Strengthening the spillway gates at Mossyrock Dam will take many years and will mean that Riffe Lake will be kept 30 feet below full level at least into the next decade, according to the hydroelectric project owner and operator.
The dam itself is not considered at risk from earthquakes. However, a 2017 U.S. Geological Survey study found that an earthquake could damage the dam’s spillway gates and release massive amounts of floodwater from Riffe Lake that would course down the Cowlitz River Valley.
“Analysis shows that specific seismic events could potentially cause the spillway piers and spillway gates to fail, resulting in an uncontrolled release of water and considerable downstream flooding. Lowering the lake level to 30 feet lower than full helps limit this impact,” according to Tacoma Power, which owns and operates the dam.
Mossyrock Dam, built in 1968 at a cost of nearly $118 million, is Washington State’s tallest hydroelectric dam, rising 606 feet from its foundation in bedrock and 365 from Cowlitz River bed. It impounds Riffe Lake, which is 23 miles long.
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When full, Riffe Lake reaches 778.5 feet above sea level. Tacoma Power lowers the lake more than 30 feet every winter to create flood storage capacity to meet its federal license requirements.
But lowered lake levels in the summer deter recreation and make some boat launches unusable.
However, a new Mossyrock Park boat launch has two long lanes usable at lower elevations: one lane is usable to approximately 720 feet; the second lane is usable to approximately 708 feet. The longer lane should be available all summer, fall and early winter (in years with normal inflows), according to Tacoma Power.
The Taidnapam North boat launch is open most of the year (when water levels permit) with one usable lane and a dock servicing elevations down to 720 feet.
The Taidnapam Park boat launch is closed. It reaches an elevation of 760 feet; there is no way to lengthen it due to a drop-off at the end, according to Tacoma Power.
The Kosmos boat launch is closed. It is not usable below 747 feet and cannot be extended.
Up-to-date information about boat launches and lake level is available at https://bit.ly/2DEG2ph or by calling (888) 502-8690.