Dams have been a fundamental part of our national agenda over the past two hundred years. Recently, however, dam removal has emerged as a significant national strategy in the United States and more than 1,100 dams have been removed since ca. 1970. However, only 130 of these removals had any ecological or geomorphic assessments, and less than half of those included before- and after-removal (BAR) studies. This gap in our knowledge inhibits predicting response to dam removal, and the restricted locations of dam removals suggest the state-of-science may be limited to distinct landscape settings. We analyzed and compared the landscape context of existing and removed dams, and assessed the biophysical response to dam removal for 63 BAR studies.
Preliminary results indicate the highest concentration of removed dams is in the Northeast and Upper Midwest. Most dams have been removed from 3rd- and 4th-order streams in low elevation (< 500 m) and low slope (<5%) watersheds that have small to moderate upstream watershed areas (10-1000 km2) with a low risk of habitat degradation. Many of the BAR-studied removals also have these characteristics, suggesting that a limited range of landscape settings influences current understanding of responses to dam removals; this limits predictive capability in other environmental settings. Biophysical responses to dam removal varied by landscape cluster, indicating that there is likely a landscape component to river response to dam removal. However, biophysical data were not equally distributed across variables or clusters, making it difficult to quantify differences. To address these inconsistencies, we provide suggestions for prioritizing and standardizing data collection associated with dam removal activities.