Relationship between stream depth and velocity

Stream Processes

relationship between stream depth and velocity

As discharge increases, generally width, depth, and velocity of the stream between bed load and suspended load as the velocity of the stream changes. .. The time difference between when heavy precipitation occurs and. A stream is any flow of water, and water flows from higher elevation to lower elevation We examined the relationship between discharge, velocity, and depth. Then, the velocity of the stream is measured using a Flow Rate Sensor. The discharge can surrounding terrain, the depth of the stream, the width of the stream.

Higher velocity currents in the stream can carry larger and denser particles. Bed Load - coarser and denser particles that remain on the bed of the stream most of the time but move by a process of saltation jumping as a result of collisions between particles, and turbulent eddies.

Note that sediment can move between bed load and suspended load as the velocity of the stream changes. Dissolved Load - ions that have been introduced into the water by chemical weathering of rocks. This load is invisible because the ions are dissolved in the water. These ions are eventually carried to the oceans and give the oceans their salty character. Streams that have a deep underground source generally have higher dissolved load than those whose source is on the Earth's surface.

The maximum size of particles that can be carried as suspended load by the stream is called stream competence.

The maximum load carried by the stream is called stream capacity. Both competence and capacity increase with increasing discharge.

At high discharge boulder and cobble size material can move with the stream and are therefore transported. At low discharge the larger fragments become stranded and only the smaller, sand, silt, and clay sized fragments move.

When flow velocity decreases the competence is reduced and sediment drops out. Sediment grain sizes are sorted by the water.

Sands are removed from gravels; muds from both. Gravels settle in channels. Sands drop out in near channel environments. Silts and clays drape floodplains away from channels. Changes Downstream As one moves along a stream in the downstream direction: Discharge increases, as noted above, because water is added to the stream from tributary streams and groundwater. As discharge increases, the width, depth, and average velocity of the stream increase.

The gradient of the stream, however, will decrease. It may seem to be counter to your observations that velocity increases in the downstream direction, since when one observes a mountain stream near the headwaters where the gradient is high, it appears to have a higher velocity than a stream flowing along a gentle gradient.

But, the water in the mountain stream is likely flowing in a turbulent manner, due to the large boulders and cobbles which make up the streambed. If the flow is turbulent, then it takes longer for the water to travel the same linear distance, and thus the average velocity is lower.

Also as one moves in the downstream direction, The size of particles that make up the bed load of the stream tends to decrease. Even though the velocity of the stream increases downstream, the bed load particle size decreases mainly because the larger particles are left in the bed load at higher elevations and abrasion of particles tends to reduce their size. The composition of the particles in the bed load tends to change along the stream as different bedrock is eroded and added to the stream's load.

Long Profile A plot of elevation versus distance. Usually shows a steep gradient or slope, near the source of the stream and a gentle gradient as the stream approaches its mouth. The long profile is concave upward, as shown by the graph below. Base Level Base level is defined as the limiting level below which a stream cannot erode its channel.

relationship between stream depth and velocity

For streams that empty into the oceans, base level is sea level. Local base levels can occur where the stream meets a resistant body of rock, where a natural or artificial dam impedes further channel erosion, or where the stream empties into a lake.

When a natural or artificial dam impedes stream flow, the stream adjusts to the new base level by adjusting its long profile. In the example here, the long profile above and below the dam are adjusted. Erosion takes place downstream from the dam especially if it is a natural dam and water can flow over the top. Just upstream from the dam the velocity of the stream is lowered so that deposition of sediment occurs causing the gradient to become lower. The dam essentially become the new base level for the part of the stream upstream from the dam.

In general, if base level is lowered, the stream cuts downward into its channel and erosion is accelerated. If base level is raised, the stream deposits sediment and readjusts its profile to the new base level.

Valleys and Canyons Land far above base level is subject to downcutting by the stream. Rapid downcutting creates an eroded trough which can become either a valley or canyon. A valley has gently sloping sidewalls that show a V-shape in cross-section. A Canyon has steep sidewalls that form cliffs. Whether or valley or canyon is formed depends on the rater of erosion and strength of the rocks. Because geologic processes stack strong and weak rocks, such stratigraphic variation often yields a stair step profile of the canyon walls, as seen in the Grand Canyon.

relationship between stream depth and velocity

Strong rocks yield vertical cliffs, whereas weak rocks produce more gently sloped canyon walls. Active downcutting flushes sediment out of channels. Only after the sediment is flushed our can further downcutting occur. Valleys store sediment when base level is raised.

Rapids Rapids are turbulent water with a rough surface. Rapids occur where the stream gradient suddenly increases, where the stream flows over large clasts in the bed of the stream, or where there is an abrupt narrowing of the channel. Sudden change in gradient may occur where an active fault crosses the stream channel.

ConcepTest: Relationship Between Stream Discharge, Depth and Velocity

Large clasts may be transported into the stream by a tributary stream resulting in rapids where the two streams join. Abrupt narrowing of the stream may occur if the stream encounters strong rock that is not easily subject to erosion. Waterfalls Waterfalls are temporary base levels caused by strong erosion resistant rocks. Upon reaching the strong rock, the stream then cascades or free falls down the steep slope to form a waterfalls.

Because the rate of flow increases on this rapid change in gradient, erosion occurs at the base of the waterfall where a plunge pool forms. This can initiate rapid erosion at the base, resulting in undercutting of the cliff that caused the waterfall.

When undercutting occurs, the cliff becomes subject to rockfalls or slides. This results in the waterfall retreating upstream and the stream eventually eroding through the cliff to remove the waterfall. Niagara Falls in upstate New York is a good example. Lake Erie drops 55 m flowing toward Lake Ontario. A dolostone caprock is resistant and the underlying shale erodes. Blocks of unsupported dolostone collapse and fall. Niagara Falls continuously erodes south toward Lake Erie.

relationship between stream depth and velocity

The rate of southward retreat of Niagara Falls is presently 0. Eventually the falls will reach Lake Erie, and when that happens Lake Erie will drain.

Channel Patterns Straight Channels - Straight stream channels are rare. Where they do occur, the channel is usually controlled by a linear zone of weakness in the underlying rock, like a fault or joint system.

Factors Affecting a River's Velocity | Sciencing

Even in straight channel segments water flows in a sinuous fashion, with the deepest part of the channel changing from near one bank to near the other.

Velocity is highest in the zone overlying the deepest part of the stream. In these areas, sediment is transported readily resulting in pools. Where the velocity of the stream is low, sediment is deposited to form bars.

The bank closest to the zone of highest velocity is usually eroded and results in a cutbank. Meandering Channels - Because of the velocity structure of a stream, and especially in streams flowing over low gradients with easily eroded banks, straight channels will eventually erode into meandering channels.

Erosion will take place on the outer parts of the meander bends where the velocity of the stream is highest. Sediment deposition will occur along the inner meander bends where the velocity is low. Such deposition of sediment results in exposed bars, called point bars. Because meandering streams are continually eroding on the outer meander bends and depositing sediment along the inner meander bends, meandering stream channels tend to migrate back and forth across their flood plain.

If erosion on the outside meander bends continues to take place, eventually a meander bend can become cut off from the rest of the stream. When this occurs, the cutoff meander bend, because it is still a depression, will collect water and form a type of lake called an oxbow lake.

Braided Channels - In streams having highly variable discharge and easily eroded banks, sediment gets deposited to form bars and islands that are exposed during periods of low discharge. In such a stream the water flows in a braided pattern around the islands and bars, dividing and reuniting as it flows downstream. Such a channel is termed a braided channel. During periods of high discharge, the entire stream channel may contain water and the islands are covered to become submerged bars.

During such high discharge, some of the islands could erode, but the sediment would be re-deposited as the discharge decreases, forming new islands or submerged bars. Islands may become resistant to erosion if they become inhabited by vegetation Stream Deposits Sudden changes in velocity can result in deposition by streams. Within a stream we have seen that the velocity varies with position, and, if sediment gets moved to the lower velocity part of the stream the sediment will come out of suspension and be deposited.

Other sudden changes in velocity that affect the whole stream can also occur. For example if the discharge is suddenly increased, as it might be during a flood, the stream will overtop its banks and flow onto the floodplain where the velocity will then suddenly decrease.

This results in deposition of such features as levees and floodplains. If the gradient of the stream suddenly changes by emptying into a flat-floored basin, an ocean basin, or a lake, the velocity of the stream will suddenly decrease resulting in deposition of sediment that can no longer be transported.

This can result in deposition of such features as alluvial fans and deltas. Floodplains and Levees - As a stream overtops its banks during a flood, the velocity of the flood will first be high, but will suddenly decrease as the water flows out over the gentle gradient of the floodplain. Because of the sudden decrease in velocity, the coarser grained suspended sediment will be deposited along the riverbank, eventually building up a natural levee.

Natural levees provide some protection from flooding because with each flood the levee is built higher and therefore discharge must be higher for the next flood to occur. Note that the levees we see along the Mississippi River here in New Orleans are not natural levees, but man made levees, built to protect the floodplain from floods.

Still, the natural levees do form the high ground as evidenced by the flooding that occurred as a result of levee breaches during Hurricane Katrina. Terraces - Terraces are exposed former floodplain deposits that result when the stream begins down cutting into its flood plain this is usually caused by regional uplift or by lowering the regional base level, such as a drop in sea level.

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Alluvial Fans - When a steep mountain stream enters a flat valley, there is a sudden decrease in gradient and velocity. Sediment transported in the stream will suddenly become deposited along the valley walls in an alluvial fan. As the velocity of the mountain stream slows it becomes choked with sediment and breaks up into numerous distributary channels.

Deltas - When a stream enters a standing body of water such as a lake or ocean, again there is a sudden decrease in velocity and the stream deposits its sediment in a deposit called a delta. Deltas build outward from the coastline, but will only survive if the ocean currents are not strong enough to remove the sediment.

As the velocity of a stream decreases on entering the delta, the stream becomes choked with sediment and conditions become favorable to those of a braided stream channel, but instead of braiding, the stream breaks into many smaller streams called distributary streams. Over the last 1, years, most of the land that makes up southern Louisiana has been built by the Mississippi River depositing sediment to form delta lobes.

These delta lobes have shifted back and forth through time as the River's course changed in response to changes in sea level and the River trying to maintain the shortest and steepest path to the Gulf of Mexico see figure Uplift sets a new base level which causes streams to cut deeper, resulting in widening of valleys and erosion of hills.

If these erosional processes were to continue, the landscape would be eroded to base level. Stream Piracy Stream piracy is where one stream erodes headward to capture the drainage of another stream. The stream with more vigorous erosion steeper gradientintercepts another stream and water from the captured stream no flows into the pirating stream see figure Drainage Reversal Drainage reversals can occur as a result of tectonic processes.

For example, in the early Mesozoic when Africa and South America were part of the same continent, South America drained westward. On the west coast, subduction began and the resulting compression caused the uplift of the Andes mountains. As the uplift occurred, the drainage had to reverse to flow to the east into the Atlantic Ocean see figure Superposed and Antecedent Streams In looking at the landscape, it is often evident that streams sometimes cut through deformed terrain seemingly ignoring the geologic structures and hardness of the rock.

If a stream initially develops on younger flat strata made of soft material and then cuts downward into the underlying deformed strata while maintaining the course developed in the younger strata, it is referred to as a superposed stream, because the stream pattern was superposed on the underlying rocks. In such cases much of the original soft strata is removed. If tectonic uplift raises the ground beneath established streams and if erosion keeps pace with uplift, the stream will cut downward and maintain its original course.

In such a case, the stream is called an antecedent stream, because the stream was present before the uplift occurred. Some antecedent streams have incised meanders.

ConcepTest: Relationship Between Stream Discharge, Depth and Velocity

The meanders initially develop on a gentle gradient then uplift raises the landscape dropping the base level and the meanders cut downward into the uplifted landscape see figure Floods Floods occur when the discharge of the stream becomes too high to be accommodated in the normal stream channel. When the discharge becomes too high, the stream widens its channel by overtopping its banks and flooding the low-lying areas surrounding the stream. The areas that become flooded are called floodplains. Floodwaters are devastating to people and property.

During a flood discharge exceeds the storage volume of the stream channel. Velocity thus, competence and capacity increase and water leaves the channel and flows onto adjacent land. Water slows away from the thalweg, dropping sediment.

Causes of Flooding Heavy rains dump large volumes of water on the landscape increasing the amount of water flowing into the stream. If the soil has become saturated as a result of rain so that there is no room in the soil for water to infiltrate, the water instead will run into stream channels and increase the discharge. In the winter, if a sudden increase in temperature rapidly melts snow causing an influx of water into the drainage system.

When a natural or artificial dam breaks or levee breaks, releasing water into a channel with a sudden increase in discharge or releases water from the channel onto the surrounding floodplain.

Flood Stage The term stage refers to the height of a river or any other body of water above a locally defined elevation. This locally defined elevation is a reference level, often referred to as datum.

For example, for the lower part of the Mississippi River, reference level or datum, is sea level 0 feet. Currently the Mississippi River is at a stage of about Other river systems have a reference level that is not sea level.

Most rivers in the United States have gaging stations where measurements are continually made of the river's stage and discharge. These are plotted on a graph called a hydrograph, which shows the stage or discharge of the river, as measured at the gaging station, versus time. When the discharge of a river increases, the channel may become completely full. Any discharge above this level will result in the river overflowing its banks and causing a flood.

The stage at which the river will overflow its banks is called bankfull stage or flood stage. For example, the graph below is a hydrograph of the Mississippi River at St.

Louis, Missouri during the time period of the flood. In other words, a gaining stream discharges water from the water table. On the other hand losing influent streams lie above the water table e. Gaining streams are perennial streams: Losing streams are typically ephemeral streams: Some streams are gaining part of the year and losing part of the year or just in particular years, as the water table drops during an extended dry season.

Streams have two sources of water: Flood Erosion and Deposition: As flood waters rise, the slope of the stream as it flows to its base level e. Also, as stream depth increases, the hydraulic radius increases thereby making the stream more free flowing. Both of these factors lead to an increase in stream velocity.

The increased velocity and the increased cross-sectional area mean that discharge increases. As discharge and velocity increase so do the stream's competence and capacity. In the rising stages of a flood much sediment is dumped into streams by overland flow and gully wash. This can result in some aggradation or building up of sediments on the stream bed. However, after the flood peaks less sediment is carried and a great deal of bed scouring erosion occurs.

As the flood subsides and competence and capacity decline sediments are deposited and the stream bed aggrades again. Even though the stream bed may return to somewhat like its pre-flood state, huge quantities of sediments have been transported downstream.

Much fine sediment has probably been deposited on the flood plain. Stream Patterns Meandering Streams: At a bend in a stream the water's momentum carries the mass of the water against the outer bank.

Water piles up on the outer bank making it a little deeper and the inner bank a little shallower. The greater depth on the outer side of the bend also leads to higher velocity at the outer bank. The greater velocity combined with the greater inertial force on the outer bank erodes a deepr channel.

The deeper channel reinforces the velocity increase. The inner bank remains shallower, increasing friction, thereby reducing the velocity. Where the depth and velocity of the water on the outer bank increase so do the competence and capacity. Erosion occurs on the outer bank or cut bank. Where velocity of the water on the inner bank decreases so do the competence and capacity. Deposition occurs, leading to the formation of a point bar.

Over time, the position of the stream changes as the bend migrates in the direction of the cut bank. As oxbow bends accentuate and migrate, two bends can erode together forming a cutoff and leaving an oxbow lake. Considering the longitudinal downstream profile of a stream: Where a stream flows down a steep slope velocity will increase which will result in increased erosion. Where that stream then flows onto a gentler slope velocity decreases and deposition will result.

This process will reduce the slope of steep stretches and increase the slope of flatter stretches resulting in a more even slope through the course of the stream. The ideal graded profile of a stream is concave upward: The reason for this is that in the upper reaches of a stream its discharge is smaller. As streams merge with other streams their discharge increases, their cross-sectional area increases, and their hydraulic radius increases.

As one goes downstream and the stream grows in size the waters flow more freely. In the upper reaches, a small stream must be steeper to transport its sediments. The extra gravitational energy on the steeper slope is needed to overcome the frictional forces in the shallow stream. If the slope is too gentle and velocity is too slow to transport the sediments being supplied by weathering and erosion, the sediments will pile up.

This increases the gradient which causes the water to flow faster which increases erosion and transport, which then reduces the gradient.