Assessing stream restoration and the influence of scale, variable choice, and comparison sites

Abstract

AbstractAssessing stream restoration remains an on‐going challenge. Conclusions about the effectiveness of stream restoration efforts vary throughout the literature. This uncertainty is likely due, in part, to the spectrum of scales at which restorations have been assessed, the different variables used for assessment, and the varying types of sites used for comparisons. Mitigating such uncertainty with stream restoration assessment requires perspectives that consider multiple sites, variables, and scales using approaches that are transferable and comparable. From 2004 to 2018, surveys of stream habitat and fish at 44 restored and unrestored sites were conducted in two adjacent watersheds in southwestern Wisconsin, USA. Using non‐parametric randomization tests, means of 42 variables were compared between (1) watershed vs. watershed sites; (2) all restored sites vs. all unrestored sites, (3) restored sites vs. pre‐treatment surveys, (4) restored sites vs. unrestored control sites, and (5) pre‐treatment surveys vs. unrestored control sites. When applicable, these comparisons were also made at nested scales ranging from all sites in both watersheds to only contiguous sites with common survey periods. Comparing sites between the two watersheds revealed natural variability between watersheds with similar geology and land‐use practices, though unexpectedly more differences were found between surveys of restored sites from the two watersheds than between unrestored sites. The most differences between surveys of restored and unrestored sites were observed at larger scales of consideration, but within a given watershed. Although relatively few variables differed between pre‐treatment surveys and control sites, there were marked differences when each type of unrestored sites was separately compared with restored sites. Habitat variable differences were more robust and consistent than fish community variables across all comparisons. Deviating from traditional multivariate approaches and independently comparing a broad spectrum of individual variable means at various, nested scales underscored the importance of context and variable choice when assessing stream restoration.