River response to dam removal

Abstract

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, the Merrimack Village Dam (MVD), was demolished and removed in August and September 2008. The modern MVD impoundment contained at least 62,000 m3 of sediment, mostly sand. Analysis of topographic and historical maps, and photographs suggests that approximately twice the area of what is now the modern impoundment has been affected by over 200 years of damming at the site. I use repeat surveys of cross sections and the river longitudinal profile, and sediment samples, to document the response of the Souhegan River to the MVD removal. A base level drop of 3.9 m caused immediate incision of the sand-sized sediment and channel widening. The 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 the presence of vegetation on the channel banks. One year after the removal, the Souhegan River has excavated 38,100 m3 (65%) of the sediment in 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 vegetation within the former impoundment and the magnitude and frequency of high discharge events.