DESCRIPTION:
North Fork Toutle River Drainage, Mount St. Helens, Washington

[Map,18K,InlineGIF)
Major Hydrologic Features in the Mount St. Helens Region
-- Modified from: Crandell and Mullineaux, 1978, USGS Bulletin 1383-C

As a result of the May 18, 1980 eruption, the source of the North Fork Toutle River was changed, and the upper part of its drainage system evolved on the dissected surface of a debris-avalanche deposit. Prior to the May 18 eruption, the source of the North Fork Toutle River was Spirit Lake, which received runoff from 46 sq.km of rugged, forested terrain, north of Mount St. Helens. The crater of Mount St. Helens was the source of the North Fork Toutle River from the time of the eruption until November 1982. The upper 24 kilometers of the North Fork Toutle valley contains the 2.8 cubic kilometers debris-avalanche deposit emplaced during the May 18, 1980 eruption. The deposit covers 60 square kilometers, with an average thickness of 45 meters. Maximum thickness of the debris-avalanche deposit is 195 meters in an area 3 kilometers west of Spirit Lake. Evolution of a through-flowing drainage network on the hummocky avalanche deposit has increased contributing drainage area above the toe of the deposit from 80 sq.km on May 19, 1980, to about 282 square kilometers on November 3, 1982 (Meyer and Janda, 1984).

In order to control sediment eroded from the debris avalanche deposit, a debris-retention structure (N-1) was constructed below the toe of the avalanche deposit during the fall and winter of water year (WY) 1981. ... Five sediment stabilization basins were also dredged on the Toutle River during the same period (Stockton, 1983).

The outlet of Spirit Lake was blocked by the debris-avalanche deposit. During the period from May 18, 1980, until November 1982, the level of Spirit Lake rose as it received runoff from rain and snow in the surrounding mountains. The rising waters threatened to breach the unstable debris-avalanche deposits damming the lake. A temporary solution, put into effect by the U.S. Army Corps of Engineers, was the construction of a pumping station and pipeline near the southwest end of the lake until a more permanent solution could be devised. The pumping station went into operation in November 1982, creating a new source for the North Fork of the Toutle River. Water was released from the pump outlet at a constant rate of 5.0 cubic m per second. It flowed across easily erodible volcanic deposits and caused rapid degradation and aggradation at cross-sections established downstream (Paine, 1984).

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.

Peak flows on the North Fork Toutle River result from storm runoff, except for some debris-flows generated by eruptive events at Mount St. Helens. On the night of February 2, 1983, two minor explosive ventings of steam and rock debris caused a small fluid-slush debris flow that entered into the upper reaches of the avalanche deposit known as the pumice plain. On May 14, 1984, an explosive eruption of Mount St. Helens caused a significant mudflow that surged down Loowit channel into the upper North Fork Toutle River, and down an adjacent gully into Spirit Lake. The flow was observed at the Elk Rock gage to be a hyperconcentrated flow, and farther downstream at N-1, it became a sediment-laden stream flow. The flows did not cause a significant rise in stage at the Kid Valley gage. A small explosive eruption on May 26, 1984, generated a small mudflow that reached Spirit Lake (Janda and Meyer, 1986).

Water and suspended-sediment discharge data have been collected at Kid Valley gaging station since June 1980, 11.1 kilometers upstream from the confluence of North Fork Toutle and South Fork Toutle Rivers. Peak discharges at Kid Valley are normally caused by storm runoff during the winter months. During WY 1985, no winter storms occurred; the peak of the year occurred during June.

Seventy-nine cross sections had been established along the North Fork Toutle and its major tributaries prior to the end of WY 1985. Most of these cross sections were surveyed annually prior to WY 1985, but 28 key indicator sections were surveyed monthly, or after large storms of the fall, winter and spring. Most of the cross sections are grouped into 19 clusters. During WY 1985, only indicator cross sections were surveyed during the summer.

Cross sections were established along South Fork Coldwater Creek during WY 1985 to monitor the effect of augmented discharge resulting from drainage of Spirit Lake.

May 18, 1980 Eruption

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