LATE-QUATERNARY SEDIMENTATION IN ANDROSCOGGIN LAKE AND ITS OUTLET DELTA, LEEDS/WAYNE, MAINE

Androscoggin Lake is a relatively large (~15 km2) and shallow (mean depth <5 m) lake in central Maine that displays elevated levels of phosphorus in the water, as well as persistent bioaccumulative toxins in fish and birds relative to surrounding lakes. During periods of low runoff, Androscoggin Lake drains through the slow-flowing Dead River, which descends about 1.5 meters along its 11-km-long course to the Androscoggin River. When the Androscoggin River floods, its elevated stage causes a reversal of flow in the Dead River, driving sediment-laden water into the shallow western side of the lake. Over millennia, these backfloods have created a narrow, elongate “reverse” or “outlet” delta that protrudes 5 km into the lake and is home to rare floodplain forest and wetland ecosystems.

A 3.5-kHz CHIRP sonar system was used to investigate sediments within the lake. Acoustic blanking caused by gas in the uppermost sediment, derived from the decomposition of organic matter, underlies ~30% of the area surveyed, especially near the delta and in the deep lake basin. In gas-free areas, an acoustically well-stratified unit up to 20 m thick, interpreted as glacial-marine sediment of the Presumpscot Formation, drapes a high-relief bedrock surface. A second unit, generally flat-lying with weak acoustic stratification, overlies glacial sediments, usually above a distinct angular unconformity. This unit is interpreted as Holocene lake deposits and is generally <3 meters thick.

Three vibracores 3-4 meters in length were taken in marshy areas of the delta, one penetrated only Holocene deposits while two contain a sharp contact with the Presumpscot Formation. Delta sediments generally comprise massive-to weakly- stratified sandy mud, with detrital and in situ organic material. Shifts to more-organic wetland deposits in the upper ~30 cm of all three cores suggest a lake-level increase, consistent with apparent shrinkage of the delta as seen with a 1912 topographic map compared to the present. This water-level rise is tentatively attributed to a pollution-control dam built on the Dead River in the 1930s. Results of ongoing sediment and metals analyses of the cores will also be presented.