Stream Flow
Streamflow is a measure of the volume of water passing a given spot in a given
amount of time. Generally, streamflow is measured in the units cubic feet per
second (cfs). To assess the volume of water in a given section of stream, three
crossections of the stream are taken. Each crosssection consists of 6 depth
measurements, which gives a rough look at the topography of the stream bed.
The distance from the first to last crossection must also be determined to find
the total volume in the stream at that point.
Finally, the average speed of the current must be taken at that spot. To do
this, a light object is placed in the current. The amount of time the object
floats from the 1st crossection to the last helps determine its speed. For greater
accuracy, the object should be floated near the left shore, in the middle, and
near the right shore when viewed downstream so that any variance in current
speed across the creek can be averaged out.
Importance of determining streamflow
The velocity of the water affects many other water quality factors in the
stream. When the velocity is high, the water has the force to erode its bank,
increasing its silt content, and increasing the turbidity (making the water
murky). If there is enough silt in the water, it may drop out on water plants
which lessens the amount of light reaching the plants. The result would be less
photosynthesis, and thus less food production by the producers and less oxygen
production, affecting the quality of life for some aquatic critters.
If the velocity is fast enough, it may create waves or large water falls which
increases the area of interface between the air and water. This helps atmospheric
gases (like oxygen) diffuse into the water faster. Fast water after storms may
initially increase the amount of the pollutants we are looking for. However,
after long rain storms, many of the pollutants have been flushed out, and the
water may actually be cleaner. High, fast water may also contribute to flooding,
creating economic havoc for neighborhoods near the stream.
As water velocity slows, there is a lesser chance of inputting atmospheric
oxygen into the water. However, clarity of the water increases, and more light
should penetrate the water so plants can provide the food web with oxygen and
food. Photosynthesis will increase as a result, but the water temperature may
also increase. Slow water velocity allows some of the water to percolate down
through the stream bed, replenishing the water table.
Slichter