From mavensnotebook.com – The Yolo Bypass, a remnant of the historical floodplain that existed here historically, plays an essential role in flood management.  The Sacramento River flows in from the north, the American River flows in from the east, and the Yolo Bypass is the flood overflow basin for the system. The Fremont Weir, at the top of the Yolo Bypass, essentially acts as a low head dam or a berm; when the flow of the Sacramento River reaches a certain point, the water spills out over the weir and onto the vast floodplains of the Yolo Bypass.  The Sacramento Weir, located further downstream by Sacramento, is used only in the highest flow years as a flood outlet valve.  The floodwaters in the Bypass eventually drain back into the system near Rio Vista. (more)