The differences are more complicated in the western United States because non-forest vegetation may also be shrublands or grasslands. Spatial variation in land cover near streams and rivers across the nation suggest some potentially large differences in sediment and contaminant loadings to streams and rivers between regions. For example, the Upper Mississippi River basin has relatively more watersheds with agricultural and urban riparian zones, and this may contribute to relatively higher levels of sediment loadings in the streams and rivers. Large forested areas of the Appalachian Mountains have high proportions of forested riparian zones, and relatively little agriculture. Sediment and contaminant loadings to streams in these areas are likely lower than in the Upper Mississippi River basin.
Water quality is also related to larger patterns of land use over entire watersheds.
For example, roads near streams affect water quality not only as direct pollution sources, but also because they represent paths for rapid runoff. The frequency of roads crossing rivers was expressed here as the number of road crossings per unit river length in each watershed. This expression helps to adjust for differences in the total length of rivers between watersheds. The mid-Atlantic region, like most of the northeast and upper midwest, has extensive road networks. Number of road-stream crossings per 100 kilometers of streams.