|Volcanic debris flows (lahars), earthquakes and submarine landslides have influenced the development of Holocene deltas at Puget Sound, Washington. We used ground-penetrating radar (GPR), seismic reflection, vibracores, soil borings, and outcrops to examine the geology of the Duwamish, Puyallup, and Nisqually River deltas, and to infer how the deltas will respond to similar events in the future. Fed by abundant sediment supply from Mount Rainier, and confined in long, narrow valleys, these deltas have grown seaward since the middle Holocene at average rates of 2.5-9.0 m/yr.
The deltas apparently did not prograde at a regular pace, instead the seaward advance was punctuated by infrequent but large pulses of volcanoclastic sediment. Pumice-rich sand fills paleochannels on the valley floors and indicates that lahars have repeatedly altered the course of rivers. Wood fragments in two channel-fill deposits gave ages similar to lahar deposits upstream. Fault zones underlie at least two of the deltas and have been active in the Holocene, as evidenced by uplifted estuarine deposits (Duwamish) and subsided coastal forests (Nisqually). Intrusive and extrusive sand record liquefaction caused by ground shaking.
The geologically young, loosely-consolidated deposits are also at risk from submarine landslides, especially along the seaward-dipping delta fronts that slope 10-20 degrees. Relatively small landslides have repeatedly struck the Puyallup delta and caused damage to the Port of Tacoma. Geomorphic evidence suggests that a much larger failure occurred on the Nisqually delta in the past, possibly triggered by an earthquake. The record of landslides on the Duwamish delta is unclear and may be obscured due to high rates of sediment accumulation and burial of any existing failure zones.