Mount St. Helens looms in the distance above the sediment-retention structure still under construction in this photograph. The 600-m wide structure spans the Toutle River valley about 35 km downstream from the volcano. The U.S. Army Corps of Engineers began work on the project in 1986, and sediment began accumulating behind the structure in Fall 1987. The entire project was completed in 1989.

Designed to trap sediment before it is transported into the Cowlitz and Columbia rivers, the structure can hold about 198 million m³ (258 million yd³) of sediment. By 1998, the structure was about 22 percent full. In 2000, the structure was about 30 percent full.

The structure traps sediment by slowing the speed of the river. Initially, the river flowed into a pond behind the structure (water behind structure in photograph), which allowed most of the suspended sediment to settle to the bottom of the pond. At the downstream end of the pond, water then poured through a row of pipes and into a plunge pool. Six rows of pipes were set into the structure at 3 m intervals to allow water to pass into the plunge pool. As the sediment settled behind the structure, the pipes became progressively blocked. The highest row of pipes was plugged in April 1998. Since then, water has poured over the structure through a spillway on the north side of the river (left bank in image).

The rate at which sediment is trapped behind the retention structure varies dramatically from year to year. For example, in the wet winters of 1996 and 1997, 30.6 million cubic yards of sediment was deposited behind the structure. That was nearly four times the amount trapped during the previous two years.