The water loss phenomena in the Washington DC metropoliton area's (WMA) Potomac River water supply basin during droughts was analyzed. Gage errors, permitted withdrawals, evaporation, and transpiration by trees along the river were investigated to account for loss. The Interstate Commission on the Potomac River Basin (ICPRB) calculated potential gage error and examined permits to determine permitted levels of consumption withdrawals from the Potomac. The result of a single slug test indicated that the soil transmissivity may not be adequate to allow passage of enough water to account for all of the calculated water loss.