Pleistocene glacial outburst flooding along the Big Lost River, east-central Idaho Item Type Thesis-Reproduction (electronic); text Authors Rathburn, Sara L. Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 01/10/2021 12:16:04 Link to Item http://hdl.handle.net/10150/191351 PLEISTOCENE GLACIAL OUTBURST FLOODING ALONG THE BIG LOST RIVER, EAST-CENTRAL IDAHO by Sara L. Rathburn A Prepublication Manuscript Submitted To the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 1989 STATEMENT BY AUTHOR This manuscript, prepared for publication in GEOLOGY, has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the Antevs Reading Room to be made available to borrowers as are copies of regular theses and dissertations. Brief quotations from this manuscript are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the department, or the graduate student coordinator, when in their judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: APPROVAL BY RESEARCH ADVISORY COMMITTEE This manuscript has been approved for submission on the date shown below: Prprd Ms-7 1 -7 4pr01 lc Graduate Student Coordinator, or Head of Department TABLE OF CONTENTS ACKNOWLEDGEMENTS i ABSTRACT ii INTRODUCTION 1 Regional Setting and Study Area 4 EVIDENCE FOR CATACLYSMIC FLOODING 5 Erosional Features 5 Streamlined Loess-Capped Hill 5 Cataracts and Scabland Topography 8 Loess Scarp 9 Depositional Features 9 Longitudinal Bars 9 PALEOHYDROLOGY AND PALEOHYDRAULICS 13 Paleostage Indicators 13 Discharge Estimates 14 Stream Power Estimates 20 FLOOD HYDRODYNAMICS 21 Erosional Patterns 24 Depositional Patterns 25 FLOOD SOURCES 29 Glacial Lake East Fork 29 Back-of-the-Envelope Flood Routing 31 Hydraulic Ponding and Other Flood Sources 33 CONCLUSIONS 34 REFERENCES 36 APPENDIX ACKNOWLEDGEMENTS Numerous people kindly contributed their time and energy to various aspects of this study. Fellow graduate students J. O'Connor, E. Wohl, K. Vincent, and J. Finley provided field assistance, helpful discussions, thorough reviews of earlier drafts, and unfailing friendship. Many of the ideas presented in this manuscript stem from conversations and guidance generously offered by J. O'Connor. My advisory committee of V. Baker, L. Mayer, and D. Hendricks substantially improved this manuscript with their insightful comments. B. Bull, committee member in abstentia, loaded abundant ideas on me during the initial phases of my work. I am also grateful to several scientists outside the University of Arizona. J. Tullis, EG&G, Idaho Falls, loaned me air photos of Box Canyon, and expressed great enthusiasm for this project. R. Smith, EG&G, Idaho Falls, provided me with an excellent set of aerial photos covering my study reach, and offered substantive input. L. Mann at the U.S. Geological Survey, Idaho Falls granted me permission to work on the INEL site, accompanied me into the field, and generously allowed me the use of numerous aerial photos. S. Anderson, and J. Putnam also of the U.S.G.S., Idaho Falls, proved to be good companions and an interesting diversion during my summer in Mackay. Thanks are also due to my computer consultants/spiritual advisors J. Finley and L. Mayer. And finally a heart-felt thanks to J. Finley, and Katie and Buz Rathburn; their calming words sustained me throughout this project. Support for this research was funded by a Sigma Xi grant to S. Rathburn, and NSF grant EAR-8805321 awarded to V. Baker. ABSTRACT Cataclysmic flood features including scabland topography, streamlined hills, a loess scarp, and flood transported boulders were mapped along Box Canyon, lower Big Lost River, eastern Snake River Plain. These features are similar to landforms within the Cheney-Palouse scabland tract, eastern Washington, formed by the great Missoula floods. Step-backwater hydraulic modeling of flow through the 11 km-long Box Canyon gorge indicates that a discharge of 60,000 m3sec-1 was required to produce the geologic paleostage evidence. Maximum stream power per unit area of bed locally attained values of 26,000 Wm-2, which is comparable to the more extensive late Pleistocene Bonneville and Missoula flows. Flood power, estimated to exceed 1000 Wm-2, induced plucking of the jointed-basalt channel banks of Box Canyon. Tracts of scabland with networks of anastomosing channels migrated headward, driven by unit stream power values in the 600-1000 Win-2 range. Deposition of the largest flood boulders occurred above a limiting unit stream power of 1400 Wm-2. This ceiling on boulder deposition indicates that entrainment of these largest boulders probably took place under maximum unit stream power conditions (26,000 Wm-2). The irregular volcanic rift topography along Box Canyon was the dominant control on removal and accumulation of flood boulders, however. Paleoflooding along Box Canyon may in part, although probably not solely, be attributed to outbursts from a glacial lake in the headwaters region located in the Pioneer Mountains. ii INTRODUCTION Climatic fluctuations and glaciation during the Pleistocene created widespread fluvial adjustments through changes in water discharge and sediment load. Episodes of deglaciation often induced cataclysmic flooding which modified large tracts of the landscape. The effects of such floods were especially marked and have been preserved in several areas of the present-day arid regions of the northwestern United States. Important examples include the Bonneville and Missoula flows (Fig. 1), first documented by Gilbert (1878) and Bretz (1923), respectively, and further described by Malde (1968), Baker (1973a), and numerous others. Bedrock gorges formed in resistant basalt of the Snake River Plain and Columbia Plateau continue to provide rich opportunities for study into the nature of Pleistocene extreme, rare floods. Box Canyon, containing a segment of the Big Lost River, east-central Idaho, is another site of Pleistocene cataclysmic flooding (Figs. 1 and 2) (Rathburn, 1988). Distinctive landforms adjacent to and within the 11 km-long canyon are strikingly similar to other well-studied terrains of flood origin, particularly the Cheney-Palouse tract, eastern Washington. The Big Lost River flooding is herein characterized through qualitative descriptions and quantitative analyses of large-scale flow processes. Erosional and depositional high water indicators combined with hydraulic modeling provide estimates of the peak discharge and power of the flood. Geomorphological interpretations of cataclysmic flood-induced landforms along Box Canyon offer insight into the nature of landform modification in response to high magnitude floods. This approach augments the existing inventory of Pleistocene cataclysmic events. 1 Figure 1. Location map showing the Big Lost River in relation and to regional features other sites of Pleistocene cataclysmic flooding. (Modified from Baker, 1983). 2 EXPI_A51AT1CN PLEISTOCENE FLOOCING Big Lost River Flood site MissouIci Flax extent Bonneville Flood extent Bitterroot Mtns. Figure 2. Location map of the Big Lost River extending from the headwaters in the Pioneer Mountains, east-central Idaho, to the Big Lost River Sinks on the Idaho National Engineering Lab (INEL). Main study reach along the lower Big Lost River is shaded. ,..)1 N.,.. .. ,,. .. 1. ....,/ .'" . '1.n —I ..... .. %. ...... X- 4'3 \ .. /Z./AN .... -1 i -'\'-s-s-.••••-......7. ''... _ — — I n .N. IN '' le i" " . .. - :41,t' , — - • ote I s 0 1 7 6,8 I 1 1 _ -, z ••• 0 2 ••• n o 65 o o CO O. o *a. I o • • " 0 "' l b" 4 Regional Setting and Study Area The Big Lost River drains an area of 3800 km2, above Arco, Idaho with streamflow originating in the Pioneer, White Knob, and Lost River Mountains (Fig. 2). The East and North Forks of the Big Lost River join to form the main stem of the Big Lost River approximately 42 km northwest of Mackay. Downstream of this confluence the Big Lost River curves southeast, flows down the Big Lost River Valley, and emerges onto the Snake River Plain at Arco as a series of alluvial braid channels. From Arco, the Big Lost River continues southeast across the nearly flat basalt- covered surface of the eastern Snake River Plain through the 11 km-long Box Canyon gorge. On average, the gorge is 40 m wide and 23 m deep with vertical walls cut into columnar-jointed, massive and vesicular basalt. Volcanic rift zones dominate this portion of the Snake River Plain with lava flow ridges, tumuli, and pressure plateaus forming irregular topography with several meters of relief (Malde, in press). Downstream of Box Canyon, the Big Lost River becomes alluvial and crosses the western boundary of the Idaho National Engineering Laboratory (INEL). Further downstream the river turns northward and surface flow ends in a playa known as the Big Lost River Sinks. Modern stream flow is often depleted before reaching Box Canyon by irrigation diversions and infiltration losses along the river. The latter recharge the Snake River Plain aquifer of Quaternary basalt flows and intercalated sediments (Nace, 1975). The major focus of this study is a 20 km reach of the Big Lost River, from southeast of Arco through Box Canyon to near the western boundary of the INEL site (Fig. 2). Cataclysmic flood evidence is most abundant and striking in this reach. 5 EVIDENCE FOR CATACLYSMIC FLOODING Flood features were mapped along the study reach using aerial photographic interpretation (scales 1:40,000 and 1:24,000), and field observation (Fig.