FLUVIAL LANDFORMS OF THE 2006 SUNCOOK RIVER AVULSION, EPSOM, NEW HAMPSHIRE

During a 100-year flood event in May 2006, a 1.5-mile reach of the Suncook River in Epsom, New Hampshire was abandoned in a complete avulsion—a rapid shift in the course of a river. In less than 24 hours, 0.5-miles of new river channel was incised into wetlands and glacial lake sediment. The avulsion shortened the Suncook channel by 0.5 miles and consolidated flow from two channels into one, leading to significant increases in channel gradient and discharge. In addition, the avulsion bypassed a bedrock-floored reach for easily eroded glacial lake sediments. These changes locally increased stream power and led to significant post-avulsion incision. Downstream of the avulsion site, aggradation filled existing channel and deposited sediment on floodplains. This work presents results from detailed GPS- and total-station based field surveys of the area immediately impacted by the avulsion, with an emphasis on characterizing fundamental pre- and post avulsion channel metrics including longitudinal and cross-sectional profiles. We also document fluvial landforms generated during the avulsion event, and quantify the amount of post-avulsion incision. Ephemeral landforms generated during the avulsion include longitudinal bars, abandoned knickpoints, erosional terraces, high water marks, and a sand sheet as much as 1 m thick deposited on floodplains downstream of the avulsion site. Detailed mapping of these landforms provides insight into the causes of the avulsion, and serves as an analog of a landscape created in a rapid, “convulsive” event. Documentation of avulsion-related incision and sand sheet deposition will refine initial estimates of sediment introduced to the Suncook by the event. The avulsion offers a significant challenge to river management. Aggradation downstream of the avulsion site is believed responsible for flood damage in spring 2007 into areas not previously flooded during larger magnitude events. And incision upstream of the avulsion site drives bank erosion, which threatens property and transportation infrastructure. This rare event also provides opportunity to document rapid incision and gain insight into the development of floodplain sedimentary architecture.