Toutle River Sediment Retention Structure (SRS) EarthCache
Toutle River Sediment Retention Structure (SRS)
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At 8:32 a.m. on May 18 1980, a magnitude 5.1 earthquake centred directly below the north slope of Mt. St helens triggered that part of the volcano to slide, approximately 7–20 seconds (about 10 seconds seems most reasonable) after the shock. One of the largest landslides in recorded history, the slide travelled at 110 to 155 miles per hour (175 to 250 km/h) and moved across Spirit Lake's west arm; part of it hit a 1,150-foot (350 m) high ridge about 6 miles (9.5 km) north. Some of the slide spilled over the ridge, but most of it moved 13 miles (21 km) down the North Fork Toutle River, filling its valley up to 600 feet (180 m) deep with avalanche debris. An area of about 24 square miles (62 km²) was covered, and the total volume of the deposit was about 0.7 cubic miles (2.9 km³). Most of St. Helens' former north side became a rubble deposit 17 miles (27 km) long, averaging 150 feet (46 m) thick; the slide was thickest at one mile (1.6 km) below Spirit Lake and thinnest at its western margin. All the water in Spirit Lake was temporarily displaced by the landslide, sending 600-foot (180 m) high waves crashing into a ridge north of the lake, adding 295 feet (90 m) of new avalanche debris above the old lakebed, and raising its surface level by about 200 feet (60 m). As the water moved back into its basin, it pulled with it thousands of trees felled by a super-heated wall of volcanic gas and searing ash and rock that overtook the landslide seconds before. In the spring of 1987, construction of a sediment retention dam on the North Fork Toutle River began. This retention dam is designed to help stop the downstream movement of the sediment near where it begins - on the debris avalanche.
A serious side effect of the Mount St. Helens 1980 eruption has been the downstream movement of enormous amounts of sediment eroded from hillslopes and from the debris-avalanche and pyroclastic-flow deposits in the upper reaches of the North Fork Toutle River. The SRS was constructed to trap this sediment before it was carried farther downstream, where it could clog the river channel and exacerbate floods along the lower Toutle and Cowlitz Rivers. An overflow channel was added to divert lahars around the dam.
To log this cache please do the following.
1. Email me (from my profile) the answer to this question: Looking at the front side of the dam - how does the dam actually work in stopping the sediment from being carried downstream.
2. When logging - post pic of yourself at dam site holding GPS.
Bonus questions....
What material is most of the sediment being carried in the river from the slide ?
Where would this materiel end up if there were no dam.
More material on the eruption and the dam....
(visit link)
(visit link)
Get there on Hwy 504 and turn to to Sediment Dam Rd. at N46 22.200, W122 34.536 - park where appropriate.
Additional Hints
(Decrypt)
Gb nafjre ybttvat dhrfgvba - ybbx hcfgernz naq guvax nobhg irybpvgl.