The Arctic is undergoing unusual and apparently progressive changes in the land, ocean, and atmospheric components of the hydrologic cycle that could have long-term consequences for both local and global climate. The prime motivation of the SEARCH/CHAMP/ASOF integration study (Solicitation NSF-02-071) is to better understand these changes and their consequences, including identification of natural modes of variability and responses to anthropogenic forcing. To achieve this goal, the combined efforts of empiricists and modelers of the linked atmosphere-land-ocean system are needed. This proposal is submitted as part of a number of independent but highly complementary research efforts and is focused on measuring the biogeochemical characteristics of river waters as they flow from land into the Arctic Ocean. The overall objective is to use river water chemistry as a means to study the origins and fates of continental runoff. Tracers in addition to temperature and salinity are needed to identify river contributions to surface waters of the Arctic Ocean because melting of sea ice confounds interpretation of temperature-salinity mixing models. Furthermore, tracers provide a means to distinguish contributions from different rivers regionally. A 5-year project is proposed in which selected parameters focusing on tracers of river water will be measured in the largest 6 rivers that drain the watershed of the Arctic Ocean: the Yenisey, Lena, Ob', Mackenzie, Yukon, and Kolyma. Tracers to be measured include H218O, barium, organic matter, alkalinity, and nutrients. Sampling will be conducted during the high flow season in Years 1 and 4 and during all seasons in Years 2 and 3. Synthesis and modeling will occur throughout the 5-year project. Samples will be collected near the mouths of the rivers (but above tidal influence) in order to get fully integrated watershed signals and the most relevant freshwater endmember values for oceanographic tracking. To investigate transport pathways of river waters through the Arctic Ocean, newly defined endmember values will be examined in the context of existing and growing oceanographic tracer databases. To study watershed sources, tracers will examined in the context of geomorphology, land cover, and hydrology models. The proposed work will also generate a synoptic pan-arctic database on river biogeochemistry for comparison with past and future data in trend analyses. Understanding sources and fates of river discharge is important because rivers make an enormous contribution to the freshwater budget of the Arctic Ocean, presently accounting for 50 to 60 percent of all freshwater inputs. General Circulation Models (GCMs) predict increased moisture transport to high latitudes and melting of ice stores with continued global warming. Should this occur, the resulting freshening of waters exported from the Arctic Ocean may significantly impact North Atlantic Deep Water (NADW) formation. This in turn has consequences for global ocean circulation and climate. Already large-scale changes in the arctic hydrologic cycle are evident, including indications of increasing discharge in major Eurasian arctic rivers. Our proposed work will provide insight into circulation of freshwater through the Arctic Ocean and help elucidate mechanisms responsible for arctic-wide changes in the hydrologic cycle.