What is a weir?
When water gets too high on the Sacramento and American rivers, the state can divert water to keep Sacramento from flooding. Both rivers merge at Discovery Park north of downtown. The Sacramento Weir is in West Sacramento along River Road.
Through the weir, the state can open the gates and actually suck water from both rivers into the Yolo Bypass under the Yolo Causeway. That's the stretch that connects Davis and West Sacramento. There are other weirs upstream including the Fremont Weir west of Sacramento International Airport.