Four periods of extreme turbidity were measured in Monterey Submarine Canyon over the past 12 yr. These turbid events occurred simultaneously with the four largest flood events of the nearby Salinas River. They filled the canyon with fresher, warmer, and, apparently, buoyant water that extended to depths below 1 km. The low-salinity signature must reflect underflow of the Salinas River plume that was driven to depth by negative buoyancy contributed from the suspended sediment. A flux estimate for the 1995 event suggests that one-half, or more, of the river's suspended sediment load was carried down the canyon in the underflow. Underflows may contribute significantly to the carbon budget of the continental slope.