The lock at Chickamauga Dam on the Tennessee River may have to close for various periods for major repair work. Such scheduled or unscheduled closures will disrupt the barge traffic serving many industries located upstream of the dam. Some of the commodities moving on barges will be diverted to trucks, depending on the nature of the closures, and the new truck traffic may result in a variety of effects, or externalities. A method is presented to estimate the effects of increased truck traffic. Delays from traffic congestion and incidents, pavement damage, truck-related accidents, and air pollution are examined. The study was performed for the Tennessee Valley Authority.