Northwest Fisheries Science Center

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Document Type: Report
Center: NWFSC
Document ID: 8956
Title: Aquatic baseline assessment of the environs of Lake Washington at Sand Pointwith predictions on the effects of developing a vessel docking center
Author/Editor: Benjamin G. Patten, Willis E. McConnaha, Lucinda Callahan
Year: 1976
Institution: Northwest Fisheries Science Center. Seattle

A portion of Sand Point, Lake Washington, Washington, has been acquired by the National Oceanographic and Atmospheric Administration and will be developed into a regional headquarters.  Developmental plans include the construction of a docking center.  This report describes the physical and biological environment of the lake from a 1–year study within the 50–ft contour; effects of development are also considered.

The bottom of the lake off the south and north part of Sand Point has a more gradual slope than is characteristic of the general body of the lake, where sharp drops to 90 ft or more are normal.  Substrate is of a compacted base of glacial till covered by sediments to 6 inches in depth.  The lake edge wash zone is of sand and gravel.  Toxic materials in the sediments are at low concentrations and should not affect aquatic life.  Wind and convection currents maintain the lake in constant motion, creating strong currents(greater than 1+ mph) at times.  Movement of drogues show that current direction frequently varies with that of the major driving force—the wind.

Important macroscopic organisms were identified and their seasonal abundances determined by broad categories of ecological association.  The aquatic biota within the 50–ft contour of Sand Point has an annual cycle of abundance where biomass is maximal from May to September.  This period encompassed the abundance maximals for fish, zooplankton, crayfish, and plants.

Direct effects of dredging will, in general, be least at the time of year when biomass is minimal.  This is during November to March.  Within this period, the least biological effects will probably take place from January to March.  Suspended sediments from the glacial till bottom at concentrations of 37,000 to 5,000 ppm (observed range for each test) did not cause significant direct effects on the fish, crayfish, or caddis larvae tested during 96–h bioassays.

The long-term effect of dredging, constructing piers and operating ships may change the productivity of the affected area.  The shallows at 10 ft are five times more productive in observed numerical biomass of benthos than the 30–ft zone.  Increasing the bottom depth to 30 ft may permanently lower the biological productivity of the affected area.  Shading of the water from the construction of piers will reduce the energy input into the lake by less than 0.0001%.  Pilings will support epiphytic life, and their physical presence should attract certain species, especially fish.  Prop wash from vessels could continuously disrupt the bottom thereby affecting productivity.