1) In earliest Holocene, sea level was low and delta existed seaward of its present position.

2) Sea-level rise caused delta to retreat up the marine embayment. Stream channel meandered across valley floor.

3) Osceola Mudflow inundated river valley at 5700 yr B.P. Braided stream channels developed in response to massive sediment load. Delta rapidly prograded down narrow valley.

4) Sediment load diminished and meandering channels re-developed. Progradation rate of delta decreased.

5) Moderate eruptions at ~2300, 1100, and 500 yr B.P. produced lahar assemblages in valley. Delta rapidly prograded again.

6) At present, sediment load is relatively low. Meandering channel is re-established. Sea level is rising slowly and delta front lies near the maximum extent of the marine embayment.

Block diagram depicting the idealized structure of Holocene deltas in southern Puget Sound, based on the Duwamish River delta. Seattle fault is blind. Uplifted estuarine deposits are known from Duwamish but not from Puyallup or Nisqually deltas. Large vertical exaggeration (approximately 20X), not drawn to scale.

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