CATEGORIZING NATURAL FLOODPLAIN FORM: A SENSITIVITY ANALYSIS

Floodplains are characterized by spatial heterogeneity that provides many ecosystem functions, e.g. floodplain storage of water, solutes, sediment, organic matter, and contaminants. Heterogeneity results from lateral channel migration and avulsion and can be delineated as patches that are spatially contiguous and distinct. Spatial heterogeneity influences storage time of water, solutes, sediment, and organic matter; contaminant storage/remobilization; and river corridor biodiversity and productivity. Centuries of flow regulation, land drainage, and artificial levees have disconnected channels and floodplains, resulting in homogenization of floodplain morphology and vegetation and loss of floodplain functions. Restoring the natural processes that perpetuate floodplain functions is of increasing scientific and practical interest. This study evaluates variables controlling river corridor dynamics (lateral confinement, drainage area, channel planform, hydrological regime, vegetation type, reach gradient, and sinuosity), and quantifies floodplain complexity. We use images from Google Earth Engine to create classified rasters in ArcGIS Pro and compare to field delineated patches for the Sol Duc and Hoh Rivers in Olympic National Park, Washington. We then calculate and compare common landscape-scale heterogeneity metrics for the classified rasters and field patches. This study builds on the results of our first study (Iskin & Wohl, in review) by enhancing the detail of the raster layers to see how the categories and heterogeneity metrics change. Enhanced layers used include calculated NDVI and other raster indices, sub-meter lidar topography, and spatially interpolated soil texture data. Preliminary results show that including vegetation and moisture index layers increases the longitudinal connectivity of river patches, and the inclusion of fine-scale topography data does not change heterogeneity metric values in a significant way. The results of the larger research of which this is a part will be applicable to many river corridors as the sites capture much of the variation in the United States. More broadly, this work can inform adaptive river management implemented by public and private practitioners by identifying processes that create and maintain floodplain heterogeneity.