Distribution of oxygen-18 and deuterium in river waters across the United States

Reconstruction of continental palaeoclimate and palaeohydrology is currentl y hampered by limited information about isotopic patterns in the modern hyd rologic cycle. To remedy this situation and to provide baseline data for ot her isotope hydrology studies, more than 4800, depth- and width-integrated, stream samples from 391 selected sites within the USGS National Stream Qua lity Accounting Network (NASQAN) and Hydrologic Benchmark Network (HBN) wer e analysed for delta O-18 and delta H-2 (http://water.usgs.gov/pubs/ofr/ofr 00-160/pdf/ofr00-160.pdf). Each site was sampled bimonthly or quarterly for 2.5 to 3 years between 1984 and 1987. The ability of this dataset to serve as a proxy for the isotopic composition of modem precipitation in the USA is supported by the excellent agreement between the river dataset and the i sotopic compositions of adjacent precipitation monitoring sites, the strong spatial coherence of the distributions of delta O-18 and delta H-2, the go od correlations of the isotopic compositions with climatic parameters, and the good agreement between the 'national' meteoric water line (MWL) generat ed from unweighted analyses of samples from the 48 contiguous states of del ta H-2 = 8.11 delta O-18 + 8.99 (r(2) = 0.98) and the unweighted global MWL of sites from the Global Network for Isotopes in Precipitation (GNIP) of t he International Atomic Energy Agency and the World Meteorological Organiza tion (WMO) of delta H-2 = 8.17 delta O-18 + 10.35. The national MWL is comp osed of water samples that arise in diverse local conditions where the loca l meteoric water lines (LMWLs) usually have much lower slopes. Adjacent sit es often have similar LMWLs, allowing the datasets to be combined into regi onal MWLs. The slopes of regional MWLs probably reflect the humidity of the local air mass, which imparts a distinctive evaporative isotopic signature to rainfall and hence to stream samples. Deuterium excess values range fro m 6 to 15% in the eastern half of the USA, along the northwest coast and on the Colorado Plateau. In the rest of the USA, these values range from -2 t o 6%, with strong spatial correlations with regional aridity. The river sam ples have successfully integrated the spatial variability in the meteorolog ical cycle and provide the best available dataset on the spatial distributi ons of delta O-18 and delta H-2 values of meteoric waters in the USA.