate climates, in which a lake's surface water changes temperature in response to seasonal weather changes and convection mixes the water to equalize temperature throughout the lake. In the winter, ice floats on the surface and temperature layering develops again. In spring, as ice melts on the lake, the cold surface water again is denser than deep lake waters, and spring turnover occurs. As summer comes, the lake again develops thermal layering. Turnover in temperate lakes illustrates an important Earth systems interaction among the atmosphere, the hydrosphere, and the biosphere. During summer and winter when the lake water is layered, bottom-dwelling organisms may use up most or all of the oxygen in deep water. At the same time, surface organisms may deplete surface waters of dissolved nutrients. However, surface water is rich in oxygen because it is in contact with the atmosphere, and deep water may be rich in nutrients because it is in contact with bottom sediment. Turnover enriches deep water with oxygen and, at the same time, supplies nutrients to the surface water. The latter effect often becomes evident in the form of an algal bloom—a sudden and obvious increase in the amount of floating green algae on a lake's surface—in spring and fall.

eutrophic lake

A relatively shallow lake characterized by abundant nutrients, thus sustaining multiple living organisms. utrophication occurs naturally as part of the life cycle of a lake. However, the addition of nutrients in the form of sewage, fertilizer runoff, and other kinds of pollution has greatly accelerated the eutrophication of many lakes.

spring

A seep or flow of groundwater onto the surface; commonly occurs where the water table intersects the land surface.

meanders

A series of twisting curves or loops in the course of a stream. As a stream flows into a meander bend, the water pushes against the outside bank of the meander, much like your body leans against the inside of our car while going around a sharp curve at highway speed. As water is pushed against the outside bank, it causes the bank to erode. The water also causes the deepest part of the channel to form adjacent to the outside bank, creating an asymmetric bottom profile from bank to bank.

hot springs

A spring formed where hot groundwater flows to the surface.

Sinuosity of a Stream Channel

A steep mountain stream usually downcuts rapidly compared with the rate of lateral erosion. As a result, it cuts a relatively straight channel with a steep-sided, V-shaped valley. The stream maintains its relatively straight path because it flows with enough energy to erode and transport any material that slumps into its channel.

intermittent streams

A stream that does not maintain some flow all the time and may be dry for long periods.

tributary

A stream that feeds water into another stream or river.

braided streams

A stream that flows in many shallow, interconnecting channels that are usually separated by emergent sediment bars; formed because the stream's capacity has been exceeded by its sediment supply. A braided stream forms where the supply of sediment exceeds the stream's capacity; that is, more sediment is available than the stream can carry. The excess sediment accumulates in the channel, forming a series of sediment bars. During low water, the stream flows simultaneously in multiple channels that pass around the bars. During floods, the bars may be completely submerged, and the higher discharge may cause the bars to migrate downstream a short distance, while changing shape in the process. As a result, braided streams often look different from year to year. Unlike mud-dominated meandering streams, braided streams are dominated by their coarser-grained bed load. As a result, braided streams usually are found in regions with higher slope gradients where coarse-grained sediment is generated through erosion. Braided streams also are common in both desert and glacial environments because both produce abundant coarse-grained sediment. A desert yields large amounts of sandy sediment because it has little or no vegetation to prevent erosion. Glaciers grind bedrock into sediment, which is carried by streams flowing from the melting ice. Unlike clay-rich banks of meandering streams, the banks of braided streams usually are made of coarser-grained sand and gravel particles that don't stick together, unlike clay-sized particles, but rather collapse at slopes steeper than the sediment's angle of repose. As a result, the relatively coarse-grained sediment forming the banks of braided rivers quickly is eroded from the bank into the channel, contributing to the high bedload characteristic of braided streams.

perennial streams

A stream that maintains some flow even during dry seasons.

graded stream

A stream with a smooth, concave-upward profile, in equilibrium with its sediment supply; it transports all the sediment supplied to it, with neither erosion nor deposition in the streambed. An idealized graded stream such as this one does not actually exist in nature, but many streams come close.

zone of saturation

A subsurface zone below the water table in which all porosity within soil and bedrock is filled with water.

zone of aeration

A subsurface zone located between the ground surface and the water table and in which the pores are mostly filled with air; also called the unsaturated zone.

drainage divide

A topographic high separating drainage basins. The most famous drainage divide in North America is the Continental Divide. Water runoff east of the Continental Divide drains into stream tributaries that ultimately lead to rivers flowing into the Atlantic Ocean, whereas runoff west of the Continental Divide eventually reaches the Pacific Ocean. Other important North American drainage divides separate the Columbia and Colorado River drainage basins west of the Continental Divide and the Rio Grande from the Mississippi River drainage basins east of the Continental Divide. Collectively these four drainage basins cover three-fourths of the lower 48 United States.

geysers

A type of hot spring that periodically erupts with violent jets of hot water and steam; eruptions occur when groundwater recharging a geyser's subsurface plumbing system becomes superheated and forces a small volume of water out of the geyser's vent. The release of this water lowers pressure at deeper levels, causing water there also to flash to steam and initiating the main eruption.

artificial levee

A wall built along the banks of a stream to prevent rising floodwater from spilling out of the channel onto the floodplain. Unfortunately, artificial levees create conditions that may increase both flood intensity and property damage. One factor is entirely human—the protection promised by levees encourages people to build in the floodplain. In the absence of a levee, people might decide to build on high ground that is safe from floods. But when levees are built, people are more likely to construct homes and businesses in harm's way. Levees also can cause higher floods along nearby reaches of a river. By restricting the channel, artificial levees form a partial dam that raises the water level and increases the risk of flooding upstream. In addition, because the stream cannot overflow levees during small floods, sediment that normally would be deposited on the floodplain is deposited within the channel, raising the level of the streambed.

artesian well

A well drilled into a confined aquifer, in which the water rises without pumping and in some cases flows to the surface.

The Water Cycle

About 1.3 billion cubic kilometers of water exist at Earth's surface. Of this huge volume, 97.5 percent is salty seawater, and another 1.8 percent is frozen into the great ice caps of Antarctica and Greenland. Although the hydrosphere contains a great volume of water, only 0.64 percent is fresh and available in streams and rivers, lakes, wetlands, and groundwater. Water evaporates from the continents and oceans to form water vapor in the atmosphere. This vapor eventually condenses and falls back to the surface as rain or snow. Most precipitation lands on the ocean, partly because it covers most of the planet. The precipitation that falls on the continents follows four paths

Stream Erosion and Mountains: How Landscapes Evolve

According to a model popular in the first half of the 20th century, streams erode Earth's surface and create landforms in an orderly sequence. At first, they cut steep, V-shaped valleys into mountains. Over time, the streams erode the mountains away and widen the valleys into broad floodplains. Eventually, the entire landscape becomes a large, featureless plain. However, if this were the only mechanism affecting Earth's surface during its 4.6-billion-year history, all landforms would have eroded to a flat plain. Streams do continuously erode the landscape, wearing down mountains and widening floodplains. But at the same time, tectonic activity may uplift the land and interrupt this simple, idealized sequence. In this way, Earth's hydrosphere and atmosphere work together with tectonic processes in the geosphere to create landforms. Geomorphology, from the Greek words for "earth forms," is a branch of geology that deals with the evolution of landforms through time.

stalactite

An icicle-like dripstone of precipitated calcite that hangs from the ceiling of a cavern.

confined aquifer

An inclined aquifer sandwiched between layers of impermeable rock; typically, the water in the lower part of the aquifer is under pressure from the weight of water above.

cavern

An underground cavity or series of chambers created when groundwater dissolves large volumes of rock, usually limestone; also called a cave. Although caverns form when limestone dissolves, most caverns also contain features formed by deposition of calcite.

Floodplain Management

As we have shown, in many cases attempts at controlling floods either do not work or they shift the problem to a different time or place. An alternative approach to flood control is to abandon some flood-control projects and let the river spill out onto its floodplain. Of course, the question is what land should be allowed to flood. Every farmer and homeowner living on the floodplain wants to maintain the levees that protect it. Currently, federal and state governments are establishing wildlife reserves in some floodplains. Because no development is allowed in these reserves, they will be permitted to flood during the next high water. However, a complete river management plan involves complex political and economic considerations.

Flood Control

Because deltas provide natural access to transportation waterways, contain level and fertile soil for agriculture and generally contain abundant wild game, deltas have long been desirable places for humans to live. However, flooding is a natural part of delta evolution, and all deltas undergo flood when sufficient discharge is supplied from the river that feeds it. As a result, humans have developed several strategies for controlling floods so that, despite their inevitable occurrence, the damage floods cause can be minimized. Flood control, while providing relief from flood waters for some, may exacerbate flood related damage for others.

distributary channels

Channels that split from the main stream feeding a delta or alluvial fan and spread out across its surface, depositing sediment in the process. Unlike tributary channels that merge in a downstream direction, distributary channels split in a downstream direction. Distributary channels on a delta carry sediment beyond the shoreline, where it accumulates in mouth bars that form where the distributary channels end. As a mouth bar grows in size, flow from the distributary must diverge to get around it. At the same time, sediment is deposited on the bed of the distributary channel, causing it to become less efficient at transporting water and sediment. Ultimately these inefficiencies become so great that the distributary channel is abandoned and the water and sediment flows down a new channel established on a different part of the delta. Although sediment is deposited on the delta top by flooding rivers, much sediment also is deposited beyond the mouths of the distributary channels, on the delta front and prodelta.

transpiration

Direct evaporation of water into the atmosphere from the leaf surfaces of plants.

downcutting

Downward erosion by a stream into its bed, usually by cutting a V-shaped valley along a relatively straight path.

geothermal energy

Energy extracted from Earth's heat.

Channel Characteristics

Features describing the shape and roughness of a stream channel. Friction between flowing water and the stream channel slows current velocity. Consequently, water flows more slowly near the banks than near the center of a stream. If you paddle a canoe down a straight stream channel, you move faster when you stay away from the banks. The amount of friction depends on the roughness and shape of the channel. Boulders on the banks or in the streambed increase friction and slow a stream down, whereas the water flows more rapidly if the bed and banks are smooth. Likewise, a stream that is deep and narrow will flow faster than a broad, shallow stream with the same discharge, and a stream that is straight flows faster than a stream with lots of curves.

Temperature Layering and Turnover in Lakes

If you have ever dived into a deep lake on a summer day, you have probably discovered that the top meter or so of lake water can be much warmer than deeper water. This occurs because sunshine warms the upper layer of water, making it less dense than the cooler, deeper water. The warm, less dense water floats on the cooler, denser water.

Flood Control, the Mississippi River Delta, and Hurricane Katrina

In August 2005, Hurricane Katrina drove storm waters over the levees protecting New Orleans, flooding the city, chasing 1.3 million people from their homes, and causing approximately $200 billion in damages. This problem was exacerbated by decades of flood-control practices along the Mississippi. In a natural system, a delta builds upward and extends seaward by sediment deposition, although the front of the delta is eroded by ocean waves and currents. In the past 50 years, deposition rates in many coastal deltas have decreased significantly, in part because dams built upstream have trapped sediment that otherwise would have reached the delta. The construction of levees along distributary channels on the delta also has caused sediment to bypass the delta rather than be deposited on it during floods. Contributing to coastal erosion is the urban and agricultural destruction of natural plant communities that normally hold the soil together and global sea level rise that causes the wave energy to reach further inland. Other problems affecting the delta include the rising global sea level and the sinking, or subsidence, of the land surface as the delta sediments compact and expel water from their pore spaces. As a result of all these processes, the Mississippi delta has shrunk. Between 1930 and 2012, nearly 5,000 square kilometers of the delta sank below sea level. One recent study estimated that by the year 2100, an additional 10,000 to 13,500 square kilometers will be submerged—an area roughly the size of Connecticut. The flooding of New Orleans in 2005 is an example of both a complex systems interaction and a threshold event. While Hurricane Katrina triggered the flood, human interference and development within a complex natural river/delta system set the stage. Over the previous decades, the global sea level rose while the delta subsided and slowly eroded. Natural buffers were removed, with no immediate ill effects. Then a catastrophic storm pushed waters inland. The levees failed, causing a huge human disaster. Although it is easy to point to the hurricane as the cause of the 2005 disaster, the other factors that combined to make this such a devastating event were known well in advance. In 1995, the senior authors of this textbook wrote: "If current rates of erosion continue, the sea will ... flood New Orleans ... causing severe economic losses." Tragically, this dire prediction was accurate.

Nutrient Balance in Lakes

In a deep lake, sunlight is available near the surface, but nutrients are abundant mainly on the bottom. Plankton (small, free-floating organisms) grow poorly on the surface due to low nutrient levels there, and bottom-rooted plants cannot grow due to lack of sunlight there. Thus, deep lakes contain low concentrations of nutrients and insufficient bottom sunlight to sustain aquatic food webs that generate much biomass. The low level of biological productivity often causes these lakes to have a deep-blue color that we associate with a clean, healthy lake. Such a lake is called oligotrophic, meaning "poorly nourished."

Groundwater

In most places, if you drill a hole in the ground between a few meters and 100 or so meters deep, the bottom of the hole fills with water, usually within a few minutes to a few days. The water appears even if no rain falls and no streams flow nearby. The water that seeps into the hole is groundwater, which saturates Earth's crust from a few meters to a few kilometers below the surface. Groundwater is exploited by digging wells and pumping the water to the surface. It provides drinking water for more than half of the population of North America and is a major source of water for irrigation and industry. However, deep wells and high-speed pumps now extract groundwater more rapidly than natural processes can replace it in many parts of the central and western United States. As a result, groundwater resources that have accumulated over thousands or tens of thousands of years are being permanently lost. In addition, industrial, agricultural, and domestic contaminants seep into groundwater in many parts of the world. Such pollution is often difficult to detect and expensive to clean up.

Lakes

Lakes and lakeshores are attractive places to live and play. Clean, sparkling water, abundant wildlife, beautiful scenery, aquatic recreation, and fresh breezes all come to mind when we think of going to the lake. Despite their great value, however, lakes are fragile and ephemeral.

Groundwater Movement

Nearly all groundwater seeps slowly through bedrock and soil. Groundwater often flows at about 4 centimeters per day (about 15 meters per year), although flow rates may be much faster or slower depending on permeability. Most aquifers are like sponges through which water seeps, rather than underground pools or streams. However, groundwater can flow very rapidly through large fractures in bedrock, and in a few regions underground rivers flow through caverns. In general, the water table mimics its surrounding topography. Groundwater flows from zones where the water table is highest toward areas where it is lowest, and so some groundwater flows roughly parallel the sloping surface of the water table toward the valley. But groundwater also flows from zones of high pressure toward zones of low pressure. Because water pressure is greatest beneath the highest part of the water table, the pressure difference forces much of the groundwater to flow downward beneath the hill, then laterally toward the valley, and finally upward beneath the lowest part of the valley, where a stream flows. This is how groundwater feeds stream flow, and why streams flow even when no rain has fallen for weeks or months.

Wetlands

Regions that are water soaked or flooded for all or part of the year; includes swamps, bogs, marshes, sloughs, mudflats, and floodplains. Some wetlands are wet only during exceptionally wet years and may be dry for several years at a time. Wetland ecosystems vary so greatly that the concept of a wetland defies a simple definition. All wetlands share certain properties, however: the ground is wet for at least part of the time; the soils reflect anaerobic (lacking oxygen) conditions; and the vegetation consists of plants such as cattails, bulrushes, mangroves, and other species adapted to periodic flooding or water saturation. North American wetlands include all stream floodplains, frozen Arctic tundra, warm Louisiana swamps, coastal Florida mangrove swamps, boggy mountain meadows of the Rockies, and the immense swamps of interior Alaska and Canada. Wetlands are among the most biologically productive environments on Earth, are important for degrading pollutants, and serve to mitigate the effects of flooding. Two-thirds of the Atlantic fish and shellfish consumed by humans rely on coastal wetlands for at least part of their life cycles. One-third of the endangered species of both plants and animals in the United States also depend on wetlands for survival. More than 400 of the 800 species of protected migratory birds and one-third of all resident bird species feed, breed, and nest in wetlands. Aquatic organisms consume many pollutants and degrade them to harmless by-products. Because these organisms abound in wetlands, the ecosystems serve as natural sewage treatment systems. Wetlands also mitigate flooding by absorbing excess water that might otherwise overrun towns and farms. Wetlands have been lost or degraded by both humans and natural causes. Many wetlands have been drained, or their sources of water cut off, through the construction of dams or dikes. Others have been filled, logged, or mined, while the introduction of nonnative invasive species or toxic levels of pollution or nutrients continues to cause widespread wetland degradation. Natural wetland loss can occur by subsidence, sea level rise, drought, and erosion by large storms.

alluvium

Sediment deposited by moving water. When alluvium is deposited, new landforms are created. For example, if a steep mountain stream flows onto a flat plain, its gradient and velocity decrease abruptly.

Stream Erosion and Sediment Transport

Streams shape Earth's surface by eroding soil and bedrock. The flowing water carries the eroded sediment downslope. A stream may deposit some of the sediment in the valley down which it flows, while it carries the remainder to a lake or to the sea, where the sediment accumulates. Stream erosion and sediment transport depend on a stream's energy. A rapidly flowing stream has more energy to erode and transport sediment than a slow stream of the same size.

runoff

Surface water that flows to the oceans in streams and rivers. This water may stop temporarily in a lake or wetland, but eventually it evaporates or flows to the oceans.

floodplain

That portion of a river valley adjacent to the channel; it is built upward by sediment deposited during floods and is covered by water during a flood.

permeability

The ability of a solid material such as a rock to transmit water or another fluid through its pore network; depends on the size, shape, and interconnectedness of the pores within the material. Water can flow rapidly through material with high permeability, but flows slowly through a low permeability material. Most materials with high porosity also have high permeability. Sand and sandstone have numerous, relatively large, well-connected pores that allow the water to flow through the material.

lateral erosion

The action of a low-gradient stream as it cuts into and erodes its outer bank while simultaneously depositing sediment onto its inner bank; results in slow lateral migration of the channel and, through time, formation of wide, flat alluvial valleys.

thermocline

The boundary between the upper warm layers and deeper cool layers of water in a lake.

base level

The deepest level to which a stream can erode its bed. The ultimate base level is usually sea level. In addition to ultimate base level, a stream may have a number of local, or temporary, base levels. For example, a stream stops flowing where it enters a lake. It then stops eroding its channel because it has reached a temporary base level. A layer of rock that resists erosion may also establish a temporary local base level because it flattens the stream gradient, causing the stream to slow down and erosion to decrease. The top of a waterfall can be a temporary base level established by resistant rock.

prodelta

The fine-grained, outermost edge of a delta, located offshore beyond the delta front.

bed

The floor of a stream channel.

capacity

The maximum quantity of sediment that a stream can transport at any one time. Thus, a large, fast stream has a greater capacity than a small, slow one. Because the ability of a stream to carry sediment is proportional to both velocity and discharge, most sediment transport occurs during the few days each year when the stream is in flood. Relatively little sediment transport occurs during the remainder of the year. So flooding streams are usually muddy and dark, but the same stream may be clear during low water. After a stream erodes soil or bedrock, it transports the products of weathering downstream in three ways.

delta front

The more steeply sloping, usually submerged, outer edge of a delta beyond the delta top.

avulsion

The process by which a stream channel is abandoned and the water and sediment diverted down a new channel.

porosity

The proportional volume of pores or open space within a material; indicates the maximum possible volume of fluid that could be held within the material. In unlithified sand or gravel, the network of open pore spaces between the individual sediment clasts results in porosity values that usually are close to 40 percent. In contrast, unlithified mud can have porosity values of up to 90 percent. The generally higher porosity of unlithified mud results from several physical and chemical characteristics of very fine-grained sediment. First, the platy shape of many clay crystals and crystal aggregates creates plentiful open space between sediment particles. Second, water is attracted to silt- and clay-sized particles through capillary action. Third, the polar water molecule is attracted to electrically charged surfaces of clay-sized particles. Finally, water is incorporated directly into the crystal structure of some clay minerals. Most rocks have lower porosities than loose sediment. Sandstone and conglomerate commonly have 5 to 30 percent porosity, and in some cases more.

drainage basin

The region that is drained by a single stream.

banks

The rising slopes bordering the sides of a stream channel.

Gradient

The steepness or vertical drop of a stream over a specific distance. A tumbling mountain stream near the continental divide may drop 50 meters or more per kilometer, whereas a river in the upper Great Plains, like the Yellowstone River in eastern Montana, has much lower gradient of about 0.5 meters per kilometer. Considerably lower yet is the gradient of the lower Mississippi River, which has a gradient of 0.01 meters (1 cm) per kilometer.

tributaries

The term river is commonly used for any large stream fed by smaller ones.

water table

The top surface of the zone of saturation; the water table separates this zone from the zone of aeration above.

suspended load

The total mass of a stream's sediment load that is carried within the flow by turbulence and is free from contact with the streambed. Clay and silt are small enough that even the slight turbulence of a slow stream keeps them in suspension. A rapidly flowing stream can carry sand in suspension.

bed load

The total mass of a stream's sediment load that is transported along the bottom or in intermittent contact with the bottom of the streambed. Sand also can move as part of the bed load by rolling or bouncing along the bottom while being carried downstream by the current between bounces.

dissolved load

The total mass of ions dissolved in and carried by a stream at any one time; the ions are derived from chemical weathering. A stream's ability to carry dissolved ions depends mostly on its discharge, its pH, and the geology over which the stream flows. A stream flowing over volcanic bedrock will have a higher dissolved load than the same stream flowing over quartzite bedrock, because the volcanic rock is more chemically reactive than the quartzite. Although dissolved ions are invisible, they comprise more than half of the total load carried by some rivers. More commonly, however, dissolved ions make up less than 20 percent of the total load.

delta top

The upper surface of a delta, including the parts above and below water.

Discharge

The volume of water flowing downstream over a specified period of time, usually measured in units of cubic meters per second (m3/sec) or cubic feet per second (cfs). At a given point in a stream, the velocity of its water increases when its discharge increases. Thus, a stream flows faster during flood, even though its gradient is unchanged. A stream's discharge can change dramatically from month to month or even during a single day.

recharge

To replenish an aquifer by the addition of water.

superheated water

Water that has a temperature above the boiling point but that does not boil because it is under pressure.