The Surface Water Supply Index (SWSI) may be considered for studying hydrologic conditions and agricultural water management. By using this indicator, water resources conditions of Colorado and Oregon basins were investigated from extremely wet to extreme drought. The SWSI values can also be plotted as a time series graph while critical years were specified. This allows the user to graphically visualize the values from year to year and to see how the current year's values change from year to year. Managers can then refer to records from critical years in determining strategies for dealing with the current years’ water supply. Also evident is whether the streamflow component or the reservoir component is the predominant driving force at any given time. SWSI's can be an excellent water management tool in determining overall risk and management strategies. It gives the water user and manager more information than simply streamflow or reservoir level alone. According to the results, obtained categories based of SWSI values are indicated hydrologic conditions for Colorado and Oregon States with two different climates. Although decisions only based on geographic and climatic information due to the insufficient and sometimes contradictory results than the SWSI can cause water loss or increase the risk of drought.