Study Tracks How Size Affects Benefits of Dam Removal

By Becky Brown

Editor's note: Becky Brown, Professor at Eastern Washington University, prepared for the Cruiser this summary of her recently published article. She has a special connection to paddling in our area, being the daughter of CCA's own Barb Brown.

As paddlers, we often encounter dams on regional rivers, some of which block downstream passage and cause significant ecological impacts. As more of these dams age, they are becoming obsolete and increasingly difficult to safely maintain, leading to their removal. As of 2023, over 2,100 dams have been removed in the U.S., according to American Rivers. However, there is a huge variation in dam size and operations, making it hard to predict the potential benefits of dam removal for river ecosystems. 

To address this knowledge gap, my colleagues and I, organized through the Patrick Center for Environmental Research, analyzed data from 16 mid-Atlantic dams to learn how variations in dam size influence their ecological impacts. Our study sites included some familiar paddling rivers like Muddy Creek and Tohickon Creek in Pennsylvania, and the Gunpowder River in Maryland. To determine impacts, we compared measurements of water quality, sediment, algae, aquatic insects, and plants from downstream and upstream of each dam impoundment, with the upstream, free-flowing reach considered an undammed reference. We measured dam size in terms of both height and hydraulic residence time (how long it takes water to move through the reservoir formed by the dam). 

In general, we found that larger dams have greater impacts on many ecosystem components, as one might expect. However, for a few components, such as sediment grain size, dams had impacts regardless of their size. Our research is published in the journal Ecological Applications, which has a photograph of Woodbine Dam, on Muddy Creek, on the cover of its September 2024 issue.

Journal article: "Size-dependent effects of dams on river ecosystems and implications for dam removal outcomes," by Rebecca L. Brown, Don Charles, Richard J. Horwitz, James E. Pizzuto, Katherine Skalak, David J. Velinsky, David D. Hart, Aug. 13, 2024

Journal cover