Western rivers are modified by construction of reservoirs to serve irrigation, flood control, and power production. Municipal supplies from these sources must be taken as is and treated to meet the varied requirements of the users. Return irrigation water, natural brine pollution, and contamination from pesticides, herbicides, and fertilizing nutrients from agricultural land runoff introduce purification problems that require adoption of new technics such as activated carbon filtration and ionic irradiation. More sophisticated analytical tools are needed to help the purification plant operator control treating systems.