Why does stream discharge vary

The length that you select will be equal to L in solving the flow equation. Twenty feet is a standard length used by many programs. Measure your length and mark the upper and lower end by running a transect line across the stream perpendicular to the shore using the string and stakes Fig. The string should be taut and near the water surface. The upstream transect is Transect 1 and the downstream one is Transect 2. Cross-sectional area A in the formula is the product of stream width multiplied by average water depth.

To calculate the average cross-sectional area for the study stream reach, volunteers should determine the cross-sectional area for each transect, add the results together, and then divide by 2 to determine the average cross-sectional area for the stream reach. Volunteers should time with a stopwatch how long it takes for an orange or some other object to float from the upstream to the downstream transect.

An orange is a good object to use because it has enough buoyancy to float just below the water surface. It is at this position that maximum velocity typically occurs. The volunteer who lets the orange go at the upstream transect should position it so it flows into the fastest current. The clock stops when the orange passes fully under the downstream transect line.

Once under the transect line, the orange can be scooped out of the water with the fishing net. This "time of travel" measurement should be conducted at least three times and the results averaged--the more trials you do, the more accurate your results will be. Drainage basins with steep sides tend to have shorter lag times than shallower basins.

This is because water flows more quickly on the steep slopes down to the river. Basins that have many streams high drainage density drain more quickly so have a shorter lag time. If the drainage basin is already saturated then surface runoff increases due to the reduction in infiltration. Rainwater enters the river quicker, reducing lag times, as surface runoff is faster than baseflow or through flow.

If a drainage basin has a significant amount of vegetation this will have a significant effect on a storm hydrograph. Vegetation intercepts precipitation and slows the movement of water into river channels.

Water is also lost due to evaporation and transpiration from the vegetation. This reduces the peak discharge of a river.

The amount of precipitation can have an effect on the storm hydrograph. Heavy storms result in more water entering the drainage basin which results in a higher discharge. The type of precipitation can also have an impact. The lag time is likely to be greater if the precipitation is snow rather than rain. This is because snow takes time to melt before the water enters the river channel. When there is rapid melting of snow the peak discharge could be high.

Drainage systems that have been created by humans lead to a short lag time and high peak discharge as water cannot evaporate or infiltrate into the soil. Areas that have been urbanised result in an increase in the use of impermeable building materials. This means infiltration levels decrease and surface runoff increases. 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. 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,.

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 is defined as the limiting level below which a stream cannot erode its channel. 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. 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.

In general, slow downcutting and weak, easily erodable rocks results in valleys and rapid downcutting in stronger rocks results in canyons. 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. 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 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. 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 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. In temporary diversion of the water that flows over the American Falls section revealed huge blocks of rock. 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. 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.

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.

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. In laminar flow, suspended particles will slowly settle to the bed. Hjulstrom's Diagram plots two curves representing 1 the minimum stream velocity required to erode sediments of varying sizes from the stream bed, and 2 the minimum velocity required to continue to transport sediments of varying sizes.

Notice that for coarser sediments sand and gravel it takes just a little higher velocity to initially erode particles than it takes to continue to transport them. For small particles clay and silt considerably higer velocities are required for erosion than for transportation because these finer particles have cohesion resulting from electrostatic attractions. Think of how sticky wet mud is. Stream competence refers to the heaviest particles a stream can carry. Stream competence depends on stream velocity as shown on the Hjulstrom diagram above.

The faster the current, the heavier the particle that can be transported. Stream capacity is the maximum amount of solid load bed and suspended a stream can carry. It depends on both the discharge and the velocity since velocity affects the competence and therefore the range of particle sizes that may be transported.

As stream velocity and discharge increase so do competence and capacity. But it is not a linear relationship e. Competence varies as approximately the sixth power of velocity. For example, doubling the velocity results in a 64 times increase in the competence. Capacity varies as the discharge squared or cubed. So tripling the discharge results in a 9 to 27 times increase in the capacity.

Therefore, most of the work of streams is accomplished during floods when stream velocity and discharge and therefore competence and capacity are many times their level during low flow regimes. This work is in the form of bed scouring erosion , sediment transport bed and suspended loads , and sediment deposition. Gaining effluent streams receive water from the groundwater. 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.