Hydrogeology of the McKinney Butte Area: Sisters, Oregon

Abstract

McKinney Butte, a late Tertiary andesite vent and flow complex, is located near the town of Sisters, Oregon, in the upper Deschutes Basin, and is situated along the structural trend that forms the eastern margin of the High Cascades graben (Sisters fault zone and Green Ridge). Rapid development and over appropriated surface water resources in this area have led to an increased dependence upon groundwater resources. A primary concern of resource managers is the potential impact of expanding groundwater use on stream flows and spring discharge. Two sets of springs (McKinney Butte Springs and Camp Polk Springs) discharge to Whychus Creek along the east flank of McKinney Butte, and during low-flow conditions supply a substantial component of the total flow in the creek. Despite their contribution to Whychus Creek, the springs along McKinney Butte are small-scale features and have received less attention than larger volume (> 2 m³/s) springs that occur in the basin (i.e., Metolius Spring and Lower Opal Springs). This study used discharge measurements in Whychus Creek upstream and downstream of the springs, and mixing models using measurements of electrical conductivity and temperature in the springs and Whychus Creek to determine the contribution of the springs to the creek. Isotopic, thermal, and geochemical signatures for the McKinney Butte and Camp Polk Springs, and local streams (Whychus Creek and Indian Ford Creek) and springs (Metolius Spring, Paulina Spring, Alder Springs, and Lower Opal Spring) were assessed to determine the source(s) of the McKinney Butte and Camp Polk Springs. The discharge and hydrochemical data along with hydraulic head data from local wells were used in the development of a conceptual model of groundwater flow for the McKinney Butte area. Discharge from the McKinney Butte Springs supplies the majority of water to Whychus Creek on the east flank of McKinney Butte (~0.20 m³/s), provides up to 46% of the flow in the creek, and is relatively stable throughout the year. Discharge from the Camp Polk Springs is less than 0.05 m³/s. Isotopic, thermal, and geochemical signatures indicate distinct sources for the McKinney Butte and Camp Polk Springs. Groundwater discharged at the McKinney Butte Springs is depleted in heavy stable isotopes (δD and δ¹⁸O) relative to the Camp Polk Springs. Recharge elevations inferred from stable isotope concentrations are 1800-1900 m for the McKinney Butte Springs and 950-1300 m for the Camp Polk Springs. Elevated water temperature in the McKinney Butte Springs relative