Send the answers to the following questions from the two waypoints
At the Thistle Lake Waypoint N 39° 59.589 W 111° 29.811
1-What can you see about the water levels that were here on site. Did it expose or bury the rocks on the hillsides?
2-Was there a lot of sediment that was deposed by the rivers into the lake here? What evidence do you have of that?
At the Thistle Slide Waypoint looking at the valley. N 40° 00.283 W 111° 29.540
3- Looking at the slide, does it appear to be stable? If not what evidence do you see of recent movement.
4-The slide blocked the river, was it cut away? or was another method to allow the lake to drain?
History
1981-1982 were very wet water years. Then in 1982 hit we had a very wet year in the state of Utah. September s waterfall was 10 times the normal. and it was followed by and vast amounts of snow falling the mountains throughout the winter of 1982-83. So as spring came the ground was saturated, and the snow was high.
Spring arrived with a bang. The cold suddenly replace with warmer weather and rains and all the snow began to melt. It flowed on the surface, supersaturated the ground under it.
Spring came
Slides began. Big and Little Cottonwood Canyon, Payson, and other canyons about Utah saw slides, slumps and other ground motion.
Thistle slump
At the bend in the river the railroad had watched this area for a number of years. There was a history of the track moving. However in April of 1983 it began to move not by fractons of an inch, but far faster. On April 13th The railroad was calling meetings and was working to keep the slowly moving slump from blocking the track. A highway patrolman hit a buckle in the road that threw him into the roof of his vehicle. by the end of the day a crew from the highway department was working to keep the road open. Trains were limited to 10 miles per hour and their maintenance personal were working constantly to keep the track in line. Within two days the tracks and the highway were closed, and they tried to determine how big the slump was. The hope was the slump would be just the hill above it.
By the 16th of April the tracks were buried and the town was ordered evacuated. The ground continued to move and the 17th they tried to keep the river from being blocked, and failed. This rail route is one of the busiest in the nation, and the rail was rerouted through Wyoming.
Eventually the slide was determined to be on a massive scale. At nearly a mile long, 250 feet deep, and 1000 feet wide. 15 million cubic meters of earth was in the slide. From the upper waypoint you can look up the long canyon that was part of the slide.
Geologic action
The slide itself was a result of the water. All the waterfall in the previous fall filled the valley. Most of these mountain valleys are like rock basins filled with dirt and the gravel from the mountains erosion. In most years water runs off, or flows down through the soil and then emerges as springs at the base of the hill.
However in 1983 all the water from the spring was running down into the soil, and upon hitting the bedrock, it backed up. It basically formed a layer of mud, with all the heavy water, and mud on top of it weighing it down. That eventually started to move.
Many people call it a landslide, however a landslide is actually considered to be fairly fast. 1 mile an hour or faster. That does not seem that fast, but consider all the material that is moving. A slump is slower. Like when your kids take mud in a bucket and tip it over. When you remove the bucket it will slowly slump and sag. That is a similar process.
It slowly crept down the hill, and then pushed the tracks, and road, until it filled the river valley and hit the other wall and blocked the river.
Slumps are usually categorized by the broken ground uphill. Like part of it just sagged. They involve sub surface movement, where the above ground sags. That leaves the ground and trees on top of it. Where a landslide or avalanche tends to be surface motion, and it buries the trees and plants.
Lake Thistle
A lake was formed behind the dame (see the lower waypoint). It reached about 160 feet deep and many people worked to stabilize the slide, and dig tunnels to divert the water. Eventually the lake was drained after a number of months. The slump was considered to unstable to allow it to be used as a dam, especially with a city just below it. (even if it was Spanish Fork Dons-my not so subtle jab at our high school rivals - Go Springville Red Devils!!)
The lake floated away some structures, others were nearly buried in the silt and debris from the spring flood waters that descended from the two canyons. Eventually when the lakes were drained the road and rail line were replaced. It was considered too dangerous for people to return and live.
This was the most expensive landslide in US history. Costing over 200 million dollars (in 1983).