SEDIMENT STORAGE BEHIND DAMS IN NEW ENGLAND RIVERS: PRESENCE OR ABSENCE, PERSISTENCE, AND IMPLICATIONS

More than ten thousand dams have been built on New England rivers over the past four centuries. Many of these do not store sediment in their reservoirs, because small dams have minor influence on channel hydraulic conditions. In formerly glaciated New England, lack of sediment supply upstream is another common reason for the absence of dam sedimentation. The presence of natural sediment traps throughout watersheds, such as lakes and wetlands, further limits the effects that dams can have on channel morphology. Where supply exists, typically from eroding glacial deposits upstream, reservoirs may infill with sediments. The base level rise caused by dams can also lead to accelerated overbank sedimentation on floodplain surfaces adjacent to reservoirs. We use ten years of monitoring after the removal of the Merrimack Village Dam on the Souhegan River in New Hampshire as an example of the response to dam removal. Initial erosion within the reservoir was extremely rapid with 50% of the impounded sediment removed within two months after the dam removal. After this phase of channel incision followed by widening, the ongoing response depended on large floods that can access sediment stored outside of the newly formed channel. Little channel change occurred during the period from 4-10 years after removal, which included no major flood events. However, considerable quantities of dam-related sediment remain stored in valley-bottom terraces, outside of the former reservoir. This legacy sediment will likely persist for decades to centuries and represents a change to valley-bottom morphology locally. While the generally sediment-supply-limited state of many New England rivers means that sediment management after dam removal can be relatively straightforward in many settings (as long as any stored sediment is uncontaminated), the implications of legacy sediment for restoration efforts need to be considered.