MID-HOLOCENE EARTHQUAKE-TRIGGERED LANDSLIDES CHANGED DRAINAGE PATTERS AND FORCED GENETIC DRIFT IN A POPULATION OF RAINBOW TROUT AT LAKE CRESCENT ON THE OLYMPIC PENINSULA, WASHINGTON STATE

Lake Crescent, a glacially carved 180-m-deep lake in NW Washington State overlies the Lake Creek-Boundary Creek fault zone with at least 56 km of post-glacial surface rupture. Seismic reflection and piston core investigation of the lake’s sediment reveals evidence that the fault beneath the lake ruptured four times in the past ~7200 years, producing unusu­ally thick coseismic deposits termed megaturbidites. A complex of several large rockslides descending the valley walls at the eastern end of Lake Crescent resulted in the separation of a larger, ancestral water body into two lakes—the modern lakes Crescent and Sutherland. Prior to the separation of the lake, ancestral drainage occurred along Indian Creek and the Elwha River, both accessible to anadromous fish. Rockslide blockage of the ancestral drainage of Lake Crescent caused its water level to increase by 24m until the lake overtopped a low divide connecting it with the Lyre River, which flows north into the Strait of Juan de Fuca, but is impassible to anadromous fish below the lake because of a 10m-high waterfall. Three lines of evidence indicate that the lake-separating landslides occurred between 4 to 5 ka; (1) the age of the penultimate megaturbidite at 4 ± 0.1 ka, (2) the 4.3 ± 0.2 ka date from a rooted tree submerged in 18 m of water in Lake Crescent and (3), a 5.1 ± 0.15 ka age of charcoal preserved in the basal sediments that accumulated in a closed depression on one of the lake-separating landslides.

Lake Crescent is home to two endemic fish populations, the Beardslee trout, a rainbow (Oncorhynchus mykiss irideus f. beardsleei), and the Lake Crescent Cutthroat trout (Oncorhynchus clarkii clarki, loc. f.) that both exhibit phenotypic variability from neighboring populations found on the Olympic Peninsula. Brenkman et al. (2014, NW Sci v. 88) document that the Beardslee Trout also exhibits genetic divergence (100% bootstrap support) from 24 Olympic Peninsula O. mykiss populations. We now know that the most distinct population of rainbow trout on the Olympic Peninsula gained its genetic distance from neighboring populations within the last 4 ka as the result of seismically-triggered mass wasting and drainage reorganization and is a powerful example of the importance of tectonics and small founding populations to shape genetic diversity at the landscape scale.