Spatial and temporal patterns of stream burial and its effect on habitat connectivity across headwater stream systems of the Potomac River Basin, USA

Background/Question/Methods

Burying headwater streams during urbanization alters their primary physical, chemical, and biological processes. Determining the cumulative impacts of reduced structure and function within buried stream networks is crucial for informing management of stream ecosystems, in light of continued urban growth and the uncertain response of freshwater ecosystems to global climate change. We utilized improved stream maps for the Potomac River Basin (PRB) to describe the extent and severity of stream burial across the basin.  Observations of stream burial made from high resolution aerial imagery and a decision tree using spatial statistics from impervious cover data were used to predict stream burial at 4 time steps between 1975 and 2006.  Beyond simple habitat loss, stream burial decreases connectivity among headwater habitats, with potential to affect a range of important ecological processes.  To quantify changes in regional headwater connectivity we applied a model based on electrical circuit theory, which functions by treating the landscape as a resistance surface representing hypothesized relationships between landscape features and their differential "resistance" to movement by organisms.  A landscape resistance layer was developed and fine-tuned in terms of the habitat use of aquatic invertebrates with winged adult stages, organisms of critical importance to aquatic ecosystem health.   

Results/Conclusions

Of the roughly 95,500 stream kilometers (km) in the PRB, approximately 4551 km (4.76%) were buried by urban development as of 2001. Analysis of county-level burial trends shows differential patterns in the timing and rates of headwater stream burial, which may be due to local development policies, topographical constraints, and/or time since development. As with previous work, consistently higher rates of stream burial were observed for smaller streams, declining with increasing stream order. Headwater streams (1st and 2nd order) are disproportionately affected, with burial rates continuing to increase over time in relation to larger stream orders. Results show a highly significant relationship between rates of stream burial and total impervious surface across PRB counties, suggesting that local development policies remain ineffective in protecting streams from burial. Output from connectivity modeling shows significant increases in landscape resistance (isolation) among headwater stream systems, and corresponding decreases in current flow (movement of organisms) across the increasingly urbanized PRB landscape. Of particular interest, the circuit model highlighted the importance of stream confluences and zero-order (non-channel) headwater areas for movement of organisms between headwater systems that are otherwise highly disconnected, and for which the latter currently receives no legal protection from development.