The replacement of Locks and Dam 26, Upper Mississippi River at Alton, Illinois, has been an extremely controversial project. One of the central points of this controversy is the capacity of these locks to process the traffic transiting this portion of the waterway system; the capacity is measured in tons of commodities moved by the tows passing through these locks. The determination of a capacity value for Locks and Dam 26 and adjacent locks is the subject of this report. Methods of determining the capacity of a lock are discussed in the first part of the report. Consideration of all factors influencing a lock's capacity and the interrelationship of these factors are discussed. Based on this consideration and the interest shown in having an easy-to-apply, straightforward method for calculating lock capacities, a new methodology for computing lock capacities was developed and is presented herein. Results from such a computation are compared with those obtained using a simulation modeling approach.