The Yolo Bypass is filling with water due to the recent heavy rains, and the Yolo Bypass Wildlife Area is in the midst of a seasonal closure as a result.
The California Department of Fish and Game announced Monday that the Wildlife Area was closing due to “forecasted flooding until further notice.”
And Ted Thomas, information officer with the California Department of Water Resources, confirmed Thursday morning that water is flowing over the Fremont Weir from the Sacramento River into the Yolo Bypass, and had been for the past day or so.
The Fremont Weir, completed in 1924, is a two-mile-long structure on the south side of the Sacramento River, not far from the confluence of the Sacramento and Feather rivers, at a point about eight miles northeast of Woodland and about 15 miles northwest of Sacramento.
When the river rises to 33.5 feet, water flows over the Fremont Weir and into the Yolo Bypass, which is roughly three miles wide and about 40 miles long. The Yolo Bypass conveys the water southward into the Sacramento-San Joaquin Delta, and the water ultimately flows into the San Francisco Bay.
The California Department of Water Resources describes the Yolo Bypass as “a great ‘safety valve’ taking huge quantities of water out of the river channel and allowing it to flow slowly and safely out to the delta.” Otherwise, there would be a high risk of flooding in the lower parts of the Sacramento urban area, a common event in the late 1800s.
When flooded, the Yolo Bypass also provides a stopover point for migrating wildfowl, and an excellent environment for fish that swim up through the delta to lay their eggs. Recent studies have shown that the slower-moving, nutrient-rich water in the seasonally flooded Yolo Bypass helps young fish grow more quickly than they would in the faster-moving, channelized Sacramento River itself.
At full flood, the Yolo Bypass also conveys an enormous amount of water — more than the river itself.
“The levee and bypass system along the Sacramento River system handle many times the normal flow of the Sacramento River,” according to the Department of Water Resources. When filled to capacity, the bypass system “carries a maximum of 600,000 cubic feet per second (cfs), many times the normal flow of the Sacramento River. Only a sixth of that flow — 110,000 cfs — is carried by the river itself. Nearly 500,000 cfs is channeled into the Yolo Bypass.”
This includes water that flows over Fremont Weir, as well as water entering the Yolo Bypass from creeks and sloughs along the side of the bypass.
The seasonal flooding of the Yolo Bypass in wet winters creates a large and comparatively shallow body of water.
“In more than half of all water years (from Oct. 1 to Sept. 30), the Yolo Bypass is inundated,” according to the Department of Water Resources. “When completely flooded, the Yolo Bypass covers an area equal to about one-third the size of San Francisco and San Pablo Bays. Water depths range from 10 feet in a heavy year to around 6 feet in a normal year.”
Seasonal flooding in the Yolo Bypass can inundate the acreage for anywhere from a few days to a few months, depending on how hard it rains and snows over the watersheds of the Sacramento River, the Feather River and their tributaries, officials said.
The Yolo Bypass includes about 59,000 acres of land, of which roughly 17,700 acres is part of the Yolo Bypass Wildlife Area. The remainder of the land includes private duck clubs and agricultural land, including a substantial amount of acreage devoted to rice.
— Reach Jeff Hudson at firstname.lastname@example.org or 530-747-8055.