Analysis of historic orthomosaic imagery collected through various agencies for mapping active channel and floodplain changes along a 22.5-km reach of the Green River at Gray Canyon near Green River, Utah from 1938-2021

The U.S. Geological Survey gathered historic aerial photography and remote sensing imagery from a variety of sources for mapping channel change along a 22.5-kilometer reach of the Green River at Gray Canyon near Green River, Utah. Imagery was downloaded in .TIF format and Structure-from-Motion (SfM) photogrammetry techniques were applied (Over et al. 2021) using Agisoft Metashape Professional (v. 1.7.3 build 12473) on applicable imagery. Orthomosaics were created for each year of imagery from 1938 to 2021. The 2006 National Agriculture Imagery Program (NAIP) imagery was used to identify a common set of Ground control points that were used in each year's imagery in the georectification process. The rectified orthomosaics were clipped using a single study area of interest polygon so the final clipped orthomosaic rasters represented the same study area extent for analysis. To improve our understanding between flows and the channel in our study reach, we digitize the active channel and floodplain boundaries of the river for each available image from 1938 to 2021, then calculate the amount of channel change occurring throughout the reach between each set of years. We utilize Soil Conservation Service Scan (SCS), NAIP, High Resolution Orthoimagery (HRO), and Worldview 2 & 3 aerial imagery to create a historical timelapse of our study area. We calculate both the floodplain formation and floodplain destruction between years. Overall, the rate of floodplain formation has increased in recent years, indicated by an encroachment of vegetation into the active channel on both the mid-channel islands and along the banks. The active channel area in our reach has been decreasing since the mid-1970s.