Background and Problem
Stream network density exerts a strong control on hydrologic processes in watersheds by dictating both the relative importance of terrestrial and aquatic influences to stream chemistry and the residence time of water in watersheds. Knowledge of stream density is critical to modeling and empirical studies aimed at understanding the impact of land use on stream water quantity and quality.
Stream network density is largely a function of the number and length of the smallest streams. Many headwater stream reaches are not included in hydrographical maps, such as the U.S. National Hydrography Dataset (NHD), either because they were buried during the course of urban development or because they were seen as smaller than the minimum mapping size at the time of map generation. These “missing streams” severely limit the effective analyses of stream network density based on the NHD, constituting a major problem for many efforts to understand land-use impacts on streams.
The study region, covering almost 60,000 km2, includes the Potomac River watershed plus 5 smaller watersheds needed to complete coverage for all of the state of Maryland (USA) west of the Chesapeake Bay. The physiographic setting spans a landscape continuum from the Appalachian Mountains to the Chesapeake Bay spanning considerable geographic and land use diversity, and includes the large metropolitan areas of Baltimore, MD and Washington, DC.
The study region includes 5 physiographic provinces, each with distinctive geology and land cover influencing stream network density.
Application: Stream Burial
As demonstrated through our stream mapping work, changes in watershed land use through urbanization often result in stream burial. Streams get buried when they are directed into culverts, pipes, concrete-lined ditches, or simply paved over. This results in the destruction of natural stream channels and contributes to downstream habitat degradation, aquatic habitat fragmentation, enhanced transport of water and toxic contaminants, and reduction of ecosystem services such as nutrient and sediment retention. Headwater streams are among those most affected by urbanization because they constitute the largest fraction of stream length and are the most economically feasible to bury.
We intersected our new stream map with remote-sensing based observations of stream burial events (training data) and maps of impervious cover (predictive variables), to map the probability of stream burial for the entire study region and found that stream burial rates have increased over time and that stream burial increases with decreasing catchment area.
Ongoing research is studying the impact of stream burial on biological connectivity between headwater stream reaches.