HIGH-RESOLUTION MAPPING OF PROGLACIAL LAKE AND MARINE WATER BODIES IN THE CHAMPLAIN AND ST. LAWRENCE LOWLANDS USING LIDAR ELEVATION AND HILLSHADE MODELS

The extent of late Pleistocene proglacial lakes Coveville and Fort Ann and the Champlain Sea are marked by deltaic sandplains and beach ridges in the Champlain Valley of northeastern New York. Pleistocene waterbody extents are well documented by conventional mapping at scales of 1:62,500 or smaller. In this study we use high resolution LiDAR elevation and hillshade models to improve the delineation of former shorelines and update estimates of post-glacial isostatic uplift in the region. Deltaic sandplains at the mouths of the largest rivers are generally well defined at map scales of 1:24,000 to 1:62,000 and provide reliable information for shoreline reconstructions. We found that LiDAR models were particularly useful in identifying subtle beach deposits and ridges, which are often difficult to identify and trace in the field. These features provide important shoreline information where deltaic deposits are absent.

To date we have identified more than 1400 beach ridges in the Champlain and St. Lawrence valleys, most of which were not previously identified as discrete ridges on published maps. Work in the St. Lawrence Valley is ongoing. Our preliminary results in the Champlain Valley indicate that Coveville beach ridges are poorly preserved. Fort Ann beach ridges are bracketed by a poorly defined upper Fort Ann level to a well-defined lower Fort Ann level. We interpret this to be the result of gradual incision of about 27 m of the Fort Ann threshold to a stable bedrock threshold. Water level dropped rapidly about 50 m to a well-defined upper marine limit of the Champlain Sea. Northward strandline elevation profiles yielded slopes of 0.87 m/km for the stable (lower) Fort Ann level and 0.92 m/km for the upper marine limit. These values are slightly higher than those of published estimates derived largely from deltaic sandplains and may indicate that beach ridge data provide a more reliable indicator of post-glacial isostatic rebound gradients in the Champlain Valley.