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WOU ES 473 - Geological Control of Spring and Channel Systems and Management Implications Cascade Range, Oregon

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M Furniss, C Clifton, and K Ronnenberg, eds., 2007. Advancing the Fundamental Sciences: Proceedings of the Forest Service National Earth Sciences Conference, San Diego, CA, 18-22 October 2004, PNW-GTR-689, Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station.A River Runs Underneath It: Geological Control of Spring and Channel Systems and Management Implications, Cascade Range, OregonAnne JeffersonDept. of Geosciences, Oregon State University, Corvallis, OregonGordon E. GrantUSDA Forest Service, Pacific Northwest Research Station, Corvallis, OregonSarah L. LewisDept. of Geosciences, Oregon State University, Corvallis, OregonLong-term sustainable management of Cascade Range watersheds requires an understanding of water sources and discharge patterns from tributary streams, particularly those sourced in large-volume cold springs of the High Cascades geologic province. Focusing on the McKenzie River watershed, measurements of discharge and stream temperature combined with laboratory analysis of spring water isotopes improve our understanding of spatial and temporal recharge and discharge patterns. Summer streamflow in the McKenzie is dominated by water from approximately ten spring-fed streams, which maintain 4 to 7°C spring water temperatures and relatively steady flow throughout the summer. In winter months, streams in the Western Cascades geologic province respond rapidly to rain and rain-on-snow events and become the major water source to the McKenzie River. Spring-fed streams also respond to precipitation events, but show muted and delayed hydrograph peaks. Summer flow behavior varies among springs, even between those that are located near each other. Isotopic data reveal that recharge to large springs occurs between 1300-1800 m in elevation, which is coincident with geologically young lava between McKenzie and Santiam Passes. Recharge elevations also suggest some disagreement between recharge areas and topographic watersheds of the springs. Because of their importance to summer streamflow, water quality, and habitat in the McKenzie River basin, water resources decision-making must differentiate between spring-fed and runoff-dominated streams. Keywords: streamflow, groundwater processes, geology/geomorphology, springs, water temperature, Oregon Cascades INTRODUCTIONThe Willamette River basin is home to 70% of Oregon’s population, and while the McKenzie River watershed covers less than 12% of the basin’s area, it provides almost 25% of the Willamette River’s water during low flow periods (PNWERC 2002). The McKenzie watershed includes threatened and endangered fish runs, a complex system of federal and private dams for flood control and hydroelectric power generation, and Oregon’s second largest city (Eugene), which draws 10 billion gallons (37.8 billion L) of drinking water per year directly from the river.Recent analyses show that the Cascade Range is particularly sensitive to current and projected climate warming trends, specifically reduced snow accumulation and earlier spring melt, leading to a decline in summer streamflow (Service 2004). By 2050, Cascade snowpacks are projected to be less than half of what they are today (Leung et al. 2004), potentially leading to major water shortages during the low-flow summer season. Although not yet as contentious as the Klamath River to the south, the stage is set in the Willamette basin for significant future conflicts and demands for water. Despite the importance of the McKenzie River’s water to the region’s quality of life, geologic and climatic controls on patterns of streamflow have been poorly understood. The watershed of the McKenzie River lies primarily within two distinct geologic provinces: the High and Western Cascades (Figure 1). The High Cascades are known for their active composite volcanoes and extensive Quaternary basaltic lavas, while the Western Cascades are the products392 A RIVER RUNS UNDERNEATH ITFigure 1. Forty-two percent of the McKenzie River watershed lies within the High Cascades geologic province, the most of any major Willamette River tributary. Geologic classification is based on mapping by Sherrod and Smith (2000).393JEFFERSON ET AL.of Tertiary volcanism, and have been extensively faulted, weathered, and dissected (Conrey et al. 2002). The Western Cascades have well-developed drainage networks and watersheds dominated by shallow-subsurface, runoff-dominated flow (Harr 1977), while in the High Cascades large areas lack drainage networks and many of the streams are fed by springs. Preliminary hydrograph analyses indicate that High Cascades streams show much more uniform flow and temperature through time compared to Western Cascades streams (Tague and Grant 2004). These differences have significant implications for water quantity and quality in headwater streams and for larger rivers, such as the McKenzie and Willamette, where both High and Western Cascades streams contribute to flow. This research represents a systematic attempt to quantify volumes and sources of discharge in the McKenzie River basin. The overall goal of the project was to provide a more complete picture of flow contributions to the McKenzie River, for use in planning the sustainable long-term water management of the basin. Our objectives were to identify sources of summer streamflow to the upper McKenzie River, obtain continuous discharge records for large spring-fed streams, and use isotopic information to characterize groundwater recharge patterns. Our field campaign focused on the upper McKenzie River basin, defined as the 2409 km2 watershed upstream of the USGS gage at Vida. This area encompasses all of the High Cascades geology in the basin, as well as two Western Cascades tributaries with Corps of Engineers flood control reservoirs. USGS stream gages operate at four locations on the mainstem of the McKenzie River and on the tributaries Blue River, Separation Creek, Smith River, and the South Fork of the McKenzie. The USGS gages for the McKenzie River at Clear Lake and Separation Creek include a substantial spring-fed component, but do not represent a pure spring signal.METHODSIn summer 2003, discharge was measured at all McKenzie River tributaries flowing


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WOU ES 473 - Geological Control of Spring and Channel Systems and Management Implications Cascade Range, Oregon

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