Monday November 30th, 2020 was a rainy day in the Ashokan watershed. A home rain gauge in Boiceville measured approximately 4 inches over the course of the day.
In response to the significant precipitation the Beaver Kill, Little Beaver Kill, Bushkill, and Esopus Creek at Cold Brook reached bankfull discharge. Bankfull discharge is the stream flow that completely fills the channel in a geomorphically stable stream. Any flow that exceeds bankfull will put water onto the adjacent floodplain.
Streams that have berms or levees, are incised, or otherwise unstable do not have such a clear relationship between bankfull discharge and channel geometry.
In the Northeast, a bankfull or greater flow happens once every 1.5 years, on average. However, “on average” means that some years see multiple bankfull events while others have none. Monday’s event was the second time in 2020 that the Little Beaver Kill has equaled or exceeded its bankfull discharge of 909 cubic feet per second (cfs).
Bankfull flow events are important because over time, these flows move more sediment than any other discharge, larger or smaller. This is because bankfull flows happen regularly, every 1.5 years on average, as opposed to big floods that move a lot of sediment but are more infrequent.
Due to the geomorphic importance of bankfull discharge events, the AWSMP regularly visits stream restoration sites, culvert replacement projects, and other stream reaches following bankfull events to take photographs and monitor any changes observed in the channel.
