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.