20-9 Quantifying the Extents and Alteration of America's Riverscapes

Abstract:

Historically, estimates of riverscape extent at national and global scales have relied on coarse geospatial datasets that primarily capture larger river systems. Because first-order streams constitute most of the drainage network, these approaches have underestimated the true extent of riverscapes. This underestimation limits our understanding of the cumulative effects of human modification, the magnitude of restoration opportunity, and flood hazards in areas lacking detailed flood hazard mapping. To quantify riverscape extents across the contiguous United States (CONUS), we used a cloud-computing platform called CyberCastor to apply the Valley Bottom Extraction Tool (VBET) to the entire drainage network. In addition to delineating valley bottoms, VBET segments riverscapes into discrete polygons and automatically computes a suite of geomorphic metrics within those polygons, providing a consistent framework for continental-scale analysis. We further quantified the anthropogenic alteration of each riverscape segment by estimating the proportion of riverscapes fragmented by transportation and other infrastructure, and converted to agricultural or urban land uses. This was accomplished by intersecting the valley bottom polygons with a nationally consistent transportation infrastructure dataset and a remotely sensed land cover raster dataset. Our analysis indicates that riverscapes occupy approximately 20% of the CONUS land surface and have an average size of 18.5 hectares per kilometer of valley length. Across different physiographic regions, riverscapes are highly variable in size (1.27 - 826 ha/km) and the proportion of the landscape they make up (4.6 - 63.7%). For all of CONUS, 29-46% of riverscapes have been fragmented by human infrastructure or land-use change. Because these metrics are calculated for individual riverscape segments, they can be aggregated to stream, subwatershed, watershed, regional, or national scales, enabling consistent assessment of riverscape extent, condition, restoration potential, and flood hazards across a wide range of management and planning applications.

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