Hydrogeology of the upper Deschutes Basin, central Oregon; a young basin adjacent to the Cascade volcanic arc

deposits thin out against relatively impermeable lower Tertiary deposits of the John Day and Clarno Formations (Fig. 2), forcing nearly all the northward-flowing ground water to discharge into the Deschutes River and its tributaries. This massive amount of ground-water discharge, exceeding 60 m3/s (2,000 ft3/s) near the confluence of the Deschutes and Crooked Rivers, is the principal reason for the remarkably stable flow of the Deschutes River. Participants on this trip will explore the visible and conceptual aspects of the regional ground-water hydrology of the upper Deschutes Basin, including the interaction between ground water and streams. The trip follows the general direction of ground-water flow northward from the headwaters of springfed streams at the margin of the Cascade Range to the principal regional discharge area near Lake Billy Chinook. This guidebook describes a 2-day trip. Day 1 begins in the La Pine Subbasin (uppermost Deschutes Basin) and proceeds through Bend, concentrating chiefly on the hydrologic controls created by Quaternary stratigraphy and structure (Fig. 3). Day 2 examines strata of the Deschutes Formation from Bend to Madras and the geologic factors that influence regional ground-water discharge. A road log for each day is at the end of the field-trip guide. The metric system is used for all scientific aspects of the guidebook except altitude and water discharge rate, which are given in both meters and feet (altitude) and cubic meters per second and cubic feet per second (rate) owing to the widespread familiarity with U.S. traditional units in these matters. The road log is reported in miles to match most car odometers. INTRODUCTION The upper Deschutes Basin encompasses about 11,700 km2 of the Deschutes River drainage basin in central Oregon (Fig. 1). Chiefly draining the east flank of the Cascade Range, the upper Deschutes Basin extends northward from a drainage divide near Chemult that separates it from the Klamath Basin to the south. The eastern margin of the basin lies along the south part of the Ochoco Mountains and through the crest of Newberry Volcano. The northern boundary is near Warm Springs, northwest of Madras. The upper Deschutes Basin is underlain by Quaternary and Tertiary volcanic and sedimentary rocks. The occurrence and movement of ground water and the interaction of ground water and streams are controlled by the distribution of permeability within the depositional sequence. The permeability distribution reflects the age, lithology, and depositional environment of the strata, along with the subsequently imposed geologic structure. The crest of the Cascade Range, including a broad upland area east of the Three Sisters, is the principal source of recharge for the groundwater system. The average annual rate of recharge from precipitation in the upper Deschutes Basin is estimated to be roughly 108 m3/s (3,800 ft3/s) (Gannett and others, 2001). Ground water moves eastward from the Cascade Range and then generally northward through permeable Quaternary and upper Tertiary deposits. North of Madras, the permeable