USING THE CLEARWATER RIVER TO TEST MODELS FOR LONG-TERM INCISION OF BEDROCK RIVERS: KIRK BRYAN AWARD

The incision of rivers in bedrock is thought to be an important factor that influences the evolution of relief in tectonically active orogens. At present, there are at least six competing models for incision of bedrock rivers, but these models have received little quantitative testing. We evaluate these models using observations from the Clearwater River in northwestern Washington State, which crosses the actively rising forearc high of the Cascadia margin. Pazzaglia and Brandon (AJS, 2001) used fluvial terraces along the Clearwater to estimate bedrock incision rates over the last ~150 k.y. They showed that incision rates have been steady over the long-term (>50 k.y.), consistent with other estimates, based on isotopic cooling ages, of steady long-term (>2 m.y.) erosion rates. The steady-state character of the river allows us to use the relatively simple time-invariant solutions for the various incision models and also to estimate long-term sediment discharge along the river, which is a critical variable for some incision models.

An interesting feature of the Clearwater River, and one that is difficult to model, is a downstream decrease in the rate of incision, from ~0.9 mm/yr in the headwater to < 0.1 mm/yr at the coast. None of the incision models, including the popular stream-power model, successfully explain this downstream pattern. Our results echo those of Stock and Montgomery (JGR, 1999), who found that the stream-power model failed to account for the pattern of long-term incision for many bedrock rivers. Our analysis goes further in considering the role of sediment discharge, but we suspect that our formulation of this important process may be too simple. Testing on other rivers is needed to see if any of the current incision models is capable of predicting long-term incision rates.