River channel response to dam removals on the lower Penobscot River, Maine, United States

Abstract

Most geomorphology studies of dam removals have focused on sites with appreciable quantities of stored sediments. There is great interest in channel responses to sediment releases because of potential effects on aquatic and riparian habitats and human uses of these areas. Yet, behind many dams in the Northeast U.S. and other regions of the world only minor accumulations of sediment are present because of small impoundments, run‐of‐river dam design and management (inflow ≈ outflow), low watershed sediment yield, and/or channel beds dominated by coarse sediment and/or bedrock. The two lowermost dams on the Penobscot River in Maine, United States, removed in 2012–2013, exemplified those conditions. Great Works and Veazie dams were about 6 and 10 m high, respectively. Pre‐project geophysical surveys showed coarse substrates dominated the reservoir beds and little sediment was stored in either impoundment—functions of reach geology, late Quaternary history, and upstream dams. Repeat cross‐section surveys in each impoundment, as well as the upstream and downstream reaches, were completed from 2009 to 2015 to evaluate channel morphology responses to the removals. Bed‐sediment grain size and turbidity were also measured to characterize changes in bed texture and suspended sediment. Pre‐ and post‐removal survey comparisons confirmed the expectation that bed elevations, channel shapes, and channel positions would not change substantially. Changes were often within, or close to, our estimated random measurement error. Our study shows that large‐scale 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 these observations an important case study that is largely unrepresented in the dam removal literature.

Watershed Suspended Sediment Supply and Potential Impacts of Dam Removals for an Estuary

“…If storage of fine-grained sediment behind a dam is appreciable (Collins et al 2020), erosion of the impounded sediment after dam removal can increase suspended sediment concentration (SSC) in the waterway downstream. The resulting increase in turbidity and reduction in water clarity can be similar in magnitude to that induced by large discharge events (Tullos et al 2016).…”

Six years of fluvial response to a large dam removal on the Carmel River, California, USA

“…The Carmel River evolved similarly to the example of the Pierre Glissotte Dam (7.3 m high) removed from the Yvonne River in France, where despite ‘intense morpho‐sedimentary dynamics in the reservoir and effective restoration of bedload continuity’, fluvial morphology below the former dam site remained largely unchanged (Gilet et al, 2021). In two other examples of similar response style, Amethyst Brook, Massachusetts remained sediment supply‐limited during its 1–2‐year geomorphic adjustment to removal of a 6 m dam (Magilligan et al, 2021), and geomorphic changes on the Penobscot River, Maine, in response to removal of two dams (6 and 10 m high), were small enough to be within measurement uncertainty (Collins et al, 2020). The changes we have documented in the Carmel River were larger than our uncertainty range but provide a worthwhile lesson that removal of even large dams does not necessarily produce major change downstream.…”

“…This is one of few dam removals accompanied by long‐term BACI investigations (cf. Collins et al, 2017; Collins et al, 2020; Duda et al, 2021; East et al, 2018; Major et al, 2012; Tang et al, 2021), and additional field data are now available from the Carmel River reflecting its disturbance and recovery. As discussed below, this river has undergone removal of a large dam but with relatively little base‐level fall and reservoir sediment release, due to intentional efforts to manage and minimize downstream sediment impacts.…”

Vegetation responses to large dam removal on the Elwha River, Washington, USA

“…Virtually all other published studies of vegetation responses to dam removal have been associated with small dam removals and restricted to former reservoirs (e.g., Orr and Stanley, 2006). Geomorphic change downstream of small damremoval sites, or large dam removals where the sediment release is deliberately limited, is typically minimal (Collins et al, 2020;Cashman et al, 2021;, which translates to minor vegetation change. Also, riparian vegetation responses to dam removal have been studied much less than physical environmental variables or other biota such as fish or aquatic macroinvertebrates (Bellmore et al, 2017).…”