TERRACE GENESIS AND FLUVIAL INCISION OVER PLEISTOCENE TIME SCALES, CLEARWATER RIVER, OLYMPIC MOUNTAINS, WASHINGTON

Six late Quaternary river terraces, preserved along the Clearwater River in northwestern Washington State, provide a ~140 ka record of long-term incision and uplift across the western side of the Cascadia forearc high. Terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio-fluvial deposits and the global eustatic curve. The terraces overlie flat bedrock surfaces, called straths, which represent uplifted segments of the river channel. Bedrock incision is measured by the height of a strath relative to the adjacent modern river channel. The straths along the Clearwater show an upstream increase in bedrock incision, ranging from ~ 0 at the coast to a maximum of 110 m in the headwaters. The incision at any point along the profile increases systematically with strath age. The calculated incision rates, ranging from < 0.1 m/k.y. at the coast, to ~0.9 m/k.y. in the central massif of the Olympic Mountains, are in close agreement with published long-term erosion rates estimated from thermochronologic data. Upstream divergence of terraces is best explained by an increase in the rate of rock uplift from the coast towards the central part of the range. The Clearwater terraces influence our views on terrace genesis over Pleistocene time scales in tectonically active landscapes which both share and differ from previously published results in other, tectonically active settings. The Clearwater River forms straths while alpine glaciers are advancing in adjacent drainages. In turn, the straths are buried during maximum glaciation and during the ensuing deglaciation of alpine valleys as upstream increases in sediment supply and downstream eustatic rise create accommodation space in the Clearwater valley and across the continental shelf as well. The fluvial system shows strong forcing by the glacial-interglacial climate cycle. Even so, the river appears to have returned to the same valley profile during each cycle of strath cutting. Thus, bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle (<105 yr.), but appears to be relatively steady when averaged over longer time scales.