Long-term Stream Monitoring Identifies Discharge Thresholds of Stream Metabolism Resistance to Storm Events.

Abstract:

Stormwater control measures (SCMs) placed in urban watersheds are commonly implemented with the goal of decreasing peak flows in streams following storm events. While SCMs are known to be effective on a site-by-site basis, it is uncertain how the cumulative effect of SCMs alter hydrologic conditions at the watershed scale and how that effect can potentially influence stream ecosystem health. To determine the effectiveness of SCMs at the watershed scale, we examined how stream metabolism responded to storm events, both in terms of peak discharge and high flow frequency, in three urban watersheds each in Cleveland, OH and Denver, CO. USGS discharge and high-frequency oxygen and temperature data were collected in six mid-order streams to make daily estimates of gross primary production (GPP) and ecosystem respiration (ER) over a two-year study period. Preliminary results indicate rates of GPP in Cleveland streams are resistant to high flows below approximately 800 cfs but drop to near zero if storm events exceed this threshold. This threshold response likely corresponds with the onset of bed-sediment mobilization which causes scouring of benthic algae. Therefore, sediment particle size is thought to influence the discharge threshold in which a stream is resistant, with larger particles being more resistant to mobilization than fine particles. Reducing peak discharge below thresholds that disrupt metabolism can be targets for groups implementing SCMs with goals aimed at improving hydrologic conditions at the watershed scale. While it may be impractical to return hydrologic conditions of an urban stream to a pre-development state, the biologically relevant thresholds identified in this study can be interpreted as a practical goal that can improve ecosystem function. Additionally, our long-term monitoring of hydrologic conditions and ecosystem health is providing key data to better inform our understanding of the cumulative effects of SCMs at the watershed scale.