Sacramento River Rising, and the Yolo Bypass Floods

Update, Monday, March 14: As forecast, the Sacramento River began overflowing the Fremont Weir over the weekend, flooding the Yolo Bypass.

The California-Nevada River Forecast Center says the river will crest later Monday about 2.5 feet over the weir, then will fall slowly and subside below the weir level by Saturday.

Original post: One sign that we have really returned, at least temporarily, to the wet season: For the first time in more than three years, the Sacramento River is about to spill over into the Yolo Bypass.

What and where is the Yolo Bypass and why in the world would you care?

If you've driven from the Bay Area to Sacramento on Interstate 80, you've seen it: In drier times, it looks like a random patchwork of marshes and farm fields as you speed across the Yolo Causeway, the long bridge between Davis and West Sacramento.

The bypass is a sort of flood relief valve for the Sacramento -- a 2- to 3-mile-wide artificial channel that runs for nearly 40 miles from a bend in the river above Sacramento to a collection of sloughs north of Rio Vista. Its principal purpose is to give the river someplace to go when it gets high instead of threatening the state capital and surrounding communities.

The California-Nevada River Forecast Center says that the Sacramento will rise above Fremont Weir, an engineered cut in the levee about 15 miles upstream from downtown Sacramento, perhaps as early as Friday night.

The forecast calls for the river, fed by a series of storms expected to continue into next week, to continue to rise for at least a few days. That in turn will send an increasing volume of flood water into the bypass.

Aside from a geeky interest in the mega-plumbing projects that control where and how water flows in Northern California, why might you care about the Yolo Bypass?

One: It looks cool when it floods. When the water is really high -- higher than it's likely to get in the coming few days -- it resembles a vast, turbulent, muddy sea. In California, we rarely see so much water moving so fast.

Two: Ironically, for a deliberately engineered feature, the bypass turns out to play a crucial role for birds, fish and other wildlife. The section of bypass immediately south of Interstate 80 and the Yolo Causeway has been turned into a 25-square-mile wildlife area that's said to be home to more than 200 species of birds.

Three: Recent research has discovered, or maybe rediscovered, the role of seasonal wetlands in supporting Central Valley salmon populations. Young chinook salmon migrating down the Sacramento River toward the Sacramento-San Joaquin Delta will stray into the bypass when flows allow. Biologists say the flooded countryside provides abundant food for the juvenile fish and protects them from predators.

And finally, the bypass is a remnant of a type of California landscape, both when it's flooded and when it's not. That alone makes it worth exploring. If you're curious, here's a site that shows the location of Fremont Weir, north of the Yolo County town of Woodland.

I was out there a little more than a year ago when the wildlife area at the northern end of the bypass was high and dry. Though the place is not far from civilization -- it's just a few miles from Sacramento International Airport -- I had it pretty much to myself. It will certainly be different in wet weather, and the one note of caution I'd add if you're inclined to go is that the smaller county roads leading to the Fremont Weir area may be a little iffy after the big rains of the last week.