Genetically Distinct Cutthroat Trout Populations above Tributary Waterfalls of the Clearwater River, Western Washington – a New Dating Tool for Landscape Evolution

An emerging field in earth science is the use of molecular genetic clocks to constrain the timing and pace of Late Cenozoic tectonic and landscape evolution. All eukaryote populations accumulate mutations in their mitochondrial DNA (mtDNA). Variations in mtDNA observed between geographically separated populations of the same species are exploited to determine the duration of genetic isolation. In the Clearwater River basin, located on the western flank of the actively uplifting Olympic Mountains, small populations of cutthroat trout (Oncorhynchus clarki clarki) are physically isolated from each other and the main river above ~10 m-high tributary waterfalls on Kunamakst (KC) and Manor (MC) Creeks. The top of both waterfalls are concordant in elevation with a prominent bedrock fluvial terrace strath (Qt2) that Pazzaglia & Brandon (2001) correlate to Marine Isotope Stage (MIS) 6 at 160 ± 30 Ka. Incision of the Clearwater River through the Qt2 strath and the formation of the waterfalls, likely occurred during the MIS 5 interglacial (130 to 80 Ka).

Adipose fin tissue was collected from individuals above both knickpoints for mtDNA sequencing. Preliminary analyses of 550 (out of 1140) base pairs of the cytochrome B gene from 3 individuals (KC = 2; MC = 1) reveals 0.18% divergence in mtDNA. Salmonid-specific mtDNA divergence rates are calibrated to 1 ± 0.25% per Ma (Smith, 1992, Syst. Biol.). By this measure, the Clearwater populations have been isolated for ~180 Ka, remarkably congruent with the geologically-estimated terrace age, especially considering potential error sources. Additional analyses of divergence will allow for a species-specific calibration of the mtDNA clock for this common western North American species. The persistence of isolated populations in small basins (<15 km²) with steep channel gradients (0.03 – 0.06) and landslide-dominated hillslopes for > 10⁵ years is a testament to the adaptive ability and survivability of O. clarki.