The Sediment Retention Structure, commonly known as the SRS, is located on the North Fork of the Toutle River in southwest Washington. It is 22 miles east of the town of Castle Rock and its purpose is to slow the flow of the North Fork Toutle River. The slower flow allows volcanic sediment from Mt St Helens to drop out of the water and settle behind the SRS.
The SRS was built in the late 1980s following the May 18, 1980 eruption of Mount St. Helens. The structure was designed with six rows of outlet pipes that allow the water to pass through the SRS and into a plunge pool and outlet channel. Each row contains five 3-foot-diameter pipes and each row is 10 feet above the next lower row. The pipes were designed to be closed off one row at a time as the sediment level rose behind the SRS. Since the top row was closed, the river flows only over the structure’s spillway. If future construction-related work is required within the spillway, the operating row could be temporarily reopened and the spillway blocked off.
A conventional dam can stop the flow of water completely. The Sediment Retention Structure does not stop water; it works with nature to stop sediment, by slowing the flow of water so that the sediment drops out, instead of traveling downstream and settling in the river channels where it can cause flooding and impede navigation. The sediment builds up behind the SRS in a single large manageable deposit.
The SRS consists of an 1800-foot-long embankment rising 184 feet above the post-eruption streambed, with concrete outlet works and an unlined spillway at one end. The embankment is made of fractured rock with a tapered core of impervious clay taken from the site. The structure rests on ancient river gravels, and water passes underneath and rises inside the embankment when the lake level behind it is high. Drainage pipes set into the embankment face, between layers of roller-compacted concrete, allow this water to run back into the lake when the level recedes.
The large block, or center monolith, of the outlet works contains six rows of five outlet pipes or conduits through which water passed into the plunge pool and outlet channels below. These pipes were closed off permanently as the sediment level rose behind the structure. It was originally estimated that by about the year 2035, when the sediment has reached the 400-foot-wide spillway at an elevation of 125 feet above the post-eruption streambed, all the conduits will be closed and the river would flow continuously over the spillway. However, it only took a little more than 10 years for the level of sediment to collect and the last row of outlet pipes were closed. There is still room to store an additional 190 million cubic yards of sediment behind the structure. There is an estimated 68 million cubic yards of volcanic sediment trapped behind the SRS.
The U.S. Army Corps of Engineers welcomes visitors to view the SRS. The viewing access area is located near milepost 22 off of State Route 504 on Sediment Dam Road. Visitors can view the dam from a viewing deck (picnic table) near the parking lot or hike the easy one mile trail and then walk along the top of the dam itself. The cache is accessible only from the south side of the dam, via the trail. You can not reach the cache from the north side of the river. Please note: there are no restrooms or potable water at this site.
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