Understanding the temporal and spatial variability of water sources within a basin is vital to our ability to interpret hydrologic controls on biogeochemical processes and to manage water resources. Water stable isotopes can be used as a tool to determine geographic and seasonal sources of water at the basin scale. Previous studies in the Coastal Range of Oregon reported that the variation in the isotopic signatures of surface water did not conform to the commonly observed "elevation effect", which exhibits a trend of increasing isotopic depletion with rising elevation. The primary purpose of this research is to investigate the mechanisms governing seasonal and spatial variations in the isotopic signature of surface waters within the Marys River Basin, located in the leeward side of the Oregon Coastal Range. Surface water and precipitation samples were collected every 2-3 weeks for isotopic analysis for one year. Our results confirmed the lack of elevational variation of surface water isotopes within this leeward basin. While we find elevational variation in precipitation in the eastern portion of the watershed, this elevation effect is counteracted by rainout with distance from the Pacific coast. In addition we found significant variation in surface water isotope values between catchments underlain predominantly by basalt or sandstone. The degree of separation was strongest during the summer when low flows reflect deeper groundwater sources. This indicates that baseflow within streams drained by each lithology is being supplied from two distinctly separate water sources. In addition, the flow of the Marys River is dominated by water originating from the sandstone water source, particularly during the low flow summer months. We interpreted that the difference in water source results from sandstone catchments having highly fractured geology or locally tipping to the east facilitating cross-basin water exchange from the windward to the leeward side of the Coast Range. Our results challenge topographic derived watershed boundaries in permeable sedimentary rocks; highlighting the overwhelming importance of underlying geology.
Keywords: Coast range; base flow; elevation; lapse isotopic rate; lithology; source water; water isotopes.