Fluvial Sedimentation in Response to Postglacial Uplift and Environmental Change, Missisquoi River, Vermont

Abstract

Three lithologically distinct alluvial units of Holocene age can be distinguished along trenched cross sections of the Missisquoi valley bottom. The oldest is of early Holocene age, and the associated floodplain had aggraded to nearly its present level by 8000 14C yr B.P. At that time, early Archaic projectile points were deposited in a fire hearth 50 cm below the surface. Abandonment of this floodplain was followed by the development of an A-E-Bt soil profile. Accumulation of a younger floodplain had begun by 6400 14C yr B.P. and local sedimentation persisted to ca. 500 14C yr B.P., as indicated by radiocarbon dates of buried woody debris (including large logs) and of charcoal. Alluvium of the modern floodplain began accreting after A.D. 1860 and contains machine-cut square nails, whiteware ceramics, and coal clinker. Previous locations of the river channel can be reconstructed from relict surfaces marked by paleosols, the preserved depositional stratigraphy, and the radiocarbon samples. Immediately after regression of the Champlain Sea from this part of the valley, and before 8000 14C yr B.P., the river incised late Fleistocene marine silts and clays at an average rate of at least 1 m/100 yr. After the interval of downcutting, episodic lateral migration became the dominant process, with the rate varying between 0 and 4 m/100 yr. The early Holocene incision was most likely a lagged response to postglacial crustal rebound, whereas strong soil development and floodplain stability between 8000 and 6400 14C yr B.P. may reflect an independently documented warmer, and perhaps drier, climate in New England at this time. Finally, the post-A.D. 1860 period of active floodplain sedimentation may have been a response to timber clear-cutting, row crop agriculture, and cattle and sheep grazing in the watershed.