Mossyrock Dam
Completed in 1968, Mossyrock Dam is Washington state’s tallest dam at 606 feet
high above bedrock (or 365 feet above the riverbed). It produces enough clean,
renewable electricity to serve more than 87,000 homes annually. The dam’s double
curvature concrete arch has three penstocks between 248 and 285 feet in length
that extend down to the powerhouse.
We replaced the 40-year-old turbine generators in the powerhouse in 2010 and
2011 with modern, efficient turbines that produce more power using the same
amount of water. Watch a video detailing how we rebuilt
the two Mossyrock powerhouse generators.
The dam created 23.5-mile-long Riffe Lake. The lake offers 52 miles of shoreline
to explore by watercraft, and two parks offering a variety of outdoor
recreation: Mossyrock
Park, located on the southwest shore near the dam, and Taidnapam
Park on the east end where the Cowlitz River flows into the lake.
These parks provide access to boat launches, swimming (Mossyrock Park only),
fishing, camping, a kitchen shelter and day-use facilities.
In 2017, the United States Geological Survey (USGS) revised its earthquake
predictions for the Cowlitz River basin. Although the probability of a large
earthquake is very low, the updated predictions showed an increase to the
potential impact a quake could have on just the spillways of Mossyrock Dam — not
to the dam structure itself.
So, to reduce this risk and protect public safety, we will hold Riffe Lake’s
elevation to about 30 feet lower than full (778 feet) at least into the next
decade.
In the event of significant rainfall or flooding, we may need to use the storage
capacity of Riffe Lake to minimize downstream flooding by temporarily allowing
the lake level to rise. After the risk of downstream flooding passes, we would
then gradually lower the reservoir to previous levels.
No concrete arch dams, such as Mossyrock Dam, have failed due to earthquakes.
However, 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.
We are currently considering seismic retrofits, but making changes to a
federally licensed hydroelectric dam is a long and complicated process. The
development of possible solutions involves substantial analysis, planning and
federal approval. We will keep you updated on this process.
For more information about Mossyrock Dam, contact us at cowlitz@cityoftacoma.org or
see the information below:
·
Riffe Lake Update
Public Meeting Presentation – 050219
·
Cowlitz/Riffe Lake
Level Fact Sheet – 11/2018
·
Cowlitz/Riffe Lake
Level Fact Sheet – 021417
·
Mossyrock Community
Center Public Meeting Presentation – 030317
· Cowlitz/Riffe Lake Level Fact Sheet – 060617 CLICK HERE
·
Year completed – 1968
·
River system – Cowlitz
·
Original investment – $117.8 million
·
Type of dam – concrete arch/gravity and embankment
·
Height – above riverbed, 365 feet; above bedrock, 606 feet
·
Length – 1,648 feet
·
Width – top, 27 feet, base, 115 feet
·
Volume of concrete in dam – 1.27 million cubic yards
·
Average flow – 5,140 cubic feet/second
·
Drainage area – 1,042 square miles
·
Reservoir length – 23.5 miles
·
Miles of shoreline – 52 miles
·
Reservoir area at maximum elevation – 11,830 acres
·
Reservoir elevation – full, 778.5 feet; lowest allowable for generation, 600
feet
·
Average annual generation – 1,100,000,000 kilowatt-hours
·
Equivalent number of NW homes served – 87,300
·
Number of generators – one 207,000 kilowatt generator and one 175,000 kilowatt
generator
·
Installed capacity (nameplate rating) – 382,000 kilowatts
Our new Mossyrock Park boat launch has two long lanes usable at lower
elevations: one lane is usable to approximately 723 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).
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 elevation 760 feet; there
is no way to lengthen it due to a drop-off at the end.
The Kosmos boat launch is closed. It is not usable below 747 feet and cannot be
extended.
Get up-to-date info about boat
launches and lake levels here or call (888) 502-8690.
Swim Areas
The new Mossyrock Park swim area is usable at approximately 728 feet, so it
should be available all summer unless there is a drought. We added an
ADA-accessible parking lot, trail to the swim area and portable toilet to help
make the area more accessible for our visitors who have disabilities.
Fishing
The North Shore Fishing Access area will be usable to 735 feet.
Riffe Lake Update meeting materials
Tacoma Power hosted a Riffe Lake Update public meeting on Thursday, May 2, 2019,
from 7 to 8 p.m. at the Mossyrock Community Center.
·
Riffe Lake Level
Facts – November 2018
Questions? Please email cowlitz@cityoftacoma.org.