Stream flow or “Discharge” is measured monthly as part of the water quality monitoring at four sampling locations on Carpenter Creek by the Stillwaters Volunteer monitoring crew. Discharge is the volume of water that passes any location in a period of time. It is usually measured in cubic feet per second, or “cfs”. One cfs is equal to 448.83 gallons per minute. That’s a fair amount of water.
The speed and amount of the water flowing in a stream segment determines what aquatic animals and plants can live in that part of the stream. Many aquatic organisms, like fish eggs and aquatic insects in early life stages, need the oxygen that fast moving water brings. Other organisms need cover and plants to use as protection, and do better in deeper, slower moving water. The speed of the water also affects how stable the sides of the stream will be. During a fast moving water event – big storms, excessive stormwater runoff entering a stream – the channel can change significantly and affect the flow and discharge at that location forever.
In order to accurately measure stream flow, the stream width, depth and water velocity are measured at many points across the stream. Stillwaters established specific “transects” – cross-sections of the stream bed – when the sampling sites were established (see map). They are marked with iron bars so that the exact same transect is measured every month. Stream width and depths along the stream vary over time. Wind, large flows of water, and woody materials or rocks can scour the bottom and sides, dramatically changing a transect. We have seen that at Site No.3.
The ten measurements at each transect are made with a flow metering device which consists of a sliding rod that is measures the depth of the water, a propeller on the end of the rod to measure the speed of the flow, and a meter that measures the velocity of the water. A tape measure is used to measure the width of the channel, and the interval between each of the ten measurements. The meter averages the velocity of the propeller over a short period of time. These measurements are put into a formula to calculate the discharge across the whole cross-section of the stream.
January and March have been the months with the highest discharge (up to 7.78 cfs or 3401.9 gallons per minute in January 2008 at site No.1). In the last 8 years, Site No.1 typically has the greatest discharge of all sites measured. Site No.4 typically has the lowest discharge. July through September are the months of lowest flow. October typically exhibits a significant shift in flow rate in most years.
The four monitoring sites have different flow characteristics because they are on different parts of the watershed. Some are draining more area; others are loosing water to their surroundings. Our continued observations of these streams will help us know the health of Carpenter Creek.
