MECHANISMS OF EROSION AND AVULSION OF THE COLD RIVER IN RESPONSE TO CLIMATE CHANGE

The Cold River (Ad 102 mi²⁾in southwestern New Hampshire flows 22 miles southwest from its source at Crescent Lake, (elev. 1213ft) through parts of Cheshire and Sullivan counties, discharging into the Connecticut River (elev. 774ft). In recent years, observations by local residents indicate that the channel of the Cold River has been shifting and widening. This study attempts to document those changes, understand the driving causes, define the river’s current morphology for future studies, and predict adjustments that may be expected as precipitation increases.

The study area covers a half-mile, mid-basin stretch of river that includes two meanders separated by a .3 mile straight stretch. The river hugs the steep eastern valley wall, composed of 30-100 feet of glacial till and lacustrine sediments. Over a 10 month period, mechanisms of erosion and avulsion were observed as well as evidence of past channel adjustments. To evaluate the river’s hydraulic geometry and classification, profiles across the river at six locations, and along its length were completed, and bedload composition quantified. Discharge from gages along the river, and from neighboring streams were evaluated to determine whether the observed channel changes might be resulting from increasing runoff related to climate change.

Records from stream gages indicate an increase in flow over the last 40 years. In addition, the intensity of storm events is increasing. In response, the Cold River is widening along straight stretches and developing a more anastomosing pattern along meander bends. Mechanisms of channel avulsion include damming induced by ice, beaver, slow moving debris flows, as well as trees introduced by undercutting and debris slides. Deflected flow escapes the channel and re-enters downstream where knickpoint erosion progressively creates new channels after repeated re-occupation. Slow moving but increasingly prevalent debris flows are progressively shifting the channel eastward, while deposition of coarse tributary flood deposits are locally forcing the channel westward. However, the bouldery bed of the river inhibits scouring and promotes channel widening and bank undercutting. Channel avulsion and widening of the Cold River will most likely continue if precipitation and the intensity of runoff events continue to increase.