A model of the circulation and quality of water in the Duwamish River estuary has been sufficiently developed to allow prediction of the effects of a proposed widening and deepening of waterways on residence time and dissolved oxygen in the estuary's salt wedge. For a low river-discharge period in August 1970, use of the model yielded an estimated residence time of wedge water to be 6.3 days in the present waterways estuary and 8.6 days in the wider and deeper proposed-waterways estuary--a 37-percent increase. June-September 1970 and for the estuary about 4 miles upstream from its mouth, the model estimates indicate that dissolved-oxygen values in the wedge of the proposed-waterways estuary would be as much as 1.4 milligrams per liter lower and would average 0.4 milligram per liter lower than (average difference significant at 95-percent confidence level) dissolved-oxygen values in the wedge of the present-waterways estuary.
Extrapolation to low dissolved-oxygen values of a regression between the predicted dissolved oxygen for both the proposed and present-waterways estuaries suggests that 4 miles upstream of the estuary mouth oxygen would be completely depleted from the proposed-waterways estuary wedge whereas there still would be 0.2 milligram of oxygen per liter of water in the wedge of the present-waterways estuary.
