If I remember my Washington state history correct It takes some where around 50-60 years to flush out the Hood canal. Where as it takes 20 years for the Puget sound to recirculate it's water. The Causes for the low O2 is the number of houses that have leaking septic tanks. The increased bio levels leads to bigger algae blooms that die off and burn off the oxygen. The state needs to crack down on home owners that don't repair these deffective septic tanks and start to limmit how close to water they can build.