Factors affecting accuracy of stream channel slope estimates derived from geographical information systems

Stream channel slope is often a critical component of geographical information systems (GIS)-based models of preferred habitat of aquatic species, but the relative accuracy of various GIS slope derivation methods is not well established. We examined the accuracy of GIS-derived stream slopes for a set of stream reaches in Idaho and Ohio. We also used the Ohio data set to examine in more detail the effects of stream reach length, source of GIS file representing the stream path (shapefile), and digital elevation model (DEM) resolution on the accuracy of GIS-derived slopes. The accuracy of GIS-derived slopes in the Ohio dataset improved with increasing reach length, but we could not draw any consistent conclusions about the effect of DEM resolution or shapefile. We present a simple and efficient method for improving GIS-derived slopes by identifying probable elevation errors in the GIS-derived longitudinal stream profiles. The resulting derived slopes were improved in all cases; the slopes derived by using a 10-m DEM and a manually traced stream shapefile were the most accurate. We demonstrate how our results can be used to evaluate the feasibility of implementing a GIS-based habitat model of sea lampreys Petromyzon marinus.