(KTXL) — During Sacramento’s centuries-long history of battling flood waters, inhabitants have devised nearly every possible method of slowing or diverting water, and one of those methods is using the Sacramento Weir.

Completed in 1916, the more than 1,900-foot long weir featuring 48 gates sits along the west bank of the Sacramento River about three miles north of the confluence of the Sacramento and American rivers.

However, the placement and purpose of the Sacramento weir differs from typical weirs found along other streams and rivers.

The University of Calgary defines a weir as a small barrier built across a stream or river that allows water to pool behind it and flow over it in a steady and controlled manner.

The Sacramento weir does not run across the Sacramento River, does not allow water to pool up behind it and does not allow for the water to run over the top.

“Its primary purpose is to protect the City of Sacramento from excessive flood stages in the Sacramento River by diverting river flows west into the two-mile-long Sacramento Bypass that connects to the Yolo Bypass,” the Sacramento Area Flood Control Agency writes.

Overflow water from the Sacramento River can make its way through the gates of the weir, but when water levels at the I Street Bridge reach 29.87 feet the gates of the weir are opened.

The raging flood waters then enter into the Sacramento Bypass and then into the Yolo Bypass which holds 80% of the flood waters from the Sacramento, American and Feather Rivers.

The last time the gates were opened along the weir was in January 2017, when significant rainfall swelled the Sacramento River and threatened to cause levee breaches.

“During a major flood event, flows from the American River channel often exceed the capacity of the Sacramento River downstream of the confluence,” the Sacramento Area Flood Control Agency writes. “When this occurs, floodwaters flow upstream from the mouth of the American River to the Sacramento Weir.”

The United States Army Corps of Engineers and the Sacramento Area Flood Agency has been working on the creation of a 1,500 foot expansion of the Sacramento weir and an additional 2-mile-long bypass, adjacent to the existing one.

This expansion is expected to reduce water surface elevation in the Sacramento River and not impact operations of the old weir, according to an April 2020 report from the U.S. Army.