Viewing River Fragmentation Across the Conterminous United States for Improved Conservation and Management of Dendritic Systems

Fragmentation of dendritic networks by dams is one of the most pervasive threats to aquatic organisms living in fluvial systems.  Besides modifying physical habitat in various ways (i.e., altered hydrologic and thermal regimes, decreased floodplain connectivity, reduced sediment transport, etc.), dams block organisms from accessing habitats necessary for critical life stages.  In spite of the pervasiveness of dam impacts on rivers, mitigating those impacts remains an important option for management to aid in conserving aquatic organisms impacted by habitat modification or loss from dams.  While decisions on dam management require a suite of information on economic and social needs and objectives, they also require the ability to understand how decisions on removing or managing dams fit in to a larger, landscape context.  Examples of the kind of information supporting such understanding include summaries of the total amount of aquatic habitat fragmented by various dams, statistics characterizing general fragmentation patterns within a region of interest, and an accounting of the influence of other disturbances on systems impacted by dams.  To provide such information at a national scale, we have quantified river fragmentation across the conterminous United States by large dams using a suite of metrics to better understand the overall degree of fragmentation and to support decision making on dam removal and dam management.  We began by linking dams represented by the 2009 National Inventory of Dams to the National Hydrography Dataset Plus, which provides a national coverage of streams throughout the conterminous United States along with their local catchment boundaries at a scale of 1:100,000.  Statistics characterizing fragmentation of river networks were then calculated including counts of dams on tributaries and mainstem river networks upstream and downstream of a given stream arc; number of river miles free of dams upstream and downstream of a given stream arc; distance to an outlet for a given stream arc; and the size of the catchment draining directly into a river network bound by dams.  By summarizing these fragmentation statistics regionally, we provide a picture of patterns of fragmentation that impact rivers across the United States.  Further, we have information that can be used directly to consider management actions within and across regions.  By incorporating fragmentation with other factors, such as landscape disturbances in catchments, we provide managers with key information to support the conservation of aquatic organisms living in fluvial systems.