Geomorphic response to dam removals on the lower Penobscot River, Maine, USA
Abstract
Most geomorphology studies of dam removals have focused on sites with appreciable quantities of stored sediments. There is great interest in channel response to sediment releases because of the potential effects on aquatic and riparian habitats and human uses of these areas. Yet, only relatively minor accumulations of sediment can be present behind many dams in the Northeast U.S. with small impoundments, run-of-river dam design and management configurations (inflow ≈ outflow), low watershed sediment yield, and/or channel beds dominated by coarse sediment and/or bedrock. The two lowermost dams on Maine's Penobscot River in New England's second largest watershed (≅22,000 km2), removed in 2012-13, were an example of those conditions. Veazie Dam (≅10 m) was located just above the head-of-tide and Great Works Dam (≅6 m) was about 12 km further upstream. These run-of-river hydroelectric dams did not have substantial water storage capacity. Pre-project geophysical surveys showed little stored sediment in either impoundment, and coarse substrates (cobbles, boulders, and bedrock) dominated the beds of the reservoirs and the reaches up and downstream—functions of reach geology, late Quaternary history, and upstream dams. However, the streambed substrata in the impoundments was poorly understood and there was a history of river modifications at the sites, suggesting unexpected geomorphic changes were possible. To evaluate physical responses to the removals, we did repeat cross-section surveys in each impoundment as well as the upstream and downstream reaches. We also measured bed sediment grain-size and collected turbidity data below the dams. Comparing pre- and post-removal surveys confirmed our expectation of little change to bed elevation, channel shape, or channel position. Indeed, changes from one survey to the next at nearly all cross-sections were often within our estimated measurement error. Our study shows that physical changes are likely to be minimal when impoundments storing relatively little sediment are removed from erosion-resistant streambeds. Many dams eligible for removal have these characteristics, making the observations an important case study largely unrepresented in the dam removal response literature.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP41C2680C
- Keywords:
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- 0481 Restoration;
- BIOGEOSCIENCESDE: 1813 Eco-hydrology;
- HYDROLOGYDE: 1825 Geomorphology: fluvial;
- HYDROLOGYDE: 1834 Human impacts;
- HYDROLOGY