Competition for water among governmental entities with common river systems has become increasingly fierce. The competition will no doubt become even more fierce as populations continue to grow and become increasingly dispersed. One of the major issues involved in water resource availability is the construction of reservoirs to provide continuous and dependable water supply. The paper uses a technique, 'deficit analysis', that matches available flow versus actual and/or projected demands. If demand exceeds available flow, a deficit occurs that must be satisfied by flow augmentation or reservoir capacity. Standard techniques such as the Ripple procedure or routing techniques can reveal total amounts of water required. By studying the shape and timing of deficits important additional design information can be gained. The technique is applied in a case study context to reservoir selection in the Washington D.C. Metropolitan Area.