Patterns of Stream Burial Due to Urbanization in the Patapsco-Gunpowder Watershed

Small, headwater streams provide important ecosystem services, including clean drinking water, habitat for aquatic biota, and rapid processing and uptake of nutrients, which can reduce delivery of nitrogen and phosphorus to downstream coastal waters. Changes in watershed land use through urbanization often result in stream burial, where streams are directed into culverts, pipes, concrete-lined ditches, or simply paved over. Stream burial 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.

There have been significant efforts to study, protect, and restore headwater resources, but in urbanizing regions of the United States, burial of natural channels (so that water is put into culverts and storm sewer drains) is ongoing. The Clean Water Act of 1972 and restrictions regarding floodplain construction provide little protection for headwater streams because they have been interpreted to apply only to permanent streams and issue permits for stream burial at specific locations with inconsistent consideration of the watershed context. More enlightened protection of headwaters requires knowledge of the extent and spatial context of stream burial.

Previous work in the Chesapeake Bay watershed using remote sensing techniques has shown a 61% increase in developed land from 1990 to 2000 with suburban/urban growth expected to increase rapidly in the future. The Gunpowder-Patapsco watershed (GPWS) selected for this study also includes older development in and around the city of Baltimore. The region is therefore likely to be representative of conditions in modern urbanizing catchments worldwide.

An estimation of stream burial (including ephemeral streams) was completed using a combination of remote sensing techniques and hydrologic modeling based on elevation to delineate hydrologic flow path. We then calibrated a map of impervious surface area using high-resolution aerial photography to build a stream channel decision-tree classification of stream channel burial (example shown at left). These methods provided an analysis of stream burial across a range in catchments from 10 ha to 104 ha for the 3.5 x 105 ha Gunpowder-Patapsco watershed.

Urbanized areas contained disproportionately more buried streams than other areas (see figure below). In Baltimore City, 66% of streams were buried. In contrast, 19% of streams were buried in counties outside Baltimore City, and 21% of streams were buried across the entire Gunpowder-Patapsco Watershed. While much of the heavy development corresponds to the main transportation corridors between rural areas and the center of Baltimore City, stream burial is apparent in most regions of the watershed.

To Learn More:

• Elmore, A.J. and S.S. Kaushal. 2008. Disappearing headwaters: patterns of stream burial due to urbanization. Frontiers in Ecology and the Environment; 6, doi:10.1890/070101. (abstract)

Project PI:

Dr. Andrew Elmore