LANDFORMS OF GLACIAL LAKE ELWHA, OLYMPIC MOUNTAINS, WASHINGTON

The Elwha River is located within Olympic National Park (OLYM) on the Olympic Peninsula of northwestern Washington state. The river drains an area of ~800 km2 making it the largest watershed in the park. During the Vashon stade of the Fraser glaciation, 17,000 cal yr BP, the advancing Cordilleran ice sheet reached its maximum extent after splitting into the Juan de Fuca and Puget lobes as it encountered the Olympic Mountains. The Juan de Fuca lobe dammed north-draining valleys, forming lakes, such as glacial Lake Elwha (GLE). Surface elevation of GLE has been originally estimated at 585 m based on erratic distribution. Our mapping of surficial geology in the watershed has identified multiple lake levels and two potential.

Mapping is completed at 1:24,000 scale and is based on identification of 37 distinct landforms, including floodplains, alluvial fans, debris cones, landslides, terraces and moraines. We have mapped two Juan de Fuca lobe end moraines in the lower Elwha valley near 530 m and 500 m elevations. Due to the fluctuating depths of GLE, valley bottom landforms, such as terraces, debris cones and deltas have been found perched throughout the valley walls of the Elwha basin. The most impressive of these features are large, perched delta systems that formed at the mouths of major tributaries to the Elwha, such as the Goldie, Lost, and Hayes Rivers. The delta systems have a variety of surface elevations (ranging between 335 m and 595 meters), many of which are consistent with surface elevations of terraces and end moraines mapped throughout the valley. These correlations provide general reconstructions of the lake's extent as well as insight into the instability of the ice dams.

Perched deltas, which are old and stable landforms have been identified as important resource management sites. They are typically composed of fine silt and sand, are excessively drained, and support lodgepole pine forests. The Elwha landform map has also identified 70 new landslides and will provide baseline data for soil models and Lake Mills restoration following Glines Canyon Dam removal.