Spatial distribution and seasonal variation in ¹⁸O/¹⁶O of modern precipitation and river water across the conterminous USA

Abstract We report a quantitative analysis of regional differences in the the oxygen isotope composition of river water and precipitation across the USA because data are now available to undertake a more geographically and temporally extensive analysis than was formerly possible. Maps of modern, mean annual δ 18 O values for both precipitation (δ 18 O PPT ) and river water (δ 18 O RIV ) across the 48 contiguous states of the USA have been generated using latitude and elevation as the primary predictors of stable isotope composition while also incorporating regional and local deviations based on available isotopic data. The difference between these two maps was calculated to determine regions where δ 18 O RIV is significantly offset from local δ 18 O PPT . Additional maps depicting seasonal and extreme values for δ 18 O RIV and δ 18 O PPT were also constructed. This exercise confirms the presence of regions characterized by differences in δ 18 O RIV and δ 18 O PPT and specifically identifies the magnitude and regional extent of these offsets. In particular, the Great Plains has δ 18 O RIV values that are more positive than precipitation, while much of the western USA is characterized by significantly lower δ 18 O RIV values in comparison with local δ 18 O PPT . The most salient feature that emerged from this comparison is the ‘catchment effect’ for the rivers. Because river water is largely derived from precipitation that fell upstream of the sample locality (i.e. at higher elevations) δ 18 O RIV values are often lower than local δ 18 O PPT values, particularly in catchments with high‐elevation gradients. Seasonal patterns in the isotopic data substantiate the generally accepted notion that amplitudes of δ 18 O variation are greatly dampened in river water relative to those of local precipitation. Copyright © 2005 John Wiley & Sons, Ltd.