Matilija Dam facts for kids
Quick facts for kids Matilija Dam |
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View of Matilija Dam from the air
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| Country | United States |
| Location | Ojai, California |
| Status | 90% silted |
| Opening date | 1947 |
| Construction cost | about $4M |
| Owner(s) | Ventura County Watershed Protection District |
| Dam and spillways | |
| Type of dam | Concrete arch dam |
| Height | 198 ft (60 m) (original) |
| Length | 620 ft (190 m) |
| Reservoir | |
| Creates | Matilija Reservoir |
| Total capacity | 7,018 acre⋅ft (8,657,000 m3) (original) 150 acre⋅ft (190,000 m3) (current) |
The Matilija Dam is a large concrete arch dam in Ventura County, California. It was finished in 1947. This dam was built to store water and help control floods. It holds back the water of Matilija Creek to create the Matilija Reservoir. This reservoir is located in the Los Padres National Forest, near Ojai.
The area that drains into the dam is about 55 square miles. When it was first built, the reservoir could hold a lot of water. Matilija Creek later flows into the Ventura River. The name Matilija comes from the Chumash people, who lived in this area long ago.
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Why Was Matilija Dam Built?
In the 1940s, the Ventura County Flood Control District was created. Its job was to manage water in the Ventura River area. People wanted a dam on Matilija Creek. This was part of a bigger plan to help refill underground water supplies around Ojai. These water supplies had been used up by farms and dry weather.
In 1941, the U.S. Army Corps of Engineers warned that the dam might not be a good idea. They said the land upstream was steep and would cause the dam to fill with dirt and sand very quickly. But the project went ahead. The county raised money to build it. Construction started in 1946 and finished in 1948. It cost almost $4 million, which was much more than first thought.
How Did the Dam Affect the Area?
The dam flooded an area that people used for fun activities. It also blocked the way for steelhead trout. These fish need to swim upstream to lay their eggs. The dam was built without a special path for fish, called a fish ladder. Before the dam, thousands of steelhead trout would lay eggs in the Ventura River each year. After the dam, only a few hundred could. In 1949, many fish died upstream because the water in the reservoir got too hot or still.
Experts later said the dam's design had problems from the start. Many groups, including the U.S. Army Corps of Engineers, were against the project. An architect even warned that the building materials would react badly together. Twenty years later, a study showed he was right. The dam was cracking and moving.
What Happened to the Dam Over Time?
Soon after it was built, the dam started filling up with dirt and sand, called silt. The dam traps about 30% of the silt that would normally flow into the Ventura River. This means less sand reaches the ocean beaches. Engineers first thought it would take 39 years for the reservoir to fill with silt. But it filled much faster.
In 1964, a study found the dam was unsafe. It suggested removing the dam. At that time, removing it would have cost $300,000. Instead, the county decided to make the dam shorter. They cut a part out of the top, making it 30 feet lower. This reduced the water pressure on the dam. They cut it again in 1978. Because of the silt and the shorter height, the reservoir was expected to be completely filled with silt by 2020.
Why Is Removing the Dam Important?
Ventura County decided to remove the dam back in 1998. In 2000, a government official visited the dam. He helped bring national attention to the idea of removing dams. Matilija Dam has been a candidate for removal ever since. Another dam, the Rindge Dam near Malibu, is also a candidate. It completely filled with silt by 1950 and also blocked fish paths.
Removing Matilija Dam has been delayed. People worry about how it might affect floods and the river's environment. A lot of silt is trapped behind the dam. If it all came out at once, it could block the river. It could also harm the Robles Diversion Dam downstream, which provides water for Lake Casitas.
In 2004, the U.S. Army Corps of Engineers estimated it would cost $200 million to remove all the silt. Moving the silt by trucks would be too hard. It would need hundreds of thousands of truck trips, and there's no good place to put the silt.
What Are the Plans for Dam Removal?
The Ventura County Watershed Protection District and the Army Corps came up with a plan. They would remove about a quarter of the silt before taking down the dam. Then, they would make some of the remaining silt stable with a special cement. This stable silt would slowly wash away during big floods over several years. This way, the river would not get too much silt at once. A bill to fund this project reached Congress in 2007, but the money did not come through.
In 2016, new ideas for removing the dam were suggested:
- Build a temporary dam: A small dam and tunnel could be built upstream. This would send Matilija Creek around the dam site while the dam is being removed. This temporary dam would break during big floods, letting silt flow to the ocean.
- Drill holes in the dam: Holes could be drilled at the bottom of the dam. Plugs could be removed during floods. This would let high water flows wash the silt out of the reservoir naturally. Once enough silt is gone, the dam could be taken down.
- Store silt upstream: Silt could be dug out and stored in piles upstream of the dam. These piles would slowly wear away during high water events. This would prevent too much silt from going into the Ventura River at once.
In 2017, the California Department of Fish and Wildlife gave $3.3 million to design the chosen plan. The most recent idea, expected to cost $111 million, involves drilling two 12-foot wide holes in the bottom of Matilija Dam. It also includes replacing two bridges on the Ventura River and improving the Robles diversion dam to handle more silt.
