How levees fail This graphic details how our current system of levees can fail: Overtopping, Through Seepage and Underseepage. Click the graphic above for a full-size view. To download a print-friendly PDF, click here. Graphic: The National Flood Insurance Program in California, Department of Water Resources. Levee failure example: Through Seepage. Click the photo above for a full-size view. Photo: Sutter County Information Office. Levee failure example: Underseepage (Sand Boil). Click the photo above for a full-size view. Photo: Bill Hampton, Levee District 1

The Yuba Basin (aka the Sutter Basin) is a flat plain below the elevation of high flows in the surrounding Sacramento, Feather, Yuba and Bear Rivers.

"Every winter and spring, rainfall and melting snow result in often-destructive stream flows coming off the mountains into the rivers. Before we began building levees and dams, Sierra runoff would form deep pools on much of the Valley floor, taking months to drain into San Francisco Bay."

David Kennedy, water engineer and former head of the State Department of Water Resources.

The first organized responses to seasonal floods were simple dirt levees, generally built by farmers to protect their crops and farm properties. The early settler's levees were often no more than berms of loose dirt, sometimes built over old lake beds. Today's levees are frequently built on top of those older leaky foundations of porous, unstable and sandy soils.

After major floods in the early part of the 20th century, the US Army Corps of Engineers constructed a comprehensive and connected set of levees and bypasses (or overflow channels) to contain the river runoff. Eventually, dams were also built that act as shock absorbers, storing sudden storm water surges to avoid overtopping levees.

Despite efforts to ward off inundation, levee breaches in 1917, 1955, 1986 and 1997 have resulted in major flooding that have affected the region, resulting in dozens of deaths and millions of dollars in property damage.

Studies show weakness in
Feather River levees

Many Central Valley levees are now under scrutiny. Some leak and slump because of water pressure forcing water through the levee; others fail because of seepage underneath because the levees were originally built on sandy, porous soils. New federal rules will call for upgrading, and may mandate flood insurance and land use controls.

California weather is changing, perhaps as a result of global climate change. More precipitation is falling in the mountains as rain, and less as snow pack. This change will increase the stress on the region's flood control system.

The State of California's agency that looks at flood protection, the Department of Water Resources, recently conducted new engineering tests of the levees that surround the Yuba City Basin, including sophisticated ground radar and soil borings. The Sutter Butte Flood Control Agency hired independent geotechnical engineers from the firm Kleinfelder, Inc. to look closely at the data from those studies. Kleinfelder's engineers believe that the entire levee along the west side of the Feather River must be rehabilitated. For more on their recommendations and the Flood Agency's Early Implementation Plan to improve local flood protections, see Flood Solutions.

Because of these levee weaknesses, the Federal Emergency Management Agency (FEMA) has been revising local maps that identify flood-prone areas, and is requiring flood insurance coverage for all federally backed mortgages (see Flood Insurance for updates on FEMA's remapping efforts and related insurance issues).