The distal lahar deposits
The lower reaches of the Toutle and Cowlitz Rivers as well as a section of the Columbia River were filled with lahar deposits after the 1980 eruption. It was imperative that these be dredged quickly to restore navigation (some 20 ocean-going vessels were trapped upstream on the Columbia). Shown below is a dredge operating in the Cowlitz River at Kelso WA in March of 1981.
Photo from USGS archive -- free use with attribution [Full-size, jpeg format]
Lahar sedimentation in the Cowlitz River at Castle Rock. Note rise in river level of 9 feet; deepest part of river bed raised by 16 feet. The artificial levees on the west side of the river were overtopped and subsequently raised in elevation.
Source:R. E. Lombard, M. B. Miles, L. M. Nelson, D. L. Kresh, and P. I. Carpenter, 1981, The impact of mudflows of May 18 on the lower Toutle and Cowlitz Rivers, in The1980 Eruptions of Mount St. Helens, Washington , edited by P. W. Lipman and D. R. Mullineaux U. S. Geological Survey Professional Paper 1250, p. 693-700.