Hydrogeologic Setting and Preliminary Estimates of Hydrologic Components for Bull Run Lake and the Bull Run Lake Drainage Basin, Multnomah and Clackamas Counties, Oregon U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 96-4064 Prepared in cooperation with CITY OF PORTLAND BUREAU OF WATER WORKS Cover. Photograph shows Bull Run Lake, viewed southeastward towards Mount Hood. The scene was described by Woodward (Portland Oregonian, August 18, 1918): “ The lake, clear, deep, and cold, ***, nestles like an emerald or turquoise with the changing lights or shadows at the base of these rocky, wooded slopes and above and beyond all, overlooking and reflected in its clear depths, lies snow-capped Mount Hood.” Photograph credit: U.S. Forest Service, circa 1960. Hydrogeologic Setting and Preliminary Estimates of Hydrologic Components for Bull Run Lake and the Bull Run Lake Drainage Basin, Multnomah and Clackamas Counties, Oregon By DANIEL T. SNYDER and DORIE L. BROWNELL U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 96-4064 Prepared in cooperation with CITY OF PORTLAND BUREAU OF WATER WORKS Portland, Oregon 1996 U. S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director The use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information Copies of this report can be write to: purchased from: District Chief U.S. Geological Survey U.S. Geological Survey, WRD Branch of Information Services 10615 S.E. Cherry Blossom Drive Box 25286, Federal Center Portland, OR 97216 Denver, CO 80225 CONTENTS Abstract .......................................................................................................................................................................................1 Introduction.................................................................................................................................................................................2 Purpose and Scope .............................................................................................................................................................3 Approach............................................................................................................................................................................3 Description of Study Area..................................................................................................................................................3 Physiography and Hydrography................................................................................................................................3 Climate ......................................................................................................................................................................5 Land Use ...................................................................................................................................................................6 Soils and Vegetation..................................................................................................................................................7 History.......................................................................................................................................................................7 Acknowledgments..............................................................................................................................................................8 Hydrogeologic Setting ................................................................................................................................................................8 Geology..............................................................................................................................................................................8 Columbia River Basalt Group...................................................................................................................................8 Younger Rocks..........................................................................................................................................................9 Formation of Bull Run Lake ..............................................................................................................................................9 Origin of the Lake .....................................................................................................................................................9 Description of the Natural Dam ..............................................................................................................................12 Ground-Water Flow System.............................................................................................................................................14 Flow Within the Landslide......................................................................................................................................14 Flow Within the Consolidated Rocks......................................................................................................................15 Implications for the Use of Ground Water in the Bull Run Watershed...................................................................17 Preliminary Estimates of Hydrologic Components ..................................................................................................................19 Water Inflows to the Bull Run Lake Drainage Basin .......................................................................................................23 Precipitation on the Lake Surface ...........................................................................................................................23 Precipitation on Land Surface.................................................................................................................................23 Rain and Snow ...............................................................................................................................................23 Fog Drip .........................................................................................................................................................25 Water Outflows from the Bull Run Lake Drainage Basin................................................................................................26 Evaporation from the Lake......................................................................................................................................27 Evapotranspiration from the Land Surface .............................................................................................................27 Ground-Water Outflow............................................................................................................................................27 Outflow through the Landslide.......................................................................................................................28 Outflow through the Consolidated Rocks ......................................................................................................33 Streamflow from the Lake.......................................................................................................................................34 Change in Storage ............................................................................................................................................................34 Bull Run Lake .........................................................................................................................................................34 Soil Moisture...........................................................................................................................................................34 Ground Water..........................................................................................................................................................35 Discussion of Hydrologic Components for the Bull Lake Drainage Basin .....................................................................35 Discussion of Hydrologic Components for Bull Run Lake .............................................................................................35 Suggestions for Further Study...................................................................................................................................................36 Methods............................................................................................................................................................................36 Summary ...................................................................................................................................................................................39 Selected References ..................................................................................................................................................................41 iii FIGURES 1.– 2. Maps showing: 1. Location of the Bull Run Watershed.......................................................................................................................2 2. Topography, probable extent of landslide, and general features of the area around Bull Run Lake ......................4 3. Map and diagrammatic section showing geologic features of the Bull Run Lake area .............................................11