GEOPHYSICAL INVESTIGATIONS OF THE COLD RIVER VALLEY, SOUTH ACWORTH, NH

The Cold River, in southwestern New Hampshire, is a southwest flowing tributary of the Connecticut River. Previous studies of landslide exposures reveal a pre-advance to post-glacial Late Wisconsinan stratigraphy. A date obtained from an abandoned meander along the floodplain established that locally the river adjusted to its current elevation around 7480±44 cal BP. However, little is known of the stratigraphy beneath the current floodplain, or depth to bedrock. Borings by the Department of Transportation within the valley reveal a possible complex subsurface stratigraphy that could be pre-Late Wisconsinan through Holocene in age.

To define the subsurface stratigraphy, this study employed seismic refraction and electrical resistivity. Data were collected along linear transects parallel and perpendicular to the current river channel on the flood plain and a higher fluvial terrace. The seismic refraction and electrical resistivity surveys used 12 geophones with a 6 metre spacing and 24 metal probes with a 5 metre spacing respectively; the spacing controlling the measured depth of each survey. The results obtained from each method were compared to locally known deposits.

The initial values from both methods distinguished two layered units, cut by both shallow and deep paleochannels, resting on bedrock ranging from 15-17 metres in depth. However, due to several velocity inversions, the exact number of units could not be determined. The upper unit, approximately 6 metres thick, has resistivity and velocity values ranging from 700-2500 ohm.m and 700-800 m/s respectively. The second unit, approximately 9 metres thick, has resistivity and velocity values ranging from 200-300 ohm.m and 1700-1900 m/s respectively. The interpreted alluvial fill in paleochannels, which are laterally discontinuous, has resistivity values ranging from 300-600 ohm.m.

In conclusion, this investigation encountered at least 2 layered stratigraphic units, cut by paleochannels, above a 15-17 metre-deep bedrock valley floor. The stratigraphy observed ranges from pre-Late Wisconsinan into the Holocene. If deeper paleochannels prove to be Holocene then rapid post-glacial down-cutting followed by a sudden increase in base level and aggradation would be required to place the river at its current level.