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The influence of lithology on surface water sources
Nickolas, L., C. Segura, AND J. Renee Brooks. The influence of lithology on surface water sources. Hydrological Processes. John Wiley & Sons, Ltd., Indianapolis, IN, 31(10):1913-1925, (2017).
Climate change is will change water availability for municipalities that rely on surface water sources, but to understand how water resources will change requires in-depth understanding of what sources of water feed streamflow. We examined the controls on the variability of source water dynamics within the Marys River Basin, whose river serves as the primary source of municipal water for the cities Corvallis and Philomath, Oregon and provides significant amounts of water to local agricultural operations. Isotopic analysis of water within the basin revealed that the underlying bedrock geology influences the water sources feeding the river seasonally. We found that basalt catchments in the central portion of the basin may be subject to a substantial decline in streamflow, yet the make up the majority of the Rock Creek watershed supplying water to the city of Corvallis. During the dry summer period, the higher permeability sandstone-drained catchments to become the primary contributors to streamflow for the river, contributing over 80% of the flow during these critical low-flow periods. Sandstone catchments along the western side of the basin may be continuously fed by highly fractured, cross-basin sandstone aquifers during these dry periods, essentially providing a subsidy of water to the Marys River besides local precipitation. Given the increasing prevalence of drought and record low flows such as those observed in the summer of 2015 (www.oregon.gov/owrd/WR/docs/Drought_Information_Statement_Nov_2015.pdf), the need to understand what regulates flow within basins supplying drinking water will be critical to municipalities providing water to their constituents. This paper contributes to ACE CIVA 2.3
Understanding the temporal and spatial variability of surface water sources within a basin is vital to our ability to manage the impacts of climate variability and land cover change. 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 does not conform to the commonly observed “rainout 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 of δ18O and δ2H for one year. Results indicate a significant difference in isotopic signature between watersheds underlain by basalt and sandstone. The degree of separation was the most distinct during the summer when low flows reflect deeper groundwater sources, whereas isotopic signatures during the rainy season (fall and winter) showed a greater degree of similarity between the two lithologies. This indicates that baseflow within streams drained by sandstone versus basalt is being supplied from two distinctly separate water sources. In addition, Marys River flow at the outlet is dominated by water originating from the sandstone water source, particularly during the low flow summer months. We posit two theories that may explain the evident difference in source: 1) local geological characteristics (e.g. permeability) lead to differences in the residence time of groundwater, or 2) sandstone formations dipping to the East within the Coast Range may facilitate cross-basin water exchange from the windward to the leeward side of the range.