the lower fox river watershed monitoring program    
University of Wisconsin Green Bay
University of Wisconsin Milwaukee

Streamflow

Streamflow, or discharge, is the volume of water in the stream flowing past a given point within a specific period of time. For the school-based monitoring program, streamflow measurements are recorded in cubic feet per second (cfs) for ease of comparison to the data reported by USGS.

Streamflow is calculated by multiplying the average width by the average depth by the average velocity by a bottom factor. To determine streamflow, a stream reach is selected that is at least 6” deep and fairly straight. The reach should contain few obstacles, and all the water should be flowing in a single channel (not braided). A run is typically a good selection for streamflow measurements because the water has sufficient speed and depth. Three transects across this stream reach provide the average width value. Along these three transects, equally spaced depth measurements are used to compute the average depth. Velocity is determined by the time it takes to float an orange or object of similar buoyancy the length of the stream reach. Oranges work well for measuring velocity because they float just below the water’s surface, where they cannot be influenced by wind. Finally, the stream bottom factor accounts for the friction of the water flowing past the stream substrate, with the assumption that average stream velocity is only 80% to 90% of the surface velocity. Rough bottom streams interspersed with submerged plants or rocks have slower water velocities as compared to smooth mud or bedrock. Thus, streams with cobble or gravel substrates or many aquatic plants have a bottom factor of 0.8, whereas streams with a smooth mud, silt, or bedrock substrate have a factor of 0.9.

The land use within a stream’s watershed greatly impacts stream discharge. Water enters the stream by direct precipitation into the channel, by surface runoff from the surrounding watershed, or from groundwater inputs. As a watershed becomes more urban, the increase in impervious surface cover causes less water to infiltrate to the groundwater and more surface runoff to enter the stream channel in a shorter period of time. Because of the reduced infiltration, the water table often drops and groundwater sources to the stream are reduced. This causes flashy conditions in which streams have higher peak discharges at the beginning of a precipitation event, and lower overall base flow conditions.

Streamflow has significant impacts on other water quality parameters. Fast-flowing, turbulent water at riffles increases dissolved oxygen levels through aeration. In contrast, low flow conditions typically result in higher water temperature and decreased oxygen levels. Periods of increased flow may result in greater turbidity, because the fast moving stream has enough energy to displace larger quantities and sizes of sediment particles. This erosive capacity is well illustrated by the exposed soil on the banks of a flashy urban stream. Finally, aquatic organisms are adapted to a variety of streamflow conditions based on feeding strategies, water temperatures, and dissolved oxygen concentrations. A diverse stream environment requires both slow and fast flowing habitats to support its biological community. Changes in streamflow dynamics, such as those brought on by an urbanizing watershed, can greatly impact the integrity of the stream ecosystem.