Water Prism is a decision support system (DSS) that evaluates alternative management plans to obtain water resource sustainability at the regional, watershed, or local levels. It considers surface, ground, and impoundment waters and all water-using sectors (industrial, agricultural, municipal, electric power, and the environment). This technical update illustrates how Water Prism is used by applying it to two large watersheds: the Muskingum River Basin (Ohio) and the Green River Basin (Kentucky).