FLOOD RECORDS, BANK EROSION MECHANISMS AND MEANDER MIGRATION IN BUTTERNUT CREEK, NEW YORK

Several natural processes erode stream cutbanks including bank collapse, fluid bank shear stress during high flows, burrowing and bioturbation, and freeze-thaw events. Which of these contribute most to meander migration rate? If fluid bank shear dominates, one might expect a direct correlation with the number of days of flows exceeding a given size and migration rate. If bank collapse dominates, perhaps smaller flows are more effective at creating undercut banks, which could increase migration rate at lower flows. We develop a test for these hypotheses by determining bank migration rate at a site along Butternut Creek, a tributary to the Susquehanna River in upstate New York, which has a well-developed Holocene floodplain nestled into glacial deposits. The cutbank is ~2 m high, extends for ~100 m, and exhibits signs of recent erosion. Several floods in the last decade have overtopped the banks here. Aerial imagery from New York State GIS warehouse shows the cutbank has migrated several meters since 1994. We extracted USGS stream gage records for nearby rivers, and determined the number of days for flows exceeding the 50, 90, and 95 percentile daily flow for the time period between aerial images. The migration rate appears to be inversely related to the number of exceedance days, suggesting that higher flows stymie migration. We hypothesize that higher bank shear stress may remove a more uniform thickness from the bank, causing a reduction in bank collapse frequency, whereas lower flows may tend to undercut more effectively. We captured overlapping photographs of the stream banks on the ground, from a canoe, and from an unmanned aerial vehicle (UAV). PhotoScan, a structure-from-motion software by Agisoft, was used to construct three dimensional topographic models of the cutbank that compare favorably with aerial based Lidar surveys. We intend to monitor the bank more closely from the river’s view with these new tools to identify the dominant erosion process.