GEOMORPHIC RESPONSE TO RAPID FLUVIAL CHANGE: THE 2006 SUNCOOK RIVER AVULSION, EPSOM, NEW HAMPSHIRE

100-year floods in May 2006 triggered a local avulsion—rapid migration of river channel— in the Suncook River of southeastern New Hampshire. In less than 24 hours, 2.4-km of channel was abandoned, and 0.8-km of new channel incised through wetlands and glacial lake sediments not previously mapped as floodplain. Analysis of digital elevation models, air photos, GPS data, ground surveys, and eyewitness accounts were employed to assess the conditions surrounding the avulsion and its aftermath. A confluence of natural and human factors triggered the event. Near the capture point, the pre-existing channel made a 180-degree turn into a reach impounded by a small dam and floored by resistant bedrock. In contrast, the new channel crosses a substrate of wetlands and unconsolidated glacial lake sediments. Sand and gravel mining in glacial deposits compromised a low ridge that acted as a natural levee and once prevented the Suncook from taking a direct route through the area now occupied by new channel. Aggregate extraction extended the 100-year floodplain and allowed floodwaters to form a knickpoint, which rapidly migrated headward through unconsolidated substrate and captured the Suncook channel. The avulsion shortened the overall course of the Suncook by 0.8-km, consolidated flow from two channels into one, and increased local gradient by as much as 50%. These changes drive ongoing rapid incision at the site, with over 3-m documented at the capture point since May 2006. Incision generated ephemeral landforms such as terraces and lateral bars, and introduced as much as 100,000 cubic meters of sediment to the river. Significant aggradation occurred downstream of the new channel, and floodplains were locally covered with a sand sheet up to 1-m thick. Aggradation exacerbated spring 2007 floods and led to channel instability downstream of the avulsion site. Continued study of this natural experiment in channel migration will yield further insight into avulsion dynamics, rapid fluvial incision, and the sedimentary architecture of avulsion deposits. At this same time, the avulsion and continued fluvial response poses a unique challenge to river management and restoration.