DESCRIPTION:
Natural Debris Dams and Debris-Dammed Lakes

[Image,32K,JPG]
Mount St. Helens May 18, 1980 debris avalanche impounding Castle Lake.
-- USGS Photo by R. Schuster, March 1984

In addition to the direct risk from debris flows and debris avalanches, voluminous deposition of volcanic material in valleys commonly forms unstable natural dams by blocking preexisting drainages. If the size and structural integrity of the blockage is insufficient to hold back the reservoir that will form (or withstand the erosion by overtopping flow), catastrophic failure of the dam will result. Such a hazardous situation may persist for months, years, or decades following an eruption. ...

The debris avalanche raised the level of Spirit Lake 64 meters and dammed its natural outlet even higher. Many small ponds filled closed depressions on top of the avalanche deposit, and several lakes formed in tributaries dammed by the avalanche; the largest lakes formed in the tributaries of Coldwater and Castle Creeks. In late 1980, some of the ponds overtopped and swiftly eroded their new outlets. The rapid release of water generated highly erosive flows on the avalanche and transported large volumes of sediment down the lower reaches of the Toutle and Cowlitz Rivers. ... Failure of the debris dams holding Spirit, Coldwater, and Castle Lakes would result in catastrophic mudflows comparable to or larger than those of May 18, 1980. Controlled outflow channels have been constructed to stabilize the water levels ...

On May 18, part of the debris avalanche slid into Spirit Lake, raising its level nearly 60 meters and damming its natural outlet to a higher level. ... Tributaries of the North Fork Toutle River were dammed by the massive debris avalanche deposit and subsequently formed lakes behind the blockages. The largest of these are Coldwater and Castle Lakes. Outlet channels were constructed in 1981 and 1982 to prevent the lakes from overtopping their dams.

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