HOLOCENE STRATH TERRACES OF THE CLEARWATER RIVER BASIN: INSIGHTS INTO TERRACE GENESIS AND THEIR SUITABILITY FOR DETERMINING LONG-TERM RATES OF DEFORMATION FOR THE OLYMPIC MOUNTAINS, WASHINGTON STATE

The ~ 400 km² Clearwater River basin, located on the west flank of the actively uplifting Olympic Mountains, contains a well-preserved flight of three paired Holocene fluvial strath terraces. Terrace ages determined from 38 ¹⁴C dates range from ~ 9,000 to 11,000 ybp, 4,000 to 8,000 ybp and 0 to 3000 ybp, respectively. We consider two models for strath terrace genesis based on terrace deposit stratigraphy, sedimentology, and age distributions. Under our favored model, terrace ages denote times of enhanced lateral incision (valley bottom widening), strath cutting, and synchronous abandonment of some of the alluvium responsible for the abrasion process. Vertical incision of the channel occurs primarily during the brief (~ 1000 years) intervals when there is no record of terraces. Under an alternative model, straths are carved as the channel incises vertically during the brief periods between dated terrace deposits; terrace ages record subsequent periods of alluviation atop the straths following termination of both lateral and vertical incision processes. Holocene rates of vertical incision along the Clearwater valley increase upstream, from ~ 0.05 – 1 mm/yr in the lower third of the basin to ~ 1 - 2 mm/yr in the medial portions, to ~2 – 3 mm/yr in the upper reaches, mimicking the pattern for Pleistocene terraces in the same basin (Pazzaglia and Brandon, 2001); however, Holocene rates are 2 to 3 times faster. The increase in fluvial incision rates between Holocene and Pleistocene terraces may indicate a Holocene channel in the process of reacquiring its characteristic, interglacial graded concavity and gradient during ongoing tectonic uplift. The increase in Holocene incision rates follows a protracted period when the river could not incise vertically into bedrock because the channel was lifted off of its bedrock base by significant late Pleistocene valley alluviation. These results illustrate how in tectonically active settings, representative rates of rock uplift inferred from studies of river incision should be integrated across at least one glacial-interglacial cycle (> ~ 50 k.y.) and that the steady state orogen concept may not be applicable over short (Holocene) time scales.