Geomorphic response of the Souhegan River to the removal of the Merrimack Village Dam (Invited)

The Souhegan River is a tributary of the Merrimack River that drains a 568 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), demolished and removed starting on August 6, 2008. The MVD was built in 1906 at a location where various dams have existed since the 18th century. Based on a pre-removal sediment-thickness survey, the MVD impounded at least 62,000 m3 of sediment, mostly sand. Analyses of topography, historical maps, and photographs suggest that approximately twice the area of the modern impoundment has been affected by over 200 years of damming at the site. We use monumented cross sections, longitudinal profiles, repeat photography, and sediment samples to document the response of the Souhegan River to the removal of the MVD. A base level drop of 3.9 m caused immediate incision of the sand-sized sediment, followed by channel widening. The former impoundment later segmented into a non-alluvial, bedrock and boulder controlled reach, and a quasi-alluvial sand and gravel reach with erosion and deposition modulated by vegetation on the channel banks. One year after the removal, the Souhegan River had excavated 40,300 m3 (65%) of sediment from the modern impoundment. Two years after the removal, two high-magnitude floods excavated another 10,600 m3 for a total of 50,900 m3 (82%) of sediment from the modern impoundment. The response of the Souhegan River was rapid and the channel and floodplain continue to evolve toward a quasi-equilibrium configuration. Continued response will be substantially influenced by the establishment of bank vegetation within the former impoundment and the magnitude and frequency of high discharge events. We explore implications of our findings in this sand-filled impoundment for future dam removals.