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

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 tempo ally extensive analysis than was formerly possible. Maps of modem, mean annual delta O-18 values for both precipitation (delta O-18(PPT)) and river water (delta O-18(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 delta O-18(RIV) is significantly offset from local delta O-18(PPT). Additional maps depicting seasonal and extreme values for delta O-18(RIV) and delta O-18(PPT) were also constructed. This exercise confirms the presence of regions characterized by differences in delta O-18(RIV) and delta O-18(PPT) and specifically identifies the magnitude and regional extent of these offsets. In particular, the Great Plains has 318ORIV values that are more positive than precipitation, while much of the western USA is characterized by significantly lower delta O-18(RIV) values in comparison with local delta O-18(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 hi-her elevations) 318 delta O-18(RFV) values are often lower than local delta O-18(PPT) values, particularly in catchments with high-elevation gradients. Seasonal patterns in the isotopic data substantiate the generally accepted notion that amplitudes of delta O-18 variation are greatly dampened in river water relative to those of local precipitation. Copyright (c) 2005 John Wiley & Sons, Ltd.