CAPITOL LAKE RESTORATION PROJECT

Abstract

ABSTRACT Capitol Lake was created by the construction of a roadway fill and tide gate dam structure across the Deschutes River in 1951. The lake, in conjunction with the state capitol, has become a valuable asset to the state of Washington as a tourist attraction, as a recreational area, and as one of Washington's most important anadromous fish-breeding facilities. In 1981, two fishkills were observed near the tide gate dam. Subsequent investigations by the Washington State Department of Ecology identified a deep, chemically stratified depression in Capitol Lake near the tide gate outlet. Marine water was leaking into the lake and settling into the depression (or crater). During certain tidal cycles, the entrapped body of marine water stagnates and toxic hydrogen sulfide gas is formed. Entranco Engineers recently evaluated various alternatives for restoring the tide gate crater and mitigating its adverse impact on water quality. The basic approaches considered were filling in the crater to prevent saltwater entrapment, preventing stratification and stagnation via mixing or aeration, and removing saltwater before stagnation occurs. Saltwater removal by a siphon (with a pump option) was selected.