Running Water

World Distribution of Water

        About 71% of the Earth's surface is covered by water. Most of the Earth's water is in the ocean (97.2%). Less than 3% of the water on Earth is located on or beneath the continents, and most of that (2.15%) is in the form of ice. About 0.001% of the Earth's water is in the atmosphere, and only 0.0001% is contained in the world's river systems. Despite the low proportion of water in streams, running water:

1. is the most important erosional agent on Earth,
2. supplies about 8% of the electricity used in North America,
3. serves as the primary source of drinking, industrial and irrigation water, and
4. is used for transportation.

        The Hydrologic Cycle describes the circulation of the Earth's surface fluids. The cycle is driven by the sun's energy and by gravity.

• Sun Energy - Causes evaporation of water from the oceans and transpiration (release of water during photosynthesis) by plants. Processes are generally lumped together (evapotranspiration).
• Wind - Wind is caused by unequal heating of the atmosphere by the sun. Wind carries evaporated water until clouds form.
• Precipitation - Rising clouds will precipitate water as rain or snow. About 80% of precipitation falls back to the ocean. Precipitated water can infiltrate (soak into the ground) or runoff (move over the ground surface). Large amounts of precipitation can be locked in glaciers as snow and ice, or beneath the surface as groundwater. Eventually, water finds its way back to the ocean. On the continents, precipitation is greater than evaporation, but evaporation exceeds precipitation for the oceans.

        Flow behavior is determined by channel smoothness and water velocity, and may be:

Laminar - Water molecules move in straight-line paths parallel to the stream channel. This behavior only occurs at slow velocity as a thin zone near a smooth channel wall.

Turbulent - Most streamflow is turbulent. Water molecules move in erratic, mixing paths at higher velocity and in rough channels.

            Runoff occurs if the amount of precipitation exceeds the infiltration capacity of the soil/rock material. Runoff moves downslope by:

• Sheet Flow - Movement as a continuous film of water moving over the surface and not confined to depressions. This type of movement causes sheet erosion.
• Channel Flow - Runoff is confined to channels (long, trough-like depressions). Channelized flow of any size is called a stream.

        Velocity is expressed as ft/sec or m/sec. It is not uniform across a channel or along stream length. Velocity typically increases downstream despite lower gradient because of larger channel size, greater channel smoothness, and higher discharge. The actual velocity depends on:

1. Channel Shape and Roughness - Velocity is greatest near the upper center part of a straight channel segment. In a curved channel segment (meander), water velocity is greatest at the outside of the bend, slowest at the inside. Semicircular channels have lowest surface area in contact with water and highest water velocities, compared to other shapes. Rough channels produce frictional drag and slow water velocity. More turbulent flow (more friction) is therefore associated with lower velocity. Channel roughness decreases downstream due to decreasing sediment particle size; the channel becomes larger and more semicircular downstream due to erosion.
2. Stream Gradient - The gradient describes the slope of the stream channel. It decreases downstream, being highest at headwaters and lowest at mouth. The longitudinal profile of a stream (cross-sectional view down the length) shows stream gradient, which can be expressed as the vertical drop of a stream channel over a fixed horizontal distance (ft/mile or m/km). Velocity is proportional to gradient (pull of gravity).
3. Discharge - This is the amount of water passing a given point during a specific time interval (ft3/sec or m3/sec). Discharge changes with the seasons as demonstrated by hydrographs (plot of stream discharge versus time). Discharge increases downstream because water is added from tributary (smaller) streams. It is proportional to velocity as shown by:

where

Q = discharge,
V = velocity, and
A = cross-sectional area of the stream channel.

        Streams possess both potential energy and kinetic energy. As water moves downstream, potential energy is converted into kinetic energy. Most of the kinetic energy is released as frictional heat, but a small amount remains to do the work of erosion and transportation.

• Stream Erosion - Streams carry material eroded from their own channel plus eroded material added from the drainage basin. Streams erode by:

1. Hydraulic Action - The force of running water can set particles in motion. Loose particles can be lifted by turbulent flow. The higher water velocity, the larger and greater quantity of particles can be lifted.
2. Abrasion - Solid particles in suspension and in the bed load can scour the channel. Bedrock can be eroded and chipped; sediment particles are smoothed and rounded. Potholes are semicircular holes scoured out in eddy currents by swirling sand and gravel.
3. Solution - Some stream channel material is eroded by rock material being dissolved away. This mechanism is minor compared to the other mechanisms.
• Transportation - Movement of sediment from one place to another can be expressed in terms of capacity and competence. Capacity is the maximum load a stream can carry (the amount of material a stream can carry at any one time), and varies with stream discharge. Competence is the maximum particle size that a stream can move, and is also determined by the water velocity. Streams transport their load in three different ways:

1. Dissolved Load - This is material carried in solution. Most dissolved material comes from groundwater seeping into streams, and is independent of stream velocity.
2. Suspended Load - Sediment particles can be carried in moving water. Suspended load typically composes the largest fraction of material transported by a stream. For a sediment particle to be transported, the water velocity must exceed the settling velocity of the particle. Coarser material is carried toward the channel bottom in the zone of highest water turbulence.
3. Bed Load - This consists of particles too large to be kept in suspension. As the particles move by sliding, rolling, and jumping by short leaps (saltation), they cause downcutting of the stream channel.
• Deposition - Streams ultimately deposit most of the material they carry. Deposition results from decreasing water velocity or from chemical changes. The coarsest material is deposited first as water velocity decreases. Stream deposited material is called alluvium.

        These are streams with channels split into many winding pathways by the deposition of sediment in the channel. They occur where a stream receives a heavy load and has variable discharge.

        Meandering stream have a winding course, which can be described in terms of several important features:
 

1. Meanders - These are curves in the stream channel formed by any obstruction to flow. The channel shape is asymmetric, as meanders migrate out and down a stream valley through erosion/deposition.
2. Cut Banks - These steep channel banks form from erosion at outer edge of meanders.
3. Point Bars - These bars form from sediment that is deposited on the inner edge of meanders where water velocity is slow.
4. Oxbow Lakes - These lakes represent abandoned meander loops. When one meander catches up with another, the narrow neck of land between them can be cutoff to form a new, shorter channel. Where the lake fills with sediment, it is called a meander scar.

        Floods occur when discharge exceeds the capacity of the channel. Long-term study of streams shows that floods of various sizes occur at fairly regular intervals and can be statistically predicted. The largest floods are more infrequent and may occur once in a few hundred years, while small floods may occur every few years. Unfortunately, we can still cannot predict precisely when a flood of a given size will occur. The annual flood damage in the US exceeds $100 million. Flood control measures encourage development of floodplain areas, and development increases the frequency and extent of flooding; as a result, flood damage continues to increase despite control measures. Deposition by floodwaters produces:
 

Floodplains - Two basic types of these wide, level floors on valley bottoms adjacent to stream channels are:

1. Erosional floodplains - These plains are created by lateral stream erosion which produces a floodplain by lateral accretion of successive point bar deposits.
2. Aggradational floodplains - The most common type of floodplain, these are created by stream deposition which builds up the valley floor by vertical accretion of relatively thin layers of fine-grained material deposited by flood waters.

Natural Levees - Levees are gently sloped ramparts paralleling the stream channel. They form as many floods deposit coarser material near the banks and finer material farther away. Because of levees, the level of water in the stream channel can lie above the level of the floodplain; in such cases, the floodplain can contain marshy areas known as back swamps. Natural levees can prevent tributary streams from flowing directly into the larger stream, resulting in a Yazoo stream that flows parallel to the main stream until it finds a break in the levee.

        Deltas develop where a stream enters a standing body of water. The sudden velocity drop causes stream to drop most of its load. This sediment eventually blocks the channel and stream seeks a new route, developing a network of distributaries (smaller channels formed by splitting of main channel). The ideal sequence of sediments laid down by a distributary from bottom to top includes: bottomset beds, foreset beds, and topset beds. Deltaic sediments can be important hosts to fossil fuel deposits (coal, oil and natural gas). There are three major types of deltas:

1. Stream-dominated deltas consist of long, finger-like sand bodies deposited as distributary channels prograde (advance) out to sea. The distributaries create a bird's-foot shaped delta.
2. Wave-dominated deltas have distributary channel sediments eroded by waves and redeposited as barrier islands. The entire delta margin progrades seaward.
3. Tide-dominated deltas develop where tidal currents erode distributary sediments and redeposit them as sand bars that parallel the direction of tidal flow.

        These are lobate deposits of alluvium and some mud flows. They develop where a stream gradient changes abruptly, such as at the foot of a mountain. The sudden velocity drop causes the stream to drop sediment in a fan-shaped deposit.

        A drainage basin is the area drained by a river system (river + tributaries). One drainage basin is separated from surrounding ones by divides (high land surrounding a stream system).

        The networks describe the connection of individual streams and stream valleys in a drainage basin. The order of a stream is determined by the number of tributary streams it has:

1. First Order - No tributaries
2. Second Order - At least two first-order tributaries.
3. Third Order - At least two second order tributaries, etc.

As stream order increases, the number of streams of that order in a given drainage basin decreases, and the size of the drainage basin increases.

        Stream networks display definite drainage patterns or geometric relationships:

1. Dendritic - This tree-like stream network develops when the underlying bedrock has uniform properties.
2. Radial - Streams, that radiate outward from a central zone, develop on newly formed volcanoes and domes.
3. Rectangular - Channels have right angle bends and tributaries join larger streams at right angles. This pattern develops where streams follow joints or faults in resistant rock.
4. Trellis - This pattern develops where short tributary streams join the main stream at right angles, and is caused by bedrock that consists of alternating bands of resistant and nonresistant rock.
5. Deranged - This drainage pattern found in recently glaciated areas involves streams that have irregular flow directions. It generally indicates a youthful drainage system that has not had time to become more organized.

        The base level of a stream is the lowest level to which a stream can erode its channel. It can represent:

Temporary (or Local) base levels - Larger streams or lakes into which a stream flows for the time being.

 

Ultimate base level (generally sea level) - All streams seek to reach sea level, but most never do. Streams never reach ultimate base level along their entire course. A change in base level causes a change in stream activity:

1. Lowering the base level causes stream to downcut its channel.
2. Raising the base level causes stream to deposit sediment and raise its channel.

The graded stream - This is a stream that has reached an equilibrium between erosion and deposition, developing a smooth, concave longitudinal profile. In reality, such conditions only exist locally and for temporary periods. Any change in base level, discharge, flow velocity, channel characteristics or gradient disturbs this equilibrium.

        Stream valleys form through stream erosion and mass wasting processes. Small rills developed by erosion become deeper and wider downstream by several processes:

• Downcutting represents downward erosion of the stream channel. It occurs when the stream energy (capacity) exceeds the stream's load, and causes valleys to be narrow and steep-sided (canyons and gorges).

• Lateral erosion undermines part of the bank or valley wall, leading to mass wasting and widening of the valley. This process is enhanced by sheet and rill erosion of the valley walls, and causes V-shaped valleys (most common type).

• Headward erosion involves lengthening of the valley upstream by erosion of headwater divide areas. It can result in stream piracy (diversion of part of one stream's drainage because of the headward erosion of another stream). Stream piracy may lead to the formation of a wind gap (an abandoned water gap).

• Superposed streams are streams that appear to have cut through a ridge or mountain lying in their path, creating water gaps (steep-walled notch). A water gap forms when streams cut down through overlying strata to expose a buried ridge or mountain.

• Stream terraces are a set of nearly flat surfaces bordering a steep slope along a stream's banks. They represent erosional remnants of an old floodplain surface left as terraces because of renewed stream downcutting. Several sets of terraces represent either successive lowering of the stream's base level or increases in stream discharge.

• Incised Meanders develop when a stream erodes downward to bedrock. Erosion creates deep, meandering canyons with no floodplains if there is little lateral erosion, or natural bridges (a span of rock across a stream valley created by a meander cut-off) if lateral erosion does take place. In order to become incised, meanders must have been established before the stream began to cut through bedrock.

• Waterfalls can form where resistant rock units are underlain by non-resistant rock or where a stream flows over cliffs formed by glaciers or faulting.

• Rapids are typically caused by resistant rock units outcropping in the stream channel.

        Urbanization affects streamflow by decreasing the amount of precipitation that can infiltrate the ground. More precipitation runs off at a faster rate and stream hydrographs are changed. The net result is a stream prone to flash flooding, such that its:

1. Lag time (time between the start of precipitation and the highest stream water level) decreases.
2. Peak discharge increases during precipitation, increasing the chances of flooding.
3. Channel size also increases to handle the peak discharge increase (increases sedimentation downstream).
4. Base flow (amount of water delivered to the stream from groundwater seepage) is decreased because less water infiltrates. A decrease in vegetative cover is similar in its effect on a hydrograph to an increase in urbanization. It can take about 35-50 years for a stream system to return to normal after clear-cutting of trees.