Coastal and Terrestrial Impact of the Isostatically Forced Late Quaternary Drainage Divide Shift, Penobscot and Kennebec Rivers, Maine, USA

Abstract

Marine features associated with Late Quaternary isostatic adjustment are well documented in central and coastal Maine, United States. However, the effects of glacially induced land-surface tilting in the interior of the state and its affects on coastal landforms are not well understood. Investigations at Moosehead Lake, Maine's largest lake, indicates that isostatically forced tilting shifted the lake's outlet from the Penobscot River drainage to the Kennebec River drainage, resulting in a significant effect on marine delta formation through changes in the discharge and competence of the Kennebec and Penobscot Rivers, two of the largest in the region. Historic stream-gage data are used as an analogue to quantify the effect of the outlet shift on the relative discharge of these two rivers. These data suggest that, in the upper portions of the watersheds, the Penobscot River lost approximately 25% of its annual discharge, while the Kennebec River experienced an approximately 74% increase in annual discharge. The timing of this change, established in the abandoned lake outlet into the Penobscot River, coincides with the end of deposition of the Penobscot paleodelta in Penobscot Bay and the onset of deposition of the western lobe of the Kennebec paleodelta. This indicates that changing river discharges were also linked to changes in sediment loads at the rivers' mouths. In addition, these findings imply that coarse-grained fluvial deposits (North Anson Formation) identified in the upper Kennebec valley may be the result of the rapid and large increase in river discharge associated with the divide shift. Recognition of both marine and fluvial features associated with isostatic adjustment emphasizes the importance of this process on early postglacial landscape formation.