Geologic Factors That Control the Occurrence and Availability of Ground Water in the Fort Rock Basin Lake County, Oregon By E. R. HAMPTON HYDROLOGY OF VOLCANIC-ROCK TERRANES GEOLOGICAL SURVEY PROFESSIONAL PAPER 383-B Prepared in cooperation with the Oregon State Engineer UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publication as follows: Hampton, Eugene R., 1934- Geologic factors that control the occurrence and availabil­ ity of ground water in the Fort Rock Basin, Lake County, Oregon. Washin,,oton, U.S. Gqvt. Print. Off., 1964. iii, 29 p. illus., maps (1 fold. col. in pocket) diagrs., tables. 30 cm. (U.1S. ·Geological Survey. Professional paper 3183-B) Hydro~ogy of volcanic-rock terranes. Prepared in cooperation with the Oregon 'State Engineer. Bibliography: p. 29. (Continued on next card) Hampton, Eugene R., 1934- Geologic factors that control the occurrence and availability of ground wa;ter in the Fort Rock Basin, Lake County, Oregon. 1964. (Card 2) 1. Geology-Oregon-Lake CO. 2. Water-supply--Oregon-Lake Co. 3. Water, Underground-Oregon-Lake Co. 4. Borings-Ore­ gon-Lake Co. I. Oregon. State Engineer. II. Title. III. Title: Fort Rock Basin, Lake County, Oregon. (Series) For sale by- the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Geology-Continued Page Abstract______ --___________ -------_________ --__ ___ _ Bl Quaternary rocks-Continued Introduction_______________________________________ 1 Unconsolidated deposits-Continued Purpose and scope of the investigation_____________ 1 Delta deposits _______ -- _______ ---- __ --- B15 Location and extent of the area___________________ 2 Dune sand___________________________ _ 15 Previous investigations__________________________ 2 Stream-valley alluvium ___ -- ____ -- -- _ - -- 16 Acknowledgm.ents_______________________________ 3 Younger basalt______________________ -- _ 16 Well-numbering system__________________ -- __ ___ _ 3 Geologic structure___________ --- ___ - ----- -------- 16 GeographY----------------------------------------- 3 Folds----- --------------------------------- 16 Climate________________________________________ 3 Faults____________________________________ _ 17 Culture and industry____________________________ 4 Ground water _____________________________ - -- ____ -- 17 Landforms and drainage_________________________ 4 Infiltration and recharge________________________ _ 17 Basin floor_________________________________ 4 Discharge-------------------------------------- 18 Mountain uplands-------------------------- 4 Water-level fluctuations _____ --- ___ - -- ___ --- ___ -- 18 Geology------------------------------------------- 5 Chemical quality of the ground water____________ _ 18 General description and relationship of rock units___ 5 Occurrence in subareas of the Fort Rock Basin____ _ 19 Tertiary rocks__________________________________ 5 Hager Mountain slope ______________________ _ 20 Picture Rock Basalt_________________________ 5 Paulina Marsh subarea_____________________ _ 21 Volcanic rocks of intermediate composition_____ 7 Thorne Lake subarea_______________________ _ 21 Fort RoCk Formation_______________________ 7 Seven Mile Ridge subarea___________________ _ 22 Hayes Butte Basalt------------------------- 11 Lost Forest subarea________________________ _ 22 Tertiary and Quaternary rocks_------------------ 12 Fort Rock-Christmas Lake Valleys ___________ _ 22 Peyerl Tuff-------------------------------- 12 Hayes Butte slope _________________________ _ 23 Paulina Basalt______________________________ 14 Horse Ranch subarea_______________________ _ 23 Quaternary rocks_______________________________ 14 Potential for additional development _____________ _ 23 Unconsolidated deposits_____________________ 14 Well construction ___________________________ ---_ 24 Lakebed and terrace deposits____________ 14 References________________________________________ _ 29 ILLUSTRATIONS [Plate is in pocket] PLATE 1. Geologic map and sections of the Fort Rock Basin, Lake County, showing location of selected wells. Page FIGURE 1. Map of Oregon showing the area of this investigation___________________________________________________ B2 2. Map of the Fort Rock Basin, L"ake County, showing general features in the area studied____________________ 3 3. Horning Bend and Hayes Butte as seen from the southwest______________________________________________ 4 4. Stratigraphic column showing the relationships of the rock units mapped in the Fort Rock Basin_ ------------ 5 5. Rhyodacite on the west side of Cougar Mountain------------------------------------------------------- 7 6. Bedded tuff of the Fort Rock Formation-------------------------------------------------------------- 9 7. Fort Rock viewed from the northeast----------------------------------------------------------------- 10 8. Annual precipitation and cumulative departure from the average precipitation at Fremont for the period 1910-59 and hydrograph of year-end water levels in well 27/15-4Gl-------------------------------------------- 18 9. Map of the Fort Rock Basin showing principal ground-water subareas______________________________________ 19 10. Map of the Fort Rock Basin showing areas irrigated from wells in 1960___________________________________ 20 TABLES Page TABLE 1. Drillers' logs of selected wells in the Fort Rock Basin, Lake County, Oreg___________________________________ B24 2. Chemical analyses of water from wells in the Fort Rock Basin, Lake County, Oreg___________________________ 28 m HYDROLOGY OF VOLCANIC-ROCK TERRANES GEOLOGIC FACTORS THAT CONTROL THE OCCURRENCE AND AVAILABILITY OF GROUND WATER IN THE FORT ROCK BASIN, LAKE COUNTY, OREGON By E. R. HAMPTON AB'STRACT slopes that surround them. Some hay and grain crops The Fort Rock Basin, in south-central Oregon, is a high are raised near the edges of the valley plains, although plateau area of interior drainage comprising about 1,500 square the climate is cool and dry and the growing season miles. The precipitation is sparse-less than 10 inches per year short. The dry climate and short growing season on the basin floor-and the growing season is less than 100 days. Consequently, the ready availability of ample water for make irrigation necessary for the production of eco­ irrigation is a requisite for successful farming in the area. nomic yields of most crops. Surface water is available Three perennial streams enter the southwestern part of the for irrigation only in the Silver Lake area. In other basin and provide water for irrigation and stock supplies for parts of the basin irrigation water must be obtained that part of the basin. In the rest of the basin, water for irri­ from wells. gation, stock, and domestic supplies is obtained from wells. About 100 irrigation wells were being used in the Geologic structural features, consisting mainly of broad, gen­ tle folds and predominantly northward-trending faults, largely area during 1960 and additional wells were being control the depth and areal distribution o.f the water-bearing drilled. The lack of infonnation on the rock mate­ rock units. Thus, the availability of ground water in any par­ rials underlying the basin has hampered prediction of ticular part of the basin is directly related to the geologic struc­ the well-drilling conditions and the water-bearing ture and the rock units present. character of the rocks at any locality, although the The rock units mapped in the basin range in age from Pliocene to Recent and are from oldest to youngest as follows: Picture position of the water table is fairly well known. Thus Rock Basalt, volcanic rocks of intermediate ·composition, Fort most wells, being largely exploratory, have been un­ Rock Form•ation, Hayes Butte Basalt, Peyerl Tuff, Paulina Ba­ duly costly and a considerable number have been un­ salt, unconsolidated deposits, and younger basalt. Of these successful, either because of improper well construction units, the volcanic rocks of intermediate composition, Peyer! or because permeable water-bearing rocks were not Tuff, and young basalt yield little <>r no water to wells; the other units yield small to large quantities of ground water present. where they are saturated. Yields as great as 4,000 gallons per The objectives of this investigation are: (1) to ob­ minute are obtained fr<>m wells tapping the Picture Rock Basalt. tain information on the extent, structure, and water­ The more productive aquifers yield water of good quality, but bearing characteristics of the rock units underlying the a few well,s tapping poorly productive zones in the Fort Rock area, (2) to determine the control of these geologic fac­ Formation yield moderately saline water. Pumpage in 1960 was tors on the occurrence, availability, and chemical qual­ about 12,000 acre-feet-about one-tenth the estimated average annual recharge of 125,000 acre-feet. ity of the ground water for the area as a whole and for Long-term water-level fluctuations lag about 10 years behind specific subareas, (3) to make these data available to the recorded long-term precipitation fluctuations. This fact in­ persons concerned with development or management of dicates that most of tbe ground-water recharge occurs on the the ground-water resources of this and similar upland sloping flanks of the basin and that the water requires years basins. to move to the areas of discharge in the