The 1300-km-long California coast receives output from watersheds that range from ∼10 km2 within the coastal mountain ranges to ∼120 000 km2 for the drainage basins of the Sacramento and San Joaquin Rivers. Results from a combination of field and remote-sensing data, modeling, and digital techniques show that during the El Niño winter of 1998, the impact of sediment and water transport on nearshore waters from coastal watersheds was spatially extensive, varied with depth, and was comparable to the impact of output from large rivers. The mass of sediment estimated from SeaWiFS data for 25 discrete and coalesced buoyant river plumes emanating from 110 watersheds was 2.2 × 106 t. Although the mass of all of the plumes represents only 1%–2% of the total sediment output from the rivers, the plumes covered an area of 29 500 km2 of coastal waters. An important cautionary note is that the surface plumes visible in the remote-sensing data (even though nearly coincident with flooding) do not represent mass output from the rivers.

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