River Sediment at Mount St. Helens: Forty Years After the 1980 Eruption

The 1980 eruption of Mount St. Helens, in Washington State, set off one of the largest debris avalanches in recorded history. It dumped 3.3 billion cubic yards of material into the upper North Fork Toutle River watershed and blocked the Columbia River shipping channel downstream. The eruption lasted hours; its effects, and the work to contain them, continue.

The North Fork Toutle River eroding the 1980 debris avalanche. Photograph by Adam Mosbrucker, U.S. Geological Survey.

From the eruption on May 18, 1980, to September 30, 2018, the Toutle River carried about 405 million tons of sediment into the lower Cowlitz River — enough to bury downtown Portland, Oregon, 300 feet deep. Since a sediment retention structure began trapping sediment in November 1987, it has carried an average of 2.8 million tons a year: still 10 times the level before the eruption, and during storms possibly 50 to 100 times.

River valleys buried

On May 18, 1980, the top and north flank of the mountain collapsed and slid, first northward and then mostly westward. The volcano lost more than 1,300 feet of height. The debris avalanche — 3.3 billion cubic yards, enough to fill 1 million Olympic-size swimming pools — spread nearly 20 miles from the volcano and partly blocked many tributaries.

It filled 23 square miles of the upper North Fork Toutle River valley to an average depth of 150 feet, and up to 500 feet. Mixed with 170 million cubic yards of glacial ice and snow, river water and groundwater, it became a huge lahar — a volcanic mudflow — that ran more than 70 miles towards the Columbia; smaller lahars ran down the South Fork Toutle and the Muddy River. The sediment they laid on the beds of the lower Cowlitz and Columbia greatly raised the flood risk downstream.

The Toutle River Basin: tributaries, monitoring stations, the sediment retention structure (SRS) and communities affected by sediment. Credit: U.S. Geological Survey.

Where the sediment comes from

• The debris-avalanche deposit. The 20-mile-long deposit supplies most of the sediment, and the North Fork Toutle River keeps eroding it. The channel bed's elevation has broadly stabilised; sediment now comes mainly from bank erosion as the channel widens more than it deepens.
• Storms. Intense rain can set off small debris flows or erode banks, beds and stored sediment across the basin.
• The crater. Debris flows, rockfalls and glacial-melt floods in the headwaters add material to the deposit.

Sediment blocking Castle Lake, September 1980. Photograph by Norm Dion, U.S. Geological Survey.

Measuring sediment

Sediment moves downstream in two ways: as suspended load, finer particles held up in the water by turbulence, and as bedload, coarser material moving along the riverbed. The suspended load is typically about three to four times the bedload. Sediment load — the mass passing a station in a given period — is usually reported in tons per day.

Suspended-sediment load takes two measurements:

• streamflow, in cubic feet per second, computed from the water's depth and velocity;
• suspended-sediment concentration, in milligrams per litre, found by lowering a specially designed sampler through the water at several points across the river.

Multiplying the two gives the load in tons per day. Samples are analysed in a laboratory and usually taken during storms, when most sediment moves — and when bad weather, fast, rising water and floating logs make sampling hard. Automated pumping samplers, turbidity sensors, acoustic backscatter devices and time-lapse cameras fill the gaps.

Tami Christianson sampling the Toutle River by hand, December 14, 2010. Photograph by Emily Roeder, U.S. Geological Survey volunteer.

Al Onions sampling with a cable-deployed sampler, December 2, 1982. Photograph by Lyn Topinka, U.S. Geological Survey.

The nearly 40-year record at the Toutle River (station 14242580) is one of the longest sediment time series in the world, used to follow how the valley's landscape evolves and to judge the mitigation work.

Managing the sediment

Excess sediment is an immediate and a long-term hazard downstream, and four decades of effort have not removed it. Managing it has cost more than any of the eruption's direct effects.

The first response. The U.S. Army Corps of Engineers (USACE) dredged the Toutle, Cowlitz and Columbia Rivers to restore their capacity.

• The lahars left 44 million cubic yards on the lower Cowlitz's bed, cutting its capacity to less than 10 percent of what it had been.
• In the Columbia they left 59 million cubic yards, reducing the channel's depth from 40 to 14 feet along 10 miles and trapping ships in upstream ports.
• The USACE dug outlets for Spirit, Coldwater and Castle Lakes, dammed by the debris, to prevent catastrophic breaches.

The sediment retention structure (SRS). For the long-term hazard, the USACE built an earthen structure on the North Fork Toutle River, upstream of its confluence with the Green River: 2,300 feet long and 180 feet tall, finished in 1989 for $65 million, and designed to trap 258 million cubic yards of sediment in the upper valley over 50 years. It began trapping sediment in autumn 1987, catching about 90 percent at first; as sediment built up, that efficiency fluctuated and declined, and at times the trapped sediment has become a source itself.

The sediment retention structure after the 2012 spillway raise, looking upstream. Photograph by Adam Mosbrucker, U.S. Geological Survey.

Adaptive management. The USACE now adjusts as conditions change:

• grade-building structures and berms upstream, built in 2010 for $4 million;
• incremental raises of the spillway as needed — the first finished in 2012 for $4.5 million;
• limited dredging of the lower Cowlitz, as needed, to keep capacity in flood-prone areas.

What 1980–2018 shows

The Toutle carries far more sediment than any other river rising in the western Cascade Range, more even than much larger basins.

• 1980–87. Before the SRS was finished, huge loads went down the Toutle to the Cowlitz and on to the Columbia: 161 million tons in 1981–87, about twice the 87 million tons of the following 31 years (1988–2018).
• Since 1981 the river has averaged 6.5 million tons a year, more than any other western Cascade river. The volume carried from 1988 to 2018 would cover a football field 960 feet high.

Suspended-sediment yield (SSY) — load divided by basin area, in tons per square mile — makes years comparable. High streamflow mainly drives it, but the SRS strongly affects it:

• yields stayed high until the SRS began holding sediment back in 1988–89, and were near background in 1989, 1992–94 and 2001 (drought and the SRS);
• the highest 5-year yield after 1989 came in 2005–09, more than 48,000 tons per square mile, once sediment had filled to the spillway and could pass over;
• after the 2012 spillway raise yields fell, until big storms in water years 2016 and 2017.

In the five years of heaviest transport since the SRS was finished — 1996, 1999, 2007, 2016 and 2017 — an average of 25 percent of the year's yield moved in a single flood.

Annual SSY and streamflow, Toutle River at Tower Road (station 14242580), water years 1981–2018 (October to September). Mean SSY: 13,200 tons per square mile for 1981–2018, 5,690 for 1988–2018. Credit: U.S. Geological Survey.

Where it comes from and where it goes

Since 1986, stations along the rivers show:

About 80 percent of the load — fine sand and smaller — washes through to the Columbia; the coarser 20 percent settles on the beds of the lower Toutle, Cowlitz and Columbia and moves again only in high flows.

The cost to communities

Forty years on, the North Fork has removed only about 20 percent of the debris-avalanche deposit. Much of the rest will probably stay put, but the river's shifting corridor still holds an enormous supply to be carried down to the SRS and past it.

• Floods. The most lasting effect, far from the volcano, is higher flood risk. More than 50,000 people along the Cowlitz River, in Castle Rock, Lexington, Kelso and Longview, could be harmed by future flooding, and businesses by lost revenue.
• Shipping. The Port of Longview plans to spend $6.4 million by 2035 removing an expected 30 million cubic yards of sediment from its berths. The Columbia is a major gateway for wheat, soy, grain, wood and cars, and any interruption hurts the whole region.

The Cowlitz River in flood at Castle Rock, Washington, January 2009. Photograph by Kate Norton, U.S. Geological Survey.

A car carrier enters the Columbia River under the bridge at Astoria, Oregon. Photograph by Mark Uhrich, U.S. Geological Survey.

The USACE projects spending $400 million on the sediment until 2035, when the congressionally mandated 50-year flood-control plan ends; forty years of monitoring suggest the spending will continue well beyond that.

Sources

• Uhrich, M.A., Spicer, K.R., Mosbrucker, A.R., Saunders, D.R., and Christianson, T.S., 2021, A 40-year story of river sediment at Mount St. Helens: U.S. Geological Survey Fact Sheet 2021–3004. https://doi.org/10.3133/fs20213004
• The photographs, map and graph are taken from the fact sheet's PDF, which also supplied the units lost from the imported text. A map of sediment loads across the western Cascades, drawn on a licensed Esri basemap, is not reproduced.
• The fact sheet says sediment is now "captured" primarily by bank erosion; from the context, it means supplied.
• Rewritten in hubnx's own words.