THE ANTHROPOGENIC BATHYMETRY OF NEARSHORE SEATTLE AS SEEN IN A HISTORIC MAP: HOW MUCH SEDIMENT DID EARLY CITY ENGINEERS SLUICE INTO ELLIOTT BAY?

A hydrographic chart produced in 1875 may yield insights into how anthropogenic alterations and natural hazards have influenced modern bathymetry in Elliott Bay, just offshore Seattle. The original depth-sounding map, drafted by the US Coast Survey, reports over 3,000 depth points and predates large-scale demolition and removal of several hills in Seattle. These “regrades” moved at least four million tons of sediment, the majority of which was dumped into Elliott Bay. Comparing the modern bathymetry to that of the relatively unaltered bay from 1875 allows us to map areas where sediment has accumulated in the past one and a half centuries. We have digitized the thousands of depth points of the historic map and use ArcGIS to compare former depths with modern bathymetry. Analyzing the differences between the historic map and modern multibeam bathymetric data may provide evidence for sedimentation, both natural and anthropogenic. These results may clarify the extent, area, and volume of the sediments emplaced via human activity from the engineered regrades. This poorly-consolidated sediment combined with the possibility of earthquakes could generate coastal hazards from sliding and slumping of the spoil deposits. The bathymetry, past and present, fits into a larger project to describe the geologic history of Elliott Bay, including its relation to the nearby Seattle Fault Zone, and provides important geologic and historical context to understand coastal impacts of climate change in Seattle.