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Track: Water Resources
William Haskins
Ecology Center
1519 Cooper Street
Missoula, MT 59802
Telephone: 406-728-5733
Fax: 406-728-9432
E-mail: ecocenter@wildrockies.org
Dave Mayhood
Stream Crossing Density as a Predictor of Watershed Impacts
Increased delivery of sediment to streams has long been recognized as one of the major environmental impacts of human development of land. Roads are an inevitable adjunct to land development for any purpose and are often by far a greater source of sediment to watercourses than all other land uses combined. Because sediment is delivered to the channels at the point that the road crosses the watercourse, there has been increasing interest, and some success, in using stream crossing density of roads as a predictor of environmental impacts of development on watersheds. Given reasonably accurate 1:20,000 digital representations of roads and streams within 90 drainage basins of southern Alberta's Rocky Mountain eastern slopes, we required an efficient GIS-based method to count the number of stream crossing points of roads. In general terms, we needed the GIS to provide the points of overlap of two coverages with line topology, but to ignore any intersections contained within each of the line coverages: we did
not want stream confluences and road intersections included in the total. This challenge is apparently not met directly by any of the standard overlay functions of ArcInfo, so we relied upon a combination of the "clean" and "intersect errors" commands in ArcInfo Version 7.03 to derive the required information. Stream crossing information was then used in a GIS-based watershed analysis that considers 11 other land use variables, including road densities, proximity of roads and logging units to streams, extent of logging, soil erodibility, steepness of slope, and proximity of development to fish-bearing streams.
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