Decadal-scale evolution of the 2006 Suncook River avulsion, New Hampshire, USA

Abstract

Avulsions are rare events that cause major realignments to fluvial systems and hold potential for significant societal consequences. We present a case study of an avulsion in a minimally-aggrading landscape that was influenced by long-term human modifications to a fluvial environment that was initially setup for an avulsion during regional Pleistocene deglaciation. The 2006 Suncook River avulsion in New Hampshire, USA, is a valuable example because minimal environmental management took place for a decade following the event, allowing natural processes to dominate initial fluvial responses. Geospatial data collected during this period allow for the evaluation of the short- and long-term morphological evolution of a river that experienced an avulsion. We review data from technical reports on the event and present a new synthesis of aerial imagery and topographic data highlighting key geomorphic features associated with the avulsion. These features include headcutting and widening associated with a 0.6-km long channel created by the avulsion. Downstream of the avulsion site, extensive channel filling, deposition of meter-scale sand sheets on floodplain surfaces, and development of a cutoff channel were major manifestations of the event. The initial setup for the avulsion can be traced to Pleistocene deglaciation, when glaciofluvial ridges deposited in the valley blocked a more direct route for the river and shifted the channel towards an area of shallow bedrock. Human modifications to the channel—including mill dam construction, floodplain narrowing, road development, and gravel mining—contributed to the avulsion setup by increasing superelevation and undermining the glaciofluvial deposits that had directed the river during the Holocene. The avulsion was triggered by a ~50-yr flood event following a 70-yr interval with no major floods but marked by continued human modification to the landscape. The unusual nature of the event combined with the wealth of available geospatial and hydrologic data make the Suncook River system an attractive target for future studies that can examine avulsion processes in greater detail.