IMAGE ANALYSIS OF RECENT COASTAL EROSION AND THERMOKARST LAKE DRAINAGE ALONG A PERMAFROST COASTLINE IN NORTHERN ALASKA

Rates of coastal erosion along the Beaufort Sea coast north of Teshekpuk Lake in the National Petroleum Reserve in Alaska (NPRA) have been documented by digital image analysis of topographic maps and Landsat Thematic Mapper (TM) data that span 1955-2005. The 2000 km² study area hosts endangered and threatened species of waterfowl, is the calving grounds for large herds of caribou and contains potentially significant petroleum resources.

Patterns of coastal land loss indicate that the dominant process of coastal erosion is wave undercutting and block collapse of mud-rich permafrost sediments. Some parts of the study area have undergone up to 0.9 km of shoreline retreat over the last 50 years. Land loss attributed to coastal erosion more than doubled from 0.48 km² per year during 1955-1985 to 1.08 km² per year during 1985-2005. The increase in coastal erosion rates may be the result of earlier seasonal melting of pack ice resulting in longer periods of open water and wave action along the coastline.

Coastal plain thermokarst lakes are susceptible to coastal erosion. In one example, topographic maps compiled in 1955 illustrate an intact lake. The 1985 TM data show that the lake has been breached by coastal erosion and is almost entirely drained. The 2005 TM data show that approximately 70% of the former lakebed that was exposed in 1985 has been inundated in 2005, presumably by marine water. Thus, thermokarst lakes breached by coastal erosion undergo drainage, followed by flooding of warmer marine water that melts permafrost and promotes inundation of the drained lakebed. Digital elevation models indicate that most coastal thermokarst lakes in study area have water elevations of 0.5 m to 4.0 m and will drain into the Beaufort Sea if breached by coastal erosion.

Coastal erosion and breaching of thermokarst lakes are changing the ecosystem and landscape of the coastal plain north of Teshekpuk Lake. The breaching of coastal-plain lakes and evolution of those thermokarst depressions into marine-influenced or fully marine bays promote the replacement of fresh-water flora and fauna with marine-influenced flora and fauna. These dynamic and interactive processes will need to be incorporated into informed decision-making as this fragile area is opened to petroleum exploration in coming years.