More than 80,000 dams exist in the U.S., and the proliferation of large dams for hydropower and flood control has led to significant hydrologic, geomorphic, and ecological adjustments in river systems. Recent geomorphic research has documented profound and sustained changes resulting from flow regulation, including changes in channel properties, sediment transport, and reduced channel complexity leading to impaired ecological habitat both within the channel and across the broader riparian zone. Several important research questions have emerged concerning the magnitude and direction of impacts across lateral and longitudinal scales caused by the interactive effects of flow regulation and the progressive downstream tributary inputs of both water and sediment. This research project will focus on subwatersheds of the Connecticut River valley, an area significantly affected by historical flow regulation and impoundment by both flood control and hydroelectric dams. The investigators will quantify (1) the length scale of impact of impoundment laterally across floodplains and progressively downstream of dams; (2) the geomorphic adjustments associated with tributary inputs; and (3) the changes in riparian community structure resulting from impoundment and downstream flow contributions. The project specifically focuses on the length scale of impact imposed by these dams through their associated flow reductions, their resulting changes to the sediment discharge regime by reservoir trapping, and the combined interactive effects of dams on sediment transport and storage as well as flow, especially as part of the progressive downstream inputs of water and sediment by tributaries. Through the use of fallout radionuclides, especially 210-Pb inventories on floodplains, stage-discharge flow modeling, and riparian vegetation sampling, this project will quantify the increased connectivity downstream of dams associated with increased tributary flow. To capture the effect of tributary inputs on sediment discharge, a field design will be employed to measure (1) the activity of 7-Be as a fingerprinting of sediment and (2) changes in bed elevation along the longitudinal profile. The gradient and aggradational adjustments then will be linked to the sediment residence time through the use of 7-Be.
This research project will elucidate the links between hydrologic changes in flow and inundation patterns, sediment inputs, and ecological responses. Results from this research can be used as a template to gage the lateral and longitudinal impacts of impoundment and can thus help provide the approaches for ecological amelioration. Moreover, from a geomorphic perspective, the project results will help ascertain the morphologic structure of watersheds and the fundamental ways that tributary inputs influence mainstem characteristics in both regulated and unregulated conditions.
Over 14,000 dams are scattered throughout New England. These dams vary in type and management, ranging from low head run-of-river impoundments to large multi-purpose structures that significantly regulate flow and store water. Flow regulation changes the timing, magnitude, and duration of flow releases, resulting in ecological repercussions both for in-channel ecological functioning as well as for out-of-bank forest communities that depend on floods to supply nutrients and exclude competitors. To understand the magnitude of flow reduction and the processes of sediment exchange between the channel and its immediately adjacent riparian zone, this research combined detailed streamflow analyses, field sampling of channel and floodplain sediments, vegetation surveys, and geochemical analyses (7Be and 210Pb inventories) to trace the movement of sediment in multiple watersheds in the Upper Connecticut River basin, especially in the highly regulated West River in VT. Given the previous success of 210Pb in determining the spatial and temporal dynamics of sedimentation rate in floodplain systems, we hypothesized that this technique can ascertain the impacts of dams on floodplain connectivity and the routing of sediments through a river system. In particular, the specific river-floodplain interactions we examined included: (1) quantifying the last time floodplain surfaces were inundated; (2) measuring the diminishing eco-geomorphic impacts with distance away from the dam; (3) documenting the progressive geomorphic changes at tributary confluences; and (4) ascertaining forest community composition due to inundation decreases following flow regulation. Based on detailed flood frequency analyses from U.S. Geological Survey stream gage data, our study indicates that the bankfull, 2-year flood at the regulated sites has been reduced by approximately 48% below flood control dams. The flow reduction is even more profound for larger flows, where the pre-dam 5-, 10-, 20- and 50- year flows were reduced by 62%, 70%, 76% and 83%, respectively. These post-dam flows have greatly reduced the ability of the mainstem to transport coarse material delivered by the unregulated tributaries, leading to progressive bar development and channel narrowing at tributary confluences. Since contemporary flows are less than the pre-dam 2-year flow, all post-dam flows are confined within the active channel, resulting in a loss of floodplain inundation area of 96 - 97% for pre-dam floodplain surfaces that had been previously inundated at least once every 5 years. In general, sites with narrow, deep channels and a higher degree of valley confinement had the least reduction in floodplain inundation area. Where the channel is wider and in a less confined valley, the loss of floodplain inundation area is significantly higher, highlighting the role of local geology in determining the magnitude and pattern of inundation. The current distribution of floodplain forest communities reflects these changes in reduced flows and subsequent channel characteristics. Flood-dependent tree species tend to dominate at elevations that are flooded between 1.2% of the year (4.5 d/y) and 26% of the year (95 d/y). Downstream mainstem reaches provided the most habitat, largely associated with low-energy features such as back swamps and meanders, and dominated by silver maple. Our field analyses identified several suitable sites in the upper part of the basin and in large tributaries, often associated with in-channel islands and bars frequently dominated by sycamore and other flood disturbance-dependent species. The long-duration floods that favored the development of distinct floodplain forests were also associated with lower abundances of most species of invasive shrubs. These invasives represent major threats to native forests, particularly in disturbed habitats. Our results emphasize the importance of both direct (via physiological tolerance) and indirect (via competition with native and non-native species) effects of hydrologic alteration on floodplain forests. Results from the radionuclide inventories of 210Pb revealed the limited exchange of overbank materials due to the reduction in inundation area. We infer from our data that flow regulation has impacted sediment deposition to floodplains below the dam; total sediment deposition is less and it is constrained to a narrower band immediately along the active channel. Regulation appears not to change the dynamics of near-channel floodplain sedimentation, but instead focuses deposition across a narrower band of the floodplain due to the compression of the differences in size between small and large floods and their corresponding floodplains. In other words, rates of overbank deposition at both our regulated and unregulated sites are principally controlled by inundation frequency regardless of the degree of flow regulation. The combination of hydrologic, geomorphic, geochemical, and ecological approaches reveals the pervasive effects of flow regulation. The limited spatial extent, flow magnitude, and flow duration combine to produce a narrower band of nutrient and sediment exchanges and an irregular mosaic of floodplain forest communities. Moreover, these results have important broader impacts as they reveal that effective river restoration and management can be scientifically assessed, ultimately leading to better policy formulation.
