Bankfull in the Ashokan Watershed

Posted on: November 2nd, 2017 by Leslie_Zucker

On Sunday, October 29 after a steady and sometimes heavy rainfall, many streams in the watershed filled their banks and were just about to spill onto their floodplain. Stream managers call this flow the “bankfull discharge”. Let’s break that down! “Bankfull” is actually an expression of the channel’s shape, specifically its width and depth when the channel is filled with water. “Discharge” is the volume of water moving down the stream at any given time. The typical unit of measurement for discharge is cubic feet per second. So “bankfull discharge” is the flow when the river is just about to spill onto its floodplain. This flow typically occurs every 1 to 2 years. The last widespread bankfull flow event happened in February 2016.

The Woodland Valley Creek just past bankfull flow (discharge) on October 30, 2017. The peak of bankfull discharge occurred during the night of October 29 and couldn't be photographed!

The Woodland Valley Creek just past bankfull flow (discharge) on October 30, 2017. The peak of bankfull discharge occurred during the night of October 29 and couldn’t be photographed! Photo by Ulster SWCD.

Why do we care? Because bankfull is considered the most effective flow for moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work that results in the shape of the channel. If a channel is naturally stable, any changes caused by a bankfull discharge should be relatively mild. Most aquatic organisms, such as our native fish and aquatic insects are well-adapted to these changes, and may even benefit from them. In fact, looking for major changes after a bankfull discharge is one way stream managers know if a channel is stable or out of balance with the surrounding environment.

If you think you have an unstable channel on your property, contact the AWSMP office for a free site visit to review your options at (845) 688-3047.

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