Characterizing Sediment Flux Using Reconstructed Topography and Bathymetry from Historical Aerial Imagery on the Willamette River, OR.

Abstract:

The Willamette is a gravel-bed river that drains ~28,800 km^2 between the Coast Range and Cascade Range in northwestern Oregon before entering the Columbia River near Portland. In the last 150 years, natural and anthropogenic drivers have altered the sediment transport regime, drastically reducing the geomorphic complexity of the river. Previously dynamic multi-threaded reaches have transformed into stable single channels to the detriment of ecosystem diversity and productivity. Flow regulation by flood-control dams, bank revetments, and conversion of riparian forests to agriculture have been key drivers of channel change. To date, little has been done to quantitatively describe temporal and spatial trends of sediment transport in the Willamette. This knowledge is critical for understanding how modern processes shape landforms and habitats. The goal of this study is to describe large-scale temporal and spatial trends in the sediment budget by reconstructing historical topography and bathymetry from aerial imagery. The area of interest for this project is a reach of the Willamette stretching from the confluence of the McKenzie River to the town of Peoria. While this reach remains one of the most dynamic sections of the river, it has exhibited a great loss in geomorphic complexity. Aerial imagery for this section of the river is available from USDA and USACE projects dating back to the 1930’s. Above water surface elevations are extracted using the Imagine Photogrammetry package in ERDAS. Bathymetry is estimated using a method known as Hydraulic Assisted Bathymetry in which hydraulic parameters are used to develop a regression between water depth and pixel values. From this, pixel values are converted to depth below the water surface. Merged together, topography and bathymetry produce a spatially continuous digital elevation model of the geomorphic floodplain. Volumetric changes in sediment stored along the study reach are then estimated for different historic periods. Additionally, HEC-RAS is used as an independent measure of sediment transport given prescribed flow, sediment and geomorphic conditions.