POSSIBLE GLACIAL MORAINES AND GRABEN ON THE FLOOR OF WALLOWA LAKE, NE OREGON

24 depth sounding profiles of the floor of Wallowa Lake in Northeast Oregon were collected during July and August 2002 using an Odom Hydrotrac depth sounder. The profiles confirmed earlier mapping by Portland State University that showed that the lake basin is divided into a southern subbasin with a maximum depth of ~90 m and a shallower (~38 m deep) subbasin to the north. The profiles revealed two west-northwest-trending underwater ridges between the south and north benches. Both ridges are compound topographic features with reliefs of up to ~6 m and are covered with boulders (erratics) and gravel. The ridges can be traced out of the water to subtle topographic benches on the late Wisconsin glacial moraine on the east side of the lake. This evidence suggests that the ridges are glacial moraines younger than those previously documented in the Wallowa Lake area. An ~0.5 km wide flat-floored graben was discovered in the deepest part of the southern lake basin. The graben is bounded by two faults which are marked in places by shallow depressions. These faults trend in a N60°E direction across the lake floor. Sediment collected with a pipe dredge in August 2002 from the lake bottom within the graben consists of silt with abundant wood fragments. This may be the product of deposition by the July 18, 2002 flash flood that diverted the Wallowa River and filled the lake with debris.