Watershed urbanization has been associated with declines in species richness and proportion of pollution-intolerant aquatic organisms. Some of these changes in community composition may be due to increased stormwater runoff from urbanized watersheds. In order to understand how stream flow patterns change along a putative stormwater impact gradient, we are continuously monitoring streamflow using Isco 2150 flow meters at six streams in the Rocky River Watershed of northeastern Ohio. Three of these streams are primary headwater streams, draining 0.41 to 1.22 km2, while the other three are larger draining 3.47 to 7.25 km2. Two of the streams are considered reference streams as they are within predominately forested watersheds, while the watersheds of the remaining streams are dominated by urbanized land use. This monitoring will help identify natural intra-annual variability in flows in reference headwater streams flowing through shale bedrock. We are testing the hypothesis that more urbanized watersheds will have streams with more variable (i.e., flashier) hydrographs and higher peak and total flow than streams in less urbanized watersheds. IHA7 (The Indicators of Hydrologic Alteration) software was used to characterize stream flow patterns at each site.
Results/Conclusions
Contrary to expectations, streams with the greatest proportion of watershed urbanization have lower peak flow rates than streams with the least urbanized catchments. From January 21 to May 3, 2010, the maximum average daily flow was 0.1167 m3/s for the most urbanized primary headwater stream and 0.2551 m3/s for the least urbanized primary headwater stream. The maximum average daily flow was 0.2246 m3/s for the most urbanized larger stream and was 0.5274 m3/s for the least urbanized larger stream. Among the primary headwater streams, the most urbanized stream had the greatest rate of rise and fall, while the most urbanized larger stream had lower rise and fall rates than the less urbanized larger streams. Other measures of hydrologic alteration such as number of high and low flow pulses were similar among streams in the same size class. These trends suggest that land use may be less useful than other factors in predicting stream flow patterns in these watersheds. Future research will quantify how riparian vegetation and stream biota vary along this urbanization gradient.