River & Lake Restoration
  Ponca State Park | Blue River | Green River | Lake Stevens

Lake Stevens is the largest recreational lake in Snohomish County.  Declining water quality was recognized as a serious problem over 20 years ago when persistent algal blooms and mats of decaying filamentous algae developed.  Phosphorus was determined to be the limiting nutrient in Lake Stevens and low dissolved oxygen concentrations were hypothesized to be the reason for the decline of the Kokanee fishery.  In the summer of 1988, Tetra Tech began working with the City of Lake Stevens and Snohomish County to assemble the comprehensive water quality plan for the Lake Stevens watershed and a restoration for the lake itself.  The plan was the first of its kind and provided a guideline for long-term water quality protection and enhancement.  The watershed water quality plan was completed in June 1989 and included an inventory of the watershed's surface resources, a catalog of surface water system characteristics, and a plan for reduction of watershed-generated non-point source pollutants. The Lake Stevens restoration was a demonstration in the multi-facetted engineering design capabilities of the Tetra Tech.  Design elements included the design of the world’s largest hypolimnetic aeration system consisting of a ten-inch airline that was 3,000 feet in length and four aerators.  Each aerator is an airlift pump that moves 80 million gallons of water per day for a total of system pumping capacity of 320 million gallons a day.  The aeration system delivers 15,500 kilograms of oxygen per day at an efficiency of 24 to 28 percent.  Other design elements include fish passage through 120-foot box culvert and open channel weirs, design of fish spawning beds, sediment control, and bank stabilization for fisheries habitat enhancement and water quality improvement.  Tetra Tech’s Public Involvement tasks included lake fairs, fact sheets, newsletters, videos, volunteer coordination, and public meetings and workshops.