Assessing stream health by evaluating stream channel stability
by Watershed Planning & Restoration | The Watershed Watch
Salt Lake County’s Watershed Planning & Restoration Program is currently conducting a comprehensive evaluation of stream channel stability. Begun in 2015, this stability survey is an update to a similar rapid stream assessment completed in 2010, and is being completed on the Jordan River and nine of the river’s tributaries in Salt Lake County.
Streams are dynamic. In a healthy stream system, stream banks move as erosive forces shape and reshape the channel and floodplain. Stream bank and bed mobility is a natural phenomenon. A stream is considered “stable” when the water flow and sediments carried by the channel do not cause excessive changes to the width, depth, cross-sectional area, and slope of the stream. The difference between stable and unstable streams is primarily marked by the rate of bank and bed mobility. The expected rate of change for a particular stream varies by stream type, which is based on steepness of the streambed and surrounding landscape, the surrounding geology, soil types, and other factors.
To evaluate channel stability, Watershed Program staff walk each stream to first delineate them into units called reaches, which are defined based on changes in geology and tributary influences. A variety of data is then collected for each reach using the Pfankuch Method (pronounced “fahn-cook”), which provides a combined assessment of physical variables of the upper bank, lower bank, and stream bed. Each variable is assigned a score, some weighted based on level of importance, and a final combined score indicates whether the overall channel stability is “Good”, “Fair”, or “Poor”, based on stream type.
This system allows us to identify weak links and to discover what, if any, opportunities exists to correct the condition. Unnaturally high rates of stream bank erosion and bed mobility can have multiple causes. These range from small-scale local causes like unrestricted livestock access
or streamside landscaping changes made by unsuspecting homeowners, to larger-scale influences such development that increases impervious surfaces (paving, rooftops) that can dramatically increase stormwater and pollutant inputs into a stream.
Stream channel stability is just one dataset being collected by the Watershed Program to help evaluate the overall health of the watershed. Ultimately, our goal is to serve as a check and a measure of the stresses put on our urban streams, as well as the success and effectiveness of management options designed to repair damage and alleviate stresses. Land managers can use the stability data to appraise channel conditions and, hopefully, respond to adverse changes before impacts to the water resource become unacceptable and unalterable.