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Yolo Bypass quietly keeps Sacramento on dry ground

When the rain begins to fall and rivers rise this week, Sacramentans should pause for a moment of gratitude to the Yolo bypass – a simple, yet highly effective flood control measure that has been preventing floods that once inundated downtown Sacramento for more than 90 years.

<p>Sacramento was under water in this 1862 photograph. Conditions like these led to the creation of the Freemont Weir and Yolo bypass.</p>

When the rain begins to fall and rivers rise this week, Sacramentans should pause for a moment of gratitude to the Yolo bypass – a simple, yet highly effective flood control measure that has been preventing floods that once inundated downtown Sacramento for more than 90 years.

The magic starts at the Fremont weir, a low, unassuming concrete structure that runs along a stretch of the Sacramento River near the confluence of the Feather River. When the river rises beyond a certain point, water begins to flow over the weir and into the Yolo bypass, a low-lying area of grassland, ponds and seasonal wetlands that drivers on I-80 pass over on their way between Sacramento and Davis.

The rivers are expected to rise to overtop the Freemont Weir as a result of Thursday’s rain by Friday, said Ted Sommer, lead scientist for the California Water Resources Department.

When people think about manmade flood control devices, they usually think of dams and levees designed to block the flow of water, Sommer said.

The weir represents a novel approach to flood control.

“It was pretty visionary,” Sommer said, adding that it’s a bit of a tourist attraction for flood managers in other regions. “We don’t have anything quite like it in the country.”

After the gold rush, when Sacramento suffered a series of devastating floods, local officials were focused on levees, which had been proven effective in holding back flooding on the Mississippi River. Will Green, a Colusa newspaperman, suggested that simply blocking the water wouldn’t be enough – a better strategy would be to work with existing flood plains to divert water.

Engineers for the predecessor of today’s U.S. Corps of Engineers found the ideal place to do that about 15 miles northwest (as the crow flies) of Sacramento near the Knight’s Landing community. By building the weir, they enhanced the diversion of water to route it into the wetlands rather than allowing it to continue downstream to threaten Sacramento and its levees. This is important because while the Sacramento River can carry 100,000 acre feet per second, the Yolo bypass has the capacity for five times that, Sommer said. The water reenters the Sacramento River delta near Rio Vista.

Although it is only one part of Sacramento’s flood control system, it’s an important part.

“Really, Yolo bypass keeps us from being underwater,” Sommer said.

The bypass is also a haven for young salmon, who find food and protection from predators in its weedy shallows. During dry periods, hikers, bikers, sportsmen and farmers use it for recreation and agriculture.

Another little-known attribute of the bypass is its beauty when seen from above while in flood stage.

“The bypass does something really visually stunning,” Sommer said.

Several waterways converge, but their streams do not mix right away, spreading down the landscape to form a striking pattern of bands or stripes.

“This is classic earth porn,” Sommer said.

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