"The Effects of Volcanic Debris Flows (Lahars), Earthquakes and Landslides on Holocene Deltas at Puget Sound, Washington: Nisqually River Delta

The Effects of Volcanic Debris Flows (Lahars), Earthquakes and Landslides on Holocene Deltas at Puget Sound, Washington


	The Nisqually River delta is in a relatively natural condition with almost 10 km2 of subaerial and intertidal wetlands preserved in a wildlife refuge. Distributary channels discharge into a constricted passage where strong tidal currents sweep the delta front. Bathymetric charts depict an irregular seafloor with small, closed depressions along the toe of the delta. A submarine terrace at a depth of -64 m may represent an older delta that formed when sea level was lower in the early Holocene.

Hummocks on the delta front may represent: 1) an imbricated series of large slide blocks, or 2) large bedforms formed by strong currents in Nisqually Reach. Both explanations are plausible. Seismic reflection profile across the toe of Nisqually River delta (see map at top for location).

Click on figure to view a larger image (84K).

Evidence for earthquake-induced subsidence on the east side of Nisqually River delta. Low marsh peat buries in-situ stumps of Douglas fir. Radiocarbon dates on the stumps and Triglochin maritima rhizomes indicate submergence occurred between 760-1270 cal. yr. B.P.

For locations of GPR profile, seismic profile, and photograph, see map above.

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Department of the Interior U.S. Geological Survey Geologic Division Coastal & Marine Geology Program Western Region Coastal & Marine Geology