VIEWS OF THE RIVER: FLOOD RECORDS, LIDAR, UAV, AND CANOE BASED MONITORING OF MEANDER MIGRATION IN UPSTATE NEW YORK

Several natural processes erode stream cutbanks including bank collapse, fluid shear stress, burrowing and bioturbation, and freeze-thaw events. Which of these dominate? How do river channels respond to large floods? Do large floods cause more bank erosion and channel migration than small floods? If fluid shear and bank collapse dominate, are they likely to increase with larger and/or more frequent floods? These are very important questions that are not easily answered without detailed measurements of stream banks. The answers have implications for bank erosion mitigation efforts and climate change. The approach taken here is to begin the work of establishing a baseline condition for streams against which changes can be measured. A long term goal is to conduct frequent surveys at a scale which reveals processes, examine changes over time, and tie these to stream gage records. While there is nothing new about this idea, the development of structure-from-motion software has opened up avenues for realizing these goals.

The field site is Butternut Creek, a tributary to the Susquehanna River in upstate New York, which has a well-developed Holocene floodplain nestled into glacial deposits. The cutbanks are ~ 2 m high, extend for ~100 m, and exhibit signs of recent erosion. Oxbows etched in the floodplain suggest long term activity. 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. We found that well-located ground control points greatly enhance the value of photographic surveys, yielding orthophoto mosaics and 3D object models that compare favorably with aerial based Lidar surveys. Thus far, lateral migration rates in the decimeter per year range are common. Less clear at this time is the relationship between stream discharge records and cutbank migration rates.