Stream & Watershed Health Indicators

Introduction | High Qualtity | Poor Quality

Land cover and land use change can have large influences on the chemical, physical and biological characteristics of streams. Forested riparian buffers are thought to preserve or increase stream quality by contributing leaf litter, regulating temperature and sunlight, deterring erosion and impacting nutrient cycling. Impervious surfaces, on the other hand, increase the amount of pollutants within and the temperature of runoff reaching streams. In past studies land cover was developed using aerial photography, but our analyses make use of high resolution satellite imagery (Landsat 30m) across large areas like the Chesapeake Bay watershed, as well as very fine resolution imagery (Ikonos 4m) for smaller watersheds (e.g., the Anacostia).

In an effort to link the land cover of watersheds with the quality of the stream life we worked with collaborators at the Maryland Department of Natural Resources, the Montgomery County Department of Environment, and the Maryland National Capitol Parks and Planning Commission. These groups sampled macro-invertebrate and fish communities in each of 246 small watersheds within Montgomery County and then combined these data with physical and chemical measurements (like temperature and dissolved oxygen) to create watershed rankings of excellent, good, fair, and poor. Using statistical regression techniques we determined that the factors accounting for the most variation in stream health rating was the proportion of impervious surface area, followed by the proportion of tree cover in a watershed. Tree cover in riparian buffer zones was also a significant predictor, but was highly correlated with total tree cover of the watersheds. We also considered additional metrics accounting for the spatial configuration of the landscape, such as the amount of impervious cover in the flow path from surrounding lands to the stream. These were also found to be significant predictors of stream health, but were second-order predictors. Our results, published in two upcoming journal articles, suggest that best management practices designed to improve stream water quality should focus on the amount of impervious area and tree cover in both the watershed and within the buffer zone.

Based on this work, we are now extending the analysis to larger areas. The following pages show where we might expect high quality stream health ratings across Maryland, as well as poor quality ratings. We emphasize that these maps do not identify these streams as good or poor quality, they merely suggest the likelihood that these streams are either likely to be impaired or likely to be biologically healthy. We also note the amount of impervious cover that these watersheds are expected to contain in the future, as predicted by a land use change model (SLEUTH) assuming different policy scenarios (managed and ecologically sustainable). In the Anacostia watershed, which we have studied in some detail, together with the Metropolitan Washington Council of Governments, confirms that the Anacostia watershed has a large proportion of impervious areas and a small proportion of tree cover, characteristic of an urban watershed. One way in which the water quality of the Anacostia can be improved is through the restoration and maintenance of riparian buffers, much of which are associated with MD-DNR's Greenways Program. These are areas considered high priority for protection in order to improve not only stream health but also promote wildlife populations and overall biological richness.

References for further reading:

Arnold, C. L., and C. J. Gibbons. 1996. Impervious surface: the emergence of a key urban environmental indicator. American Planning Association Journal 62:243-258.

Basnyat, P., L.D. Teeter, B.G. Lockaby, and K.M. Flynn. 2000. The use of remote sensing and GIS in watershed level analyses of non-point source pollution problems. Forest Ecology and Management. 128:65-73.

Goetz, S. J., R. Wright, A. J. Smith, E. Zinecker, and E. Schaub. Ikonos imagery for resource management: tree cover, impervious surfaces and riparian buffer analyses in the mid-Atlantic region. Remote Sensing of Environment (in press).

Snyder, M., S. J. Goetz, and R. Wright. Stream health rankings predicted by satellite-derived land cover metrics: impervious area, forest buffers and landscape configuration. Journal of the American Water Resources Association (in press).