Hood Canal is located in western Washington State and is part of the Puget Sound water body. The depths of the canal vary between a depth of 175m and 40m or less. A 180 degree bend called the Great Bend and other geographical features affect the water flow in and out of Hood Canal. Historically the shape and geography of Hood Canal produces periods of low oxygen levels.
Dissolved oxygen from the Great Bend to the end of the canal are the lowest in Hood Canal, and over the years the level of oxygen have been decreasing. Low dissolved oxygen levels are alarming because they can cause fish kills. Low oxygen is caused by eutrophication, a process where organisms in the water use up the oxygen. An abundance of nitrogen lead to an abundance in oxygen using organisms. According to a USGS report on Hood Canal, identified sources of nitrogen are: leaking septic tanks, overuse of chemical fertilizers, animal waste and improper waste discharge from boats and water vessels. As population and human activity increase in the Hood Canal area, the oxygen levels are consistently lower and fish kills are occurring more frequently.
Source of information: http://wa.water.usgs.gov/projects/hoodcanal/data/HC.pdf