Surface water oxygen and hydrogen isotopic values are commonly used as proxies of precipitation isotopic values to track modern hydrologic processes while proxies of water isotopic values preserved in lake and river sediments are used for paleoclimate and paleoaltimetry studies. Previous work has been able to explain variability in USA river-water and meteoric-precipitation oxygen isotope variability with geographic variables. These studies show that in the western United States, river-water isotopic values are depleted relative to precipitation values. In comparison, the controls on lake-water isotopic values are not well constrained. It has been documented that western United States lake-water input values, unlike river water, reflect the monthly weighted mean isotopic value of precipitation. To understand the differing controls on lake- and river-water isotopic values in the western United States, we examine the seasonal distribution of precipitation, evaporation and snowmelt across a range of seasonality regimes. We generate new predictive equations based on easily measured factors for western United States lake-water, which are able to explain 69–63% of the variability in lake-water hydrogen and oxygen isotopic values. In addition to the geographic factors that can explain river and precipitation values, lake-water isotopic values need factors related to local hydrologic and climatic characteristics to explain variability. Study results suggest that the spring snowmelt runs off the landscape via rivers and streams, depleting river and stream-water isotopic values. By contrast, lakes receive seasonal contributions of precipitation in proportion to the seasonal fraction of total annual precipitation within their watershed. Climate change may alter the ratio of snow to rain fall, affecting water resource partitioning between rivers and lakes and by implication of groundwater. Paleolimnological studies must account for the multiple drivers of water isotopic values; likewise, studies based on the isotopic composition of fossil material need to distinguish between species that are associated with rivers versus lakes. Copyright © 2010 John Wiley & Sons, Ltd.