DESCRIPTION:
Mount St. Helens Lakes and Reservoirs

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Map, Major hydrologic features in the Mount St. Helens Region
-- Modified from Crandell and Mullineaux, 1978, USGS Bulletin 1383-C

A number of natural and human-made lakes exist close to the volcano in the North Fork Toutle and Lewis River valleys. The uppermost lake in the Lewis River valley, Swift Reservoir, receives drainage from the volcano via Swift Creek, Pine Creek, and Muddy River. In 1980, lahars descending these streams dumped about 14 million cubic meters (18 million cubic yards) of sediment and water into the lake, abruptly raising the lake level 0.85 meters (2.8 feet). Because the operators of the reservoir, Pacific Power and Light, lowered the lake level about 18 meters (23 feet) below normal in anticipation of possible lahars, the small lake-level rise and the 0.4 meter (1.3 feet) accompanying wave posed no threat to the dam. It is assumed that (1) future lahars reaching Swift Reservoir would not be appreciably larger than those of May 18, 1980, and (2) dam operators would again take precautionary steps to lower lake level if Mount St. Helens were to show signs of imminent eruption. Therefore, Swift Reservoir and the downstream lakes (Yale Lake and Lake Merwin) are not considered to be at risk from lahars.

Three natural lakes in the North Fork Toutle River, formed by natural debris dams during the 1980 eruption, have required modifications to their outlets in order to prevent catastrophic outbreaks. The U.S. Army Corps of Engineers provided (1) a tunnel outlet to Spirit Lake, (2) a bedrock spillway channel at Coldwater Lake, and (3) a reinforced spillway channel at Castle Lake to hold the levels of these lakes constant and to prevent them from overtopping their erodible natural dams. A recent study (Roeloffs, 1994), however, has verified earlier conclusions that the natural dam at Castle Lake is potentially susceptible to modes of failure other than overtopping and, under certain conditions, is only marginally stable. Castle Lake contains about 23 million cubic meters (30 million cubic yards) of water and would produce a large lahar if the blockage were to fail. We assume that an outbreak of Castle Lake is a potential hazard ...

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The May 18, 1980 debris avalanche from Mount St. Helens covered over 24 square miles (62 square kilometers) of the upper Toutle River valley and blocked tributaries of the North Fork Toutle River. New lakes such as Castle Lake (pictured here) and Coldwater Lake were created.
-- USGS Photo by Robert L. Schuster, March 1984

Castle Lake: Located in the Mount St. Helens National Volcanic Monument, Castle Lake requires a 20-plus mile drive over rough logging roads, followed by a tough hike down a steep hillside to the lake. The key to finding the lake is to first find the Weyerhaeuser 3000 road. Would-be anglers not familiar with the area should get a St. Helens West hunting map, published by the Washington Forest Protection Association. Those who put forth the effort have a chance to catch dandy-size rainbow trout; fish up to 10 pounds have been reported. This is a self-sustaining fishery, with no fish planted, so selective gear rules are in effect, along with a one-fish, 16-inch minimum size limit. The lake is open year-round, but the road in is usually blocked by snow until at least May.

When Mount St. Helens erupted in May, 1980, the debris flow that swept down the Toutle River blocked South Fork Castle Creek, impounding Castle Lake. In order to prevent the rising lake from overtopping the debris dam, a spillway was constructed in 1981 to stabilize the elevation of the lake, which now contains approximately 19,000 acre-feet (acre-ft) of water. If the natural dam were to fail, the ensuing breakout of Castle Lake might seriously affect communities downstream along the Toutle River. Although the dam and spillway have performed well to date, piezometers in the dam reveal hydraulic heads 30 feet or more above lake level in the dam crest. In this respect, the hydraulic head distribution in the natural dam does not resemble that in a engineered embankment dam.

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Aerial view, Coldwater Lake, with debris avalanche hummocks in the foreground. Coldwater Ridge Visitor Center sits on the ridge to the west (left), overlooking the lake.
-- USGS Photo by Lyn Topinka, January 13, 1984

Coldwater Lake (750 acres): Located in the Mount St. Helens National Volcanic Monument off SR 504, Coldwater Lake was formed by a debris dam resulting from the eruption of Mount St. Helens. Skamania and Cowlitz counties share this lake, but the main access is in Cowlitz County. Rainbow trout fingerlings stocked in the late 1980s have grown to impressive size, and are reproducing on their own. No additional planting is planned. Cutthroat trout also inhabit the lake. Selective gear rules are in effect, with a one-fish limit and 16-inch minimum size limit. Electric fishing motors are allowed. Shoreline access is limited, but a boat launch, fish cleaning station, and nature walk are available. The Forest Service charges an access fee to use the Coldwater facility; contact the Forest Service's Mount St. Helens monument headquarters (360/274-2131) for more information. Although the lake is open to fishing all year, access may not be available during winter. Disabled accessibility - Level 2 at the boat ramp. WCT, Blk Tpa, steep ramp.

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Merrill Lake (344 acres): This fly-fishing-only lake is located north of Cougar near Mount St. Helens. Merrill contains coastal cutthroat, brown trout to several pounds, and some rainbow trout. Special bag and size limits are in effect; please check the regulations pamphlet. Internal combustion engines are prohibited on boats that are used for fishing. Merrill is open year-round, but action is usually slow during the winter months. There is a DNR campground and boat ramp.

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Merwin Reservoir is formed by a combination gravity-concrete-arch dam. Some storage began March 1931; completed May 13, 1931. Usable capacity: 245,600 acre-feet between elevations 165 feet (lower limit of regulation set by Federal Energy Regulatory Comission), and 235 feet (top of spillway gates). Additional storage of 18,200 acre-feet is provided by flashboards to elevation 239.6 feet. Unused storage below elevation 165 fett: 159,000 acre-feet. Water is used by PacifiCorp for power development.

Merwin (Lake) Reservoir (4,090 acres): This North Fork Lewis River impoundment located near the town of Ariel is shared by Clark and Cowlitz counties, with 2,400 acres in Clark County and 1,690 in Cowlitz. Kokanee are the main target. Angling is best in early spring, with fish in the 10-inch class. Small northern squawfish can be numerous and pesky. Tiger muskies were planted in 1995 to help control squawfish populations. There is a 36-inch minimum size limit on the muskies, which they will not reach for several years. The lake is open to fishing year-round.

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This site is within the May 18, 1980, blast zone, about 8.5 miles from the volcano; to the northeast, the blast swept out to a distance of 13 miles. A short trail leads from the road to Meta Lake, an interesting path through downed timber, to a lake that was only moderately disturbed by the blast. Small trees on the shore and fish in the lake survived because they were under the snowpack. One blown-down tree near the start of the trail shows tightly clustered rings for a few years after A.D.1800 (the year when pumice of the T tephra was erupted from Mount St. Helens).

A car damaged by the blast belonged to a family killed at their small mine, on the ridge 1 1/2 miles to the west. Thick tephra layers are exposed along USFS Road 99 from here to the Windy Ridge parking lot.

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Ryan Lake Viewpoint and the 1/2-mile loop trail provide a good overview of the effects and dynamics of the Mount St. Helens May 18, 1980 blast at a point 12 miles north of the crater. Research on the effects of heat on conifer needles shows that the temperature of the blast at this site reached an estimated 300 degrees C (572 degrees F). As much as 6 inches of ash fell here. Two people died in this area on May 18, 1980, as a result of asphyxiation caused by inhaling the ash. Another person somehow hiked nearly 10 miles farther north and then succumbed, also to asphyxiation. Notice that locally trees on the lower parts of the valley wall have been blown down, whereas those on upper parts of the valley wall still stand. By the time the blast reached this location, particle segregation within it had created a heavier "flow" phase, which blew down trees in lower areas, and a lighter "surge" phase, which left trees standing in higher areas.

Mount St. Helens tephra layers Y, W, and T crop out in cuts along this trail. Rock of the Spirit Lake pluton was quarried for road construction at a site on the south edge of the parking area.

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View across Silver Lake towards Mount St. Helens.
-- USGS Photo by Lyn Topinka, September 15, 1980

Silver Lake is shallow (maximum depth about 16 feet or 5 meters) and was formed and is partially underlain by lahar deposits. About 2,500 years ago during the Pine Creek eruptive period, a series of very large lahars traveled down the Toutle River from Mount St. Helens. The lahars flowed into Outlet Creek (east of the lake) and dammed its valley to produce Silver Lake. These lahars were generated by the catastrophic draining of a lake (presumably an older Spirit Lake) or lakes that had been dammed by debris avalanches from Mount St. Helens. The level of Silver Lake is now controlled by a dam.

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Spirit Lake, once surrounded by lush forest, is within the area devastated by the May 18, 1980 blast. Remnants of the forest now float on the surface of the lake. Another Cascade volcano, Mount Rainier (14,410 feet [4,392 meters]), is in the distance. The view is from the south.
-- USGS Photo by Lyn Topinka, October 4, 1980

The 1980 eruptions of Mount St. Helens in southwestern Washington marked the re-awakening of a relatively young (40,000 years) volcano that had been dormant since 1857. Frequent dacitic eruptions during the previous 2,500 years had produced pyroclastic flows, ash falls, debris flows, lava domes, and lava flows of andesite and basalt. Pyroclastic flows and lahars accompanied most eruptive periods and were largely responsible for forming fans around the base of the volcano, some of which dammed the North Fork Toutle River to form Spirit Lake between 3,300 and 4,000 years ago.

The debris avalanche that triggered the (Mount St. Helens May 18, 1980) eruption slid north into Spirit Lake and west 25 kilometers down the North Fork Toutle River valley, covering the valley floor with unconsolidated debris to an average depth of 45 meters and as much as 180 meters in some places. ... The debris avalanche raised the level of Spirit Lake 64 meters and dammed its natural outlet even higher.

A comparison of aerial photographs taken before and after the eruption shows that the lake increased in size and rose in altitude. The surface area of the lake increased from 1,300 acres to about 2,200 acres. Soundings of the east bay of the lake on October 16, 1980, indicated that the depth at the midpoint of the east bay, originally 190 feet, was less than 50 feet. Because of an extensive cover of logs on the surface of Spirit Lake, a post-eruption bathymetric map could not be constructed. The rise in lake level, estimated to be about 240 feet, is due to the blockage of the North Fork Toutle River, floods from melting snow on the volcano, and partial filling of the lake with debris (Youd and Wilson, 1980). Because of the blockage of the original outlet, surface outflow to North Fork Toutle River will not occur until the lake rises from its present altitude of about 3,440 feet to about 3,600 feet. The pre-eruption altitude of the lake was 2,198 feet.

In May 1985, a tunnel was opened connecting Spirit Lake and South Coldwater Creek. Water was released from Spirit Lake into South Coldwater Creek at controlled rates of 5.7 cubic meters per second, 8.5 cubic meters per second, 11.3 cubic meters per second, and 14.2 cubic meters per second for approximately one-week periods during May 1985. As the level of the lake approached the tunnel gate elevation (1048.5 meters above NGVD of 1929) during June through August 1985, tunnel discharge declined from about 11.3 cubic meters per second to less than 5 cubic meters per second, the natural summer discharge from Spirit Lake.

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Aerial view, Swift Reservoir.
-- USGS Photo by Lyn Topinka, June 22, 1984

Swift Reservoir is formed by a rock and earthfill dam. Storage began September 19, 1958; the dam was completed in December 1958. Usable capacity: 446,600 acre-feet between elevations 878 feet (lower limit for economic operation), and 1,000.5 feet (maximum operating limit). Dead storage is unknown. Water is used by PacifiCorp for power development.

Swift Reservoir (4590 acres): Large plants of fingerling rainbow trout usually produce excellent trout fishing in this 10-mile-long reservoir on the North Fork of the Lewis River. Watch local newspapers for reports on fish survival. Improved water clarity will be a key to fishing success. During April and May fish tend to congregate near the dam and around streams entering the reservoir. By June, anglers should be able to catch trout throughout the reservoir. A public boat launch is available. Call Pacific Power and Light at 1-800-547-1501 for reservoir level information. Remember that all bull trout/Dolly Varden must be released in this region. Open season runs from the last Saturday in April through October 31.

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Yale Reservoir is formed by a rock and earthfill dam. Storage began July 31, 1952; the dam was completed in 1952. Usable capacity: 189,500 acre-feet between elevations 430 feet (lower limit for economic operation), and 490 feet (top of spillway gates). Dead storage below elevation 417 feet: 178,000 acre-feet. Water is used by PacifiCorp for power development.

Yale Reservoir (3,802 acres): This North Fork Lewis River reservoir has 2,023 acres in Clark County and 1,779 acres in Cowlitz County. Kokanee fishing is the main attraction, but success this year will likely still be down because of the severe winter flooding in 1996, and heavy rainfall and snowpack in 1997. Shallow fishing for kokanee is best in the spring and fall; in the summer, deep fishing is the rule. Some cutthroat trout are caught near the mouth of Siouxon Creek, and Yale also has a few bull trout. All bull trout and Dolly Varden must be released. Boat ramps are available at Saddle Dam, Yale, Cougar Camp and Beaver Bay. The lake is open to fishing year-round.

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