Inundation of State Highway 162 across Butte Basin at the latitude of Butte City results from overland floodflow from the Sacramento River and flooding on Butte Creek. Flooding of Butte Basin from the Sacramento River will occur whenever flow in the main channel at Butte City exceeds 90,000 cubic feet per second (2550 cubic metres per second), a discharge with a recurrence interval of about 3 years. The distribution of floodflow across the basin is not uniform. During the flood of January 24, 1970, 84 percent of the total discharge resulted from overland flow from the Sacramento River and 16 percent from flooding on Butte Creek. When flooding in Butte Basin is severe enough to affect State Highway 162, overflow across the road first occurs between bridges 11-21 and 11-22. The construction of bridges or culverts at two locations between bridges 11-21 and 11-22 would increase the period of time that the road is usable. Analysis of the present roadway, bridge geometry, and ground elevations adjacent to the roadway indicates that backwater is less than 0.6 foot (0.2 metre) for flows of the magnitude experienced during the flood of January 24, 1970. The concurrent maximum velocity of flow at the bridges is 6.8 feet per second (2.1 metres per second). Part of the backwater is caused by ground elevations adjacent to the roadway that are, at many locations, higher than the road crown. If the roadway embankment were raised to prevent overtopping by a flood equivalent to that of January 24, 1970, without increasing the capacity or number of the bridges, backwater greater than 0.5 foot (0.2 metre) would result upstream from 6 of the 15 bridges on State Highway 162, and velocities would be excessive. Additional bridge openings to discharge a total of 37,800 cubic feet per second (1070 cubic metres per second) would be required for at least six locations if backwater and velocity were to be kept to levels similar to those observed for present conditions. During the flood of November 12, 1973, water-surface elevations recorded in several of the overflow channels of Butte Creek were higher than those observed during the January 1970 flood even though the discharge was less. The higher water-surface elevations in 1973 probably resulted from vegetal growth in the channels and consequent reduction in the capacity of the channel to carry floodflows.