Effects of Land Cover Change on Water Quality in Urban Streams at Two Spatial Scales

This study examines the relationships between land cover change and water quality change in three urbanizing watersheds in the Pacific Northwest region of the United States: Burnt Bridge Creek, Salmon Creek, and the Tualatin River. All three watersheds have had many of their water quality parameters exceeding Total Maximum Daily Loads as required by their state’s environmental agencies in the past decades. By using the National Land Cover Datasets classified by the United States Geological Survey (USGS) for 1992, 2001 and 2006 and water quality data for a period between 1991 and 2010, this paper aims to examine whether changes in land cover are causing changes in water quality at two different spatial scales at the sub-watershed scale and at a 100 meter riparian buffer scale. We used spatial regression models to identity the major determinants of changes in water temperature (WT), total suspended solids (TSS), dissolved oxygen (DO), and total phosphorus (TP) over time at different scales. The results show that each parameter reacts differently to land cover change depending on the scale of analysis. Both DO and WT showed significant relationships with land cover parameters on the watershed scale but not as much on the riparian buffer scale. TP shows significant relationships at the watershed scale, but TSS shows no significant relationships at the watershed scale. WT shows the only significant change in water quality over the past twenty years and is positively related to change in urban land cover. Topographic variables become significant in explaining the variations in WT and TP at the riparian scale. DO is mostly explained by mean slope for both 1992 and 2001 at both scales, but urban land cover became an important predictor in 2006 at both scales. Our analysis also suggested that there may be a potential lag between changes in land management and changes in water quality across different scales.