Thursday, August 18, 2016

Naselle River Notes

I was digging through some historic sources on the Naselle River area in southwest Washington and spotted this classic tree-like drainage pattern within a farm field.

2013 (USGS)

The field is protected from salt water inundation by a dike along the river bank which presents tidal water from flooding the fields. But this is a wet area and with low elevation and very shallow groundwater the filed id covered with drainage patterns consistent with a tidal marsh.

This particular field has not changes much over the aerial photographic period>

1953 (USGS)

These lands were settled fairly early by American settlers. Fishing, oysters, calms and timber with ready access to water for transport attracted early resource extractors. First nations peoples (Chinooks) had already used these bays and inlets for oyster gathering and trade. The farming soon followed with dikes and flood gates converting the rich alluvial estuary soils to pasture.

Investigations by Brian Atwater starting in the 1980s identified drowned forests along tidal areas of Willapa Bay and in the tidal areas of the Naselle River. These areas had been higher in elevation and then subsided during a large Cascadia subduction zone earthquake. The trees were killed by the flooding of the forest with salt water. Tsunami related sediments are present along many of these tidal areas. Post earthquake, alluvial sediment from floods has also covered these low lying areas with a layer of silt and mud.  Excellent soil if one can keep the saltwater out. 

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