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History of Levee Construction
Sacramento-San Joaquin levees were built in the mid to late 19th Century to prevent flooding on prime agricultural land. Most of the land was at sea level, and levees were frequently constructed on top of natural dirt barriers that formed along rivers and sloughs. Many of the levees were built by Chinese laborers using hand shovels and wheelbarrows. By the turn of the 20th Century, the sidedraft-clamshell dredge was used to remove material from riverbeds to increase the size of levee barriers. These Delta levees are built on a foundation of sand, silt and peat which make them susceptible to erosion, seepage and breaks. In 1917 Congress authorized the Sacramento Flood Control Project, which was completed by the U.S. Army Corps of Engineers in 1960. “Project levees” built by the Corps were designed to provide superior flood control protection. Once completed, the general upkeep was turned over to local entities. There are over 1600 miles of State-federal project levees in the Central Valley, 385 miles of which are located in the Delta. Over 700 miles, or 65 percent, of Delta levees are classified as “non-project.” These flood control structures have been built and maintained by landowners or reclamation districts to protect agricultural lands. Frequently, they are not as stable as the project levees. History of Levees
One hundred fifty years ago, the levees of the Sacramento-San Joaquin Delta were raised to prevent flooding on some of the most fertile farmland in the nation. Though the peat soils were excellent for agriculture, they didn’t make strong foundations for barriers meant to contain a constant flow of river water. As farmers settled the valleys, gold prospectors were drawn to the hills. Mining in the Sierra Nevada washed entire mountainsides into local streams and rivers. This silt deposited in riverbeds of the central valley which increased flood risk. As a remedy to rising riverbeds, levees were built very close to the channel to keep water velocity high and scour away sediment. However, these narrow channels have been too successful. The gold rush silt is long gone, but the erosive force of the river continues to eat away at the levee system. In addition, the peat soils of the Delta have subsided, gradually lowering the elevations of Delta islands. Some of these parcels are now more than 20 feet below sea level Many changes have taken place in the Delta in the last 150 years. Today, the levees protect not only farms, but also hundreds of thousands of people who live and work in central valley communities. Both the federal Central Valley Project and the State Water Project, built to deliver water to millions of Californians, are vitally dependent on fragile Delta levees to protect water supply and water quality. State highways, railroad lines, water supply pipelines that serve much of the San Francisco Bay area, energy transmission lines, and petroleum pipelines now cross the Delta, and rely on the continued stability of Delta levees. All together, more than $47 billion in infrastructure is protected by central valley levees. |
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