RIVER RESPONSE TO DAM REMOVAL: INITIAL RESULTS FROM THE SOUHEGAN RIVER AND THE MERRIMACK VILLAGE DAM, MERRIMACK, NEW HAMPSHIRE

The Souhegan River is a tributary of the Merrimack River that drains a 443 km² watershed in southern New Hampshire. The lowermost barrier on the Souhegan River was the ~4-m high Merrimack Village Dam (MVD, ~500 m upstream of the confluence with the Merrimack River), which was breached and removed starting on August 6, 2008. The MVD impounded at least 62,000 m³ of sediment, mostly sand. After the breach, the Souhegan River immediately began to incise a new channel into these legacy sediments. We use 12 monumented cross-sections, longitudinal profiles, repeat photography, and sediment samples to document the response of the Souhegan River to the removal of the MVD. Our study is part of the first full application of a newly published guide to stream barrier removal monitoring. Prior to dam removal, in August 2007 and June 2008, we surveyed the cross-sections and longitudinal profile, and collected thalweg sediment samples. After removal, in August 2008, we surveyed 11 cross-sections and the longitudinal profile, and collected sediment samples. We surveyed 10 of the cross-sections again in October 2008. Comparison of the August 2008 cross-sections with those from June 2008 showed that, in a 495-m reach upstream of the former location of the MVD, the Souhegan River eroded a relatively narrow channel 0.6 to 3.0 m into the legacy sediments, in some places exposing bedrock and boulders likely from the pre-dam riverbed. Downstream of the former dam location, a sand deposit up to 1.0 to 3.5 m thick prograded almost to the confluence with the Merrimack River. Between the August and October 2008 surveys, upstream of the former dam, the river continued to incise (up to 1.0 m) and began to widen its channel. Downstream, the river began removing the initial pulse of stored sediment, with up to 1.2 m of incision. Our ongoing work in this dynamic system will document and analyze longer-term response of the channel to the removal.