SOURCES OF SPRINGS DISCHARGING FROM MCKINNEY BUTTE, UPPER DESCHUTES BASIN, OREGON

Several springs emanate from the east flank of McKinney Butte, a late Tertiary andesite vent and flow complex, located near the town of Sisters, Oregon in the upper Deschutes Basin. The origin of the springs has been the subject of speculation by area water managers for years. The springs discharge at various elevations along the east side of the butte, and are tributary to Whychus Creek. Whychus Creek originates in the Cascade Mountains and flows generally northeast, but abruptly jogs north around the east flank of McKinney Butte adjacent to the spring complexes. On the west side of the butte, Indian Ford Creek, another tributary of Whychus Creek, loses water. Stable isotope (δD and δ¹⁸O) and major ion concentrations in Whychus Creek, Indian Ford Creek, and springs on the east side of McKinney Butte (Frank springs complex and Camp Polk springhouse) are used to infer approximate recharge elevations and flow patterns for groundwater discharging from the springs. Stable isotope concentrations in Frank springs complex (δD = -106.6, δ¹⁸O = -14.2) are similar to Whychus Creek (δD = -106.0, δ¹⁸O = -14.5) and are depleted in heavy isotopes relative to Camp Polk springhouse (δD = -96.8, δ¹⁸O = -12.8) and Indian Ford Creek (δD = -92.4, δ¹⁸O = -12.7). Average recharge elevations inferred from stable isotopes are 1050 m for Camp Polk springhouse and 1850 m for Frank springs complex. Camp Polk springhouse samples contain higher concentrations of Cl^- = 5.10 mg/L, SO₄^2- = 2.24 mg/L, and NO₃^- = 3.18 mg/L than Frank springs complex (Cl^- = 1.65 mg/L, SO₄^2- = 1.14 mg/L; NO₃^- = 1.19 mg/L), suggesting Camp Polk springhouse discharges groundwater that is more affected by anthropogenic sources (e.g. fertilizer and septic systems) than Frank springs complex. Although Frank springs complex and Camp Polk springhouse are separated by only a few hundred meters, stable isotope and major ion data suggest recharge supplying Frank springs complex follows relatively deeper flowpaths and originates at high elevations close to the crest of the Cascades Range (~20 km away) while Camp Polk springhouse discharges locally derived shallow groundwater.