Mountain erosion over 10 yr, 10 k.y., and 10 m.y. time scales

We used cosmogenic Be-10 to measure erosion rates over 10 k.y. time scales at 32 Idaho mountain catchments, ranging from small experimental watersheds (0.2 km(2)) to large river basins (35 000 km(2)), These long-term sediment yields are, on average, 17 times higher than stream sediment fluxes measur ed over 10-84 yr, but are consistent with 10 m.y. erosion rates measured by apatite fission tracks. Our results imply that conventional sediment-yield measurements - even those made over decades - can greatly underestimate lo ng-term average rates of sediment delivery and thus overestimate the life s pans of engineered reservoirs. Our observations also suggest that sediment delivery from mountainous terrain is extremely episodic, sporadically subje cting mountain stream ecosystems to extensive disturbance.