Pleistocene glacial outburst flooding along the Big Lost River, east-central Idaho
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Authors Rathburn, Sara L.
Publisher The University of Arizona.
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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
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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
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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. 3). The
nomenclature and mapping units used by Baker (1978a) in his map of the Cheney-
Palouse scabland tract, eastern Washington, were adopted for the Box Canyon reach to
allow visual comparison with a well-studied flood landscape. Examples of Box Canyon
erosional and depositional landforms, diagnostic of cataclysmic floods, are described.
Erosional Features
Streamlined Loess-Capped Hill
One lemniscate-shaped hill abutting the downstream end of a bedrock outcrop is
present within the study site (Figs. 3 and 4). This streamlined loess-capped hill sits
slightly oblique to the inferred direction of flow (parallel to the Big Lost River), and
has a length and width of approximately 770 m and 400 m, respectively. A prominent,
blunt upstream prow contrasts sharply with its smoothly tapered downstream end.
Arcuate-shaped cataracts and pour-overs of varying sizes bound the upstream sides of
the hill.
Streamlined features have been interpreted to form as fluid flow splits around a
resistant landform and rejoins on the downstream end, thus minimizing flow resistance
(Baker, 1973b; Komar, 1983). The basalt obstacle at the head of the streamlined loess-
capped hill (Fig. 3) is moderately plucked and scoured, and probably caused the divided
flow. The Box Canyon streamlined hill has a length-width ratio of about 2, somewhat smaller than the corresponding Channeled Scabland forms which have a length-width ratio of about 3 (Baker, 1978a). Figure 3. Geomorphic map of cataclysmic flood features along Box Canyon, lower Big Lost River. LMC denotes Lost Moon Cataract. Complied from 1:24,000 scale base maps. Boulder train Loess C7=' Boulder bar Sc Scabland topography Cataract Major scour area Slightly scoured bedrock 4144'7.1"' ÇTh Minor basalt pour-over Basalt bedrock Inferred paleoflow direction
5H -L Streamlined loess-capped hill • Erratic
Loess scarp 5H -B Streamlined boulder-mantled hills
0 1 2 3 kilometers 1111111111111111•111111111.11M Figure 4. Oblique aerial photograph of the streamlined loess-capped hill with approximate dimensions of 400 m wide by 770 m long. View is upstream (NW) toward Arco, Idaho, with the Arco Hills in the background. LMC - Lost Moon Cataract. 7
L)
..J 8
Cataracts and Scabland Topography Sizeable tracts of irregular, loess-stripped topography exist within the study reach. These tracts are mapped as scabland topography (Fig. 3) denoting highly scoured terrain denuded of a soil mantle similar to that originally described by Bretz
(1923). Two distinct types of features are abundant within these scabland areas: cataracts, and minor pour-overs. Both are characterized by an arcuate shape and an exposed basalt lip, often columnar-jointed, which opens downstream. Minor pour-overs are small, generally less than 1 m high, and are found on the basalt uplands. Cataracts are much larger than pour-overs, and typically occur as plucked embayments marginal to the present canyon. An exception is Lost Moon Cataract (Fig. 3), which is located well away from the Box Canyon gorge and bounds the northern side of the streamlined hill (Fig. 4). Lost Moon Cataract is one of the largest and best developed cataracts within the study reach, and has the typically associated features of plunge-pool, abandoned channel, and boulder accumulation downstream of the pour-over face (Fig. 6). Boulders up to 3 m (long axis) were transported from, or over the 9 m high lip of the cataract and deposited along the abandoned channel. Cataracts and scabland topography develop simultaneously once plucking and quarrying of the basaltic surface creates an initial cavity (Baker, 1973a). The vertical face of this cavity migrates upstream as a headcut, eroding the basalt upland, producing tracts of scabland and anastomosed channels ending at a cataract face. Minor pour-overs and the slight scarring of the basalt to the north of Box Canyon are inferred to be incipient cataracts and scabland terrain, respectively. 9
Loess Scarp
An aerial photograph covering the central portion of the study reach (Fig. 5) provides an excellent view of a scarp eroded into loess, extending for nearly 2 km along the southern side of the lower Big Lost River. This scarp separates thick loessial cover (>1 m), from either slightly scoured bedrock or scabland topography (Fig. 3), and is inferred to represent the limit of flood-eroded terrain.
Depositional Features
Longitudinal Bars Longitudinal bars of various sizes, composed predominantly of boulders, form the dominant depositional landform along the study reach. These longitudinal bars, elongate parallel to inferred paleoflow, can be categorized as 1) boulder-mantled hills, 2) boulder bars, and 3) boulder trains. This bar terminology has been developed specifically for the Box Canyon study reach. On the south side of Box Canyon are two large streamlined boulder-mantled hills
(Fig. 3) spanning 600-800 m in length. Boulder-mantled hills are distinguished from boulder bars and trains on the basis of having less relief and fewer surface boulders. One such feature abuts the south side of the streamlined hill, with boulders covering its downstream flank, and is interpreted to have formed in the shadow zone of flow obstructed by the streamlined hill. Boulder bars form at locales of flow deceleration such as on the insides of Box Canyon meander bends, as pictured in Baker (1984, Fig. 2B), or in cataracts marginal to the modern channel (Fig. 3). Several of the largest boulders observed within the study reach occur in boulder bars located within these marginal channel cataracts.
Boulder trains are common on the basalt upland portion of the study site (Fig. 3).
Deposition of a boulder train requires a strong longitudinal component of vortex flow Figure 5. Aerial photograph covering the middle portion of the Box Canyon study reach (Cross Sections 23 to 5), lower Big Lost River, where cataclysmic flooding evidence is most abundant. LMC - Lost Moon Cataract; LS - loess scarp. Big Lost River flow from left to right. Scale is 1:56,000 (reduced from 1:40,000). 10
LS Figure 6. Long profile of Lost Moon Cataract, the largest, best developed cataract in the study reach. Boulders were measured within six plots along the length of the abandoned channel. For the five largest basalt boulders within each plot mean intermediate diameter generally decreases and particle roundness increases downstream of the jointed basalt pour-over face. The columnar-jointed basalt pour-over face and plunge pool of Lost Moon Cataract are shown to the far left of the figure. 11
NW SE
1585 Intermediate diameter (mm)
roundness '-scr; 1580 S- W !I III IV E 1575
o (z 1570 1090 970 2.1 2.8 G) 1140 910 LLJ 2.85 2.75 1565
600 0 100 200 300 400 500 Distance (meters) 12 over a planar surface. Parallel rollers, formed due to flow irregularities, deposit material on the upward sweep of the helical motion (Baker, 1978b). Boulder bars and boulder trains typically do not exceed 300 m in length, and there are abundant examples of imbrication within both boulder bars and trains. Locally derived basalt comprises a majority of the boulders; however, clasts of exotic lithologies are also present within the bars and scattered about the basalt upland (Fig. 3). Often these erratics exist as small, monolithologic piles of gravel-, cobble-and boulder-sized material. Banded augen gneiss and quartz monzonite represent the most common lithologies, and were probably derived from outcrops of the Pioneer Mountains metamorphic core complex, located 100 km upstream of Box Canyon. Monolithologic piles of exotic rocks are inferred to be 'berg mounds', originally defined by Bretz (1923) to describe piles of debris transported fluvially within a block of ice and melted out to form anomalous deposits. The minor amount of less than 256 millimeter material present on the basalt upland proximal to the modern channel was not mapped. The lack of sand, silt, and clay within the study reach suggests that this finer material transported by flood waters was carried through Box Canyon. Nace (1975) finds that coarse gravel is the chief component of Big Lost River alluvium at the INEL, with locally varying amounts
of sand and silt. Maximum gravel thickness above basalt is about 21 m (Nace, 1975). Although alluvium downstream of Box Canyon was not mapped, no flood deltas were
observed.
A complex assemblage of scabland topography, cataracts, streamlined hills, and
boulder accumulations exists along Box Canyon which closely resembles portions of the Missoula flood-produced Channeled Scabland. Although the Box Canyon features are
small, subtle, and relatively few in number, the overwhelming similarities between the 13 two sites suggest the coursing of highly turbulent flood waters through Box Canyon, lower Big Lost River. Several of the features resulting from this flood serve as a lasting document of maximum Box Canyon flood stage and provide critical evidence for paleohydrologic reconstruction.
PALEOHYDROLOGY AND PALEOHYDRAULICS
Geologic evidence of paleostage, combined with step-backwater hydraulic modeling, provides estimates of peak discharge, mean flow velocities, energy gradient, and stream power per unit area of bed (unit stream power) for the Big Lost River flood.
Comparison with late Pleistocene Missoula and Bonneville floods illustrate the power of large-magnitude flow events of differing scales. Spatial fluctuations in unit stream power for the Box Canyon reach relate to general areas of flood-induced erosion and deposition.
Paleostage Indicators
The most useful evidence of maximum paleostage along Box Canyon is the highest- altitude ice-rafted erratics and the loess scarp (Fig. 3). Because an unknown depth of water is required to emplace the erratics and erode the loess scarp, the evidence serve as minimum constraints of the water-surface profile. However, both types of features probably represent altitudes very close to maximum flood stage. Paleostage indicators were surveyed using spot elevations on U.S.G.S. 7.5' topographic maps. Of these, the altitude of the loess scarp, and four of the highest erratics were actually taken as minimum indicators of maximum stage. All other flood evidence is assumed to be either the result of the largest event preserved at altitudes 14 of less-than-maximum stage, or the result of smaller flow events. Because of the ambiguity of preservation, the high stage of only the most extensive flood is used.
Discharge Estimates
Paleodischarge was determined by using step-backwater modeling techniques. Flow was modeled through Box Canyon with the U.S. Army Corps of Engineers' HEC-2 Water
Surface Profiles Computer Program (Hydrologic Engineering Center, 1982). This program computes an energy-balanced water-surface profile for a series of specified channel cross sections. Step-backwater calculations assume steady, gradually varied flow and employ the standard-step method of profile computation while solving the one dimensional energy equation (Chow, 1959). O'Connor and Webb (1988) discuss in detail
the hydraulic modeling calculations and application for paleoflood analyses.
To determine a discharge associated with a set of high water indicators, water-
surface profiles are computed by the modeling package for various discharges. The
best estimate of paleodischarge is the one that produces a water-surface profile most
closely matching the altitudes of the high water indicators. Examples of application of
step-backwater techniques to cataclysmic paleoflooding include Jarrett and Malde
(1987), and O'Connor and Baker (1987).
This method of estimating discharge assumes: 1) minimal flood-related channel
widening and downcutting so that measured channel cross sections accurately reflect channel geometry during emplacement of the highest stage indicators; 2) application of
the empirical Manning equation to evaluate friction slope is reasonable for floods of
this magnitude; and 3) flow regime is primarily subcritical.
Flow was modeled through 31 cross sections which were measured from 7.5'
U.S.G.S. maps along a 20 km distance of the lower Big Lost River (Appendix I). Cross
sections were oriented perpendicular to the inferred flow direction with spacing 15 dependent on channel complexity. The dense network of cross sections along the Big
Lost River reflects the irregular, tortuous channel geometry, and serves to keep the subreaches between cross sections short enough to satisfy the assumption of gradually varied flow.
Subcritical flow was modeled starting with the downstream-most Cross Section 1, and proceeding upstream. An initial water-surface elevation of 1555 m was assumed for Cross Section 1 by extrapolating the water-surface slope from upstream areas with paleostage evidence. Mannings coefficient of roughness (n) was assessed on-site, and values were selected for Box Canyon which are within range of similar analyses completed by Jarrett and Malde (1987) on the Snake River, and by O'Connor and Baker
(1987) at two sites along the Columbia River; these rivers are also joint-controlled basalt gorges. For the initial computer runs, n values of 0.05 and 0.1 were assigned for flow within the channel and overbank areas, respectively. Coefficients of expansion
(ke) and contraction (ke) were considered to be 0.1 and 0.3, respectively, as recommended by HEC (1982).
Figure 7 shows the water-surface profiles computed for various discharges, and the "best fit" profile corresponds to a discharge estimate of approximately 60,000 m3sec-1 . Because the loess scarp data point falls within the area of steepest water- surface slopes, between Cross Sections 11 and 14, and matches several of the profiles, the highest erratic (Cross Section 18) was used to constrain the peak discharge estimate. This discharge is considerably larger than the estimated 2,400 m3sec-1 necessary to inundate the narrow, deep Box Canyon gorge (K. Koslow, written communication, 1987).
Since there is uncertainty involved in estimating Mannings n and contraction/expansion coefficients, a sensitivity analysis was conducted to quantify the effect of these parameters on the computed water-surface profile for a 60,000 m3sec-1 Figure 7. Box Canyon water-surface profiles for five discharge trials generated by HEC-2 step-backwater computer modeling program. Circles refer to paleostage indicators: closed circles, erratics; open circle, loess scarp. Numbers and astrix denote cross section locations alon:z the thalweg of the Box Canyon study reach. A discharge of approximately 60,000 m-5sec -1 , constrained between the loess scarp and the highest erratic, generates the most reasonable profile for the lower Big Lost River flooding. 16
1640 -
DISCHARGE (ems) Erratic 1620 -
C/3 a) 1600
- o 0 1560 - >
1540 - Thalweg-
30 20 10 1 • ••• •••• ****** • • • • • • • • • • • • • 1520 1111111111 ***** 111111111111111 111111111111111111 0 2000 6000 10000 14000 18000 Distance (meters) 17 discharge. Varying n by a factor of + 25% causes only a 1.2% change in discharge which indicates that the roughness coefficient has a minimal effect on the modeling results. This agrees with sensitivity analyses conducted in similar studies (Jarrett and
Malde, 1987; O'Connor and Webb, 1988). The effect of contraction/expansion variations on the modeled flow magnitude is not as sensitive as in some other paleoflood studies, however. A conservative k=0.3 and ke=0.7 does not significantly change the profile.
This relative insensitivity to expansion and contraction coefficients for Box Canyon is probably related to a more constant flow width in comparison with the geometry of modeled reaches of other river canyons. For example, energy losses along the Snake
River (Jarrett and Malde, 1987) are greater because of expansion and contraction within a sequence of basins and narrows. Flow was subjected to one relatively major width change along Box Canyon, at Cross Section 14, otherwise the present-day canyon acted as an inner channel during peak flow, and conveyed generally less than 25% of the total discharge. Mean channel velocities during the flood peak range from 4 to 12 ms-1 , mean channel depths vary from a minimum of 9 m to a maximum of 31 m (at
Cross Section 14), and the energy slope computed for each cross section ranges from
0.0006 to 0.0093. Calculated profiles between Cross Section 10 and 18 are physically reasonable.
This reach of Box Canyon has the steepest thalweg slope, and a strikingly steep water-
surface gradient mimics that terrain (Fig. 7). The change in energy slope from 0.0034
(Cross Section 14) to 0.0029 (Cross Section 10), with intervening energy slopes as high
as 0.0093 (Cross Section 11), is a function of the steep bed slope and the change in
width from a relatively wide area at Cross Section 14, which conveyed 42% of the total discharge within the main channel, to a more constricted reach including Cross
Sections 13, 12, and 11. At Cross Section 10, flow exited the confines of the basalt
gorge. According to the modeling results, flow approached critical conditions between 18
Cross Sections 11 and 14, losing approximately 30 m of head. Fronde numbers for these four cross sections range from 0.9 to 0.5, indicating that flow during peak conditions was subcritical. During passage of the flood peak, mean channel flow depth was probably great enough to suppress the formation of supercritical flow. Upstream and downstream of this steep reach water-surface profiles are relatively flat and velocities are low, presumably reflecting hydraulic ponding as flood waters backed up behind the narrow constriction of Box Canyon and expanded again after exiting the gorge (Fig. 8).
Assumptions concerning the stability of the bedrock gorge and application of the
Manning equation to floods of cataclysmic scale warrant further explanation. Changes in channel morphology could be expected during extreme flooding, however Box Canyon gorge cuts resistant Snake River Plain basalt, and conveyed generally less than 25% of the total peak discharge. As a result, unit stream power for the Big Lost River flooding attained high (>5000 Wm -2) values only locally, thus flood-related widening and downcutting may have been relatively minor. Uncertainties inherent in utilizing the Manning equation for indirect estimation of cataclysmic discharges are presently unresolved.
To date, field observations suggest only a single flood, although multiple cataclysmic floods have not been ruled out. The age of the flood is not presently known, however, there is promise in obtaining age estimates from five sources:
1) Analyzing flood features within the context of the loess stratigraphy developed by
Pierce and others (1982) for the eastern Snake River Plain. They suggest two major loess layers, separated by a buried soil, correlate with the Wisconsin Glaciation (loess unit A), and to a period approximately 130,000 to 70,000 years ago (loess unit B), respectively. Their Kettle Buttes loess section may be useful for the dating the loess scarp south of Box Canyon. Figure 8. Representative Box Canyon channel cross sections showing the constricted nature of the basalt gorge. Cross Section 31 is located upstream of the entrance to Box Canyon, 24 and 13 are located within the basalt gorge, and 8 is located downstream of the confining canyon. Flood waters were ponded upstream of the entrance to Box Canyon, and expanded downstream of the deep, narrow gorge. Cross sections were taken from U.S.G.S. 7.5' topographic maps shown in Appendix 1.
19 1590-
1570 Cross Section 8
1550 0 15 '00 300000 4500' 6000' 7500 9000 1610
1590
Cross Section 13 (r) 1570
1550 0 1500 3000 4500 6000 7500 9000
0 1630
,a) bi 1610
1590
163
161
159 0 1500 3000 4500 6000 7500 9000 Distance (meters) 20
2) Dating of sediments collected within the plunge pool of Lost Moon Cataract. This may provide a date of the most recent sediment filling event. 3) Weathering rinds on flood transported boulders. Rinds several millimeters thick were observed on massive, basaltic flood boulders along Box Canyon, and could be calibrated to a basalt flow of known age near the study site.
4) Helium isotopes may be useful for dating the time since surficial exposure of flood- transported basalt boulders and pour-over faces worn smooth by floodwaters (T.
Cerling, personal communication, 1988). 5) Examination of fan fronts along the length of the Big Lost River. The alluvial fan southeast of Mackay Reservoir is a composite feature with a truncated front approximately 10 m high. Brief field inspection revealed that the mid to late Pleistocene portions of the fan are trimmed, possibly by cataclysmic flooding, whereas the latest Pleistocene fan surface is not truncated but grades smoothly to the level of the modern flood plain. The latter is no farther from the influence of the Big Lost
River than are the older flood-trimmed(?) fan surfaces.
Stream Power Estimates Channel boundary shear stress and total stream power are measures of the drag force per unit area of bed surface, and of the rate of energy expenditure per unit length of channel, respectively. Both values are closely related to sediment transport capability, and have been related to the potential rate of sediment movement, or the potential work performed by rivers (Bagnold, 1966; Baker and Ritter, 1975).
Since total stream power scales with the discharge of a river, stream power per unit area of bed (unit stream power) better quantifies the power of a flood (Baker and
Costa, 1987) in terms of erosion and transport potential, and is expressed as 21
w = ,ydSv = rv (1)
in Wm-2 , where "I is the specific weight of the fluid (9800 Nrn-3 for clear water), d is flow depth (m), S is the energy slope, v is mean channel velocity (ms-1 ), and r is channel boundary shear stress (Nm-2). Unit stream power estimates were calculated using equation (1) and information provided by the hydraulic modeling, which reports flow conditions for both the channel and adjacent basalt upland areas. Estimated
maximum channel unit stream power for Box Canyon is 26,000 Wm-2 at Cross Section
13 (Fig. 9). This value is exceeded only by the two largest known floods in the world
-- late Pleistocene Missoula and Bonneville floods. Despite the tremendous disparity in drainage areas and peak discharges, the power of the Big Lost River flooding approached that of Bonneville and Missoula flows, perhaps explaining landform similarities between the three sites.
FLOOD HYDRODYNAMICS
Maximum channel unit stream power for the Box Canyon gorge (26,000 Wm-2) is
not easily related to the erosional and depositional landforms on the basalt uplands. In
order to define a relation between peak flood power and these out-of-channel, flood- derived landforms (Fig. 3) an alternative approach was used by mapping segment unit stream power along Box Canyon as an isopleth map (Fig. 10). Relatively few studies have addressed cataclysmic flood-derived landforms from
the perspective of spatial fluctuations in flood power. Scott and Graylee (1968) documented thalweg tractive force for a flood surge on the Rubicon River, CA, and O'Connor and others (1986) suggest a model for riffle development at Boulder Creek, Utah, based on spatial variations in thalweg unit stream power. Results from these Figure 9. Flood power-imeter comparing unit stream power values of the Big Lost River to other cataclysmic floods around the world. This Big Lost River maximum unit stream power occurred within the Box Canyon gorge at Cross Section 13. Table modified from Baker and Costa (1987). 22 ZT,r?/ -41tEssizszszemanagss a Missoula Flood Power-imeter n75.0Cef Bonneville (Watts / m2 )
Flood Channel Peak Slope Depth Velocity Shear Flood Type Discharge Stress Power (m3sec-1) (m) (msec-1) (Nm-2) (Wm12)
Amazon Alluvial 300000 0.00001 60 2 6 12
Big Lost Bedrock 60000 0.008 31 11.5 2300 26000 River
Bonneville Bedrock 900000 0.004 63 10-26 2500 75000
Katherine Bedrock 6000 0.003 45 7.5 1500 10000 Gorge
Missoula Bedrock 5000000 0.01 100 30 10000 300000
Pecos Bedrock 27000 0.002 30 12 600 7000 River
Big Lost River
Katherine Gorge
Pecos River
Amazon Figure 10. Isopleth map illustrating spatial fluctuations in segment unit stream power during peak discharge of the Box Canyon cataclysmic flood. Estimates of segment unit stream power were calculated for every segment along each of the 31 Box Canyon cross sections. These estimates were then plotted at the mid-point of each segment on a map of channel cross sections of the lower Big Lost River (cross sections have been removed for clarity). Isopleth lines were extended to adjacent segment boundaries and half way between adjacent cross sections. All points within any particular colored area have a constant range of segment unit stream power. Letters A, B, C, and D refer to sites discussed in the text. Segment Unit Stream Power (Wm -2 )
1000-5000
600-1000
100-600
less than 100
27 25
o 2 3 kilometers 24 two studies suggest deposition occurred in areas of reduced flow competence. This study investigated the relationship between the cataclysmically flooded Box Canyon landscape and the areal distribution of segment unit stream power during the flood peak. In general, areas of highest, rapidly changing flood power (Fig. 10) correspond with the most flood-modified portion of the study reach (Fig. 3), between Cross
Sections 8 through 14. Segment unit stream power decreases both up- and downstream of this area.
Erosional Patterns
Areas of boulder entrainment and transport are generally associated with segment
unit stream power estimates in excess of 1000 Wm-2 (Fig. 10). These zones occur
primarily adjacent to the gorge where plucking and quarrying of the exposed basalt
columns commenced, creating marginal channel cataracts. The network of cataracts on
the right bank of the modern channel, at point A on Figure 10, initiated the
development of the bare, rugged scabland topography in that area as cataracts migrated
upstream. Segment unit stream powers of 600-1000 Wm-2 were apparently sufficient to
drive the headward recession of the vertical basalt faces, forming a series of
anastomosed scour patterns or channels, one of which ends at Lost Moon Cataract. All
but a few of the cataracts, a majority of which lie on the south side of the modern
channel, are located within the zones of highest segment unit stream power (Cross
Sections 8-14).
Flow modeling indicates that the streamlining of the loess-capped hill took place
subfluvially, under approximately 3 to 4 m of water. The loess cap present today
suggests either post-flood accumulation, or only minimal erosion behind the protective
basalt outcrop during flooding. Segment unit stream power on top of the loess hill
was about 300-500 Wm-2 at peak flood stage. 25
One explanation for the concentration of erosional flood features south of the
Box Canyon gorge and their absence to the north, also an area of high flood power
(Fig. 10), may lie in the character of the topography. Higher topography near the
southern end of Cross Section 11 probably forced flow to the northeast, impinging
against the bend in the Big Lost River at point A (Fig. 10). Flow pouring over the
basalt upland into Box Canyon at this site may have induced a form of macroturbulent
vortex known as a 'kolk' (Matthes, 1947; Baker, 1978b) resulting in headward erosion
and cataract formation. To the north of the canyon, however, the necessary lift
forces provided by the abrupt increase in depth as flow poured into the canyon were
not present to instigate more than incipient development of scabland topography and
minor basalt pour-overs. This interpretation suggests a rather complicated relationship
between the resistance and structural control of the bedrock, and local fluctuations in
segment unit stream power. At the present scale of modeling, this relationship remains undefined, though both obviously combined to influence the erosional and depositional
processes during the Big Lost River cataclysmic flood.
Depositional Patterns
Most boulder-laden areas are located within zones of segment unit stream power
values of less than 600 Wm-2, and generally these zones have lower segment unit
stream power relative to adjacent upstream areas (Fig. 10). This suggests that boulder
deposition in Box Canyon may in part be controlled by reversals in the segment unit
stream power gradient. For example, boulders within the boulder-mantled hill (point B;
Fig. 10) were deposited under conditions of transitional segment unit stream power,
with the flow unable to sustain boulder transport. However, the largest basalt boulders observed within the Box Canyon study reach are located north of the channel in the 26 narrow strip of highest segment unit stream power (1000-5000 Wm-2) relative to surrounding zones on the basalt upland (point C; Fig. 10).
The eastern-most series of boulder trains south of the channel (point D; Fig. 10) is another example of deposition under conditions of reduced flood power. Deposition corresponds with segment unit stream power in the 100-600 Wm-2 range, an area of low segment unit stream power relative to upstream sites.
More detailed aspects of the controls on boulder deposition along Box Canyon were undoubtedly influenced by local flow phenomena which are below the resolution of the isopleth map. Flow perturbations formed in response to the irregular topography probably created pockets of extreme turbulence which cannot be fully characterized.
For example, changes in boulder size and the degree of rounding within the abandoned channel of Lost Moon Cataract (Fig. 6) are evident, yet can not be fully explained by the 600-1000 Wm-2 zone in which they are shown on Figure 10.
In order to assess these changes in boulder size and roundness, detailed surveying down the long profile of the abandoned channel of Lost Moon Cataract was conducted in the field, and boulder measurements were made at the mid-points of six plots along the survey. Distance between plots varied and was based on the distribution of boulders along the profile. Individual plots were laid out as squares and ranged between 100 and 150 m on a side. A minimum of ten of the largest boulders were visually selected and measured to ensure that 5 of the largest were included for analysis. Only the largest boulders were measured because they presumably reflect
upper limits of transport capability. Long, intermediate, and short axes of the boulders, bearing of the long axis, angle of imbrication, and particle roundness (Folk,
1955) were measured.
There is a systematic decrease in the mean intermediate diameter of the five
largest basalt boulders within a plot, and an increase in mean roundness downstream of 27 the cataract face (Fig. 6). Acceleration of convergent flow over the jointed cataract face probably induced plucking of several of the largest boulders measured within Plot
I; their size and angularity indicate minimal transport. The local increase in boulder size to 1140 mm at Plot V is probably caused by backwater from the adjacent abandoned channel sweeping around the streamlined hill (Fig. 3). Coarse particles carried by the flood waters were deposited at the juncture of the two channels, a site of expanding flow and reduced power.
Several empirical relations that approximate minimum flow conditions for particle entrainment (Bagnold, 1980; Costa, 1983; Williams, 1983) do not include data sets for
Pleistocene outburst floods. According to Costa (1983, p. 991) this is due to the
"uncertainties involved in estimating Pleistocene flood velocities as well as the limitations of particle sizes available for transport during these enormous floods". With the present level of accuracy in hydraulic flow modeling better estimates of hydraulic variables can be produced, allowing for the inclusion of Pleistocene flood data in these compilations. Also, concerns regarding particle size limitations do not apply to this study which emphasizes conditions of boulder deposition rather than entrainment.
Limiting power conditions for Box Canyon flood-induced deposition were defined from point estimates of unit stream power for seven bars containing the largest flood boulders measured in the field (Figure 11). The estimates of flood power for each bar are distance weighted averages using surrounding segment unit stream power values.
Two of the bars containing the largest measured boulders occur in marginal channel cataracts where channel unit stream power influenced deposition. Only the largest boulders are of hydrodynamic significance since trapping of sediments behind obstacles during flood transport is less important for the largest particles than for smaller particles. Therefore, the largest boulders along Box Canyon are probably not deposited prematurely but were carried as far as the power of the flood was sufficient to Figure 11. Plot of unit stream power vs. mean intermediate diameter of the five largest flood boulders for seven sites along Box Canyon. Under peak flow conditions of 60,000 m3sec-1 deposition of the largest Box Canyon flood-transported boulders occurred above the limiting line. 28
1 0 4 _
-
-
- e•-n - C•11 - E
0.1 •
• •
Limit of Deposition
t-1 - C D
I lu 2 I I ' ' I . L ' 500 10 0 0 15 0 0 I 2000 ' 2500 Mean Intermediate Diameter (mm) 29 continue transport. Deposition of these largest boulders at any particular site is thus assumed to better reflect the flood power at that site since reintrainment is unlikely.
Figure 11 illustrates this relationship between segment unit stream power associated with deposition and the largest flood boulders observed within the Box
Canyon study reach. Under peak flow conditions of 60,000 m3sec-1 , deposition of the largest Box Canyon boulders occurred above a segment unit stream power limit of 1400
Wm-2 (Fig. 11). Contrasted with the isopleth map estimates (<600 Wm-2) for areas of widespread deposition of smaller boulders, this value offers considerably more constraint on the limit of boulder deposition, and suggests that maximum flood power
(26,000 Wm-2) was probably the dominant transporting agent for these large flood
boulders.
FLOOD SOURCES
Results of the hydraulic modeling indicate a peak discharge of approximately
60,000 m3sec- 1 was required to produce the flood features along Box Canyon. The source, or cause of flooding has not yet been determined. An outburst flood from a glacier-dammed lake along the East Fork Big Lost River (Evenson and others, 1982)
may have been at least partially responsible, although preliminary field work and routing calculations indicate that is probably was not the sole source of Box Canyon
flooding.
Glacial Lake East Fork
In a study of alpine glaciations in the Pioneer Mountains, Evenson and others
(1982) cite evidence for the existence of a glacier-dammed lake along the East Fork
Big Lost River. Westward drainage of the East Fork was apparently blocked by an ice 30 lobe emerging from Wildhorse Canyon (Fig. 2), creating "Glacial Lake East Fork".
Evenson and others (1982) suggest two high stands during alpine glaciation, once during
Bull Lake-, and again during Pinedale-equivalent full glacial stages.
Evidence for the existence of a lake lies in the erratics of migmatitic gneiss lining the valley hillslopes of the East Fork Big Lost River. The single source of these rocks is an exposure at the head of Wildhorse Canyon (mapped by Wust and
Link, 1988), suggesting that the hillside boulders must have been ice-rafted up the East
Fork Valley and subsequently melted out on the shores of Glacial Lake East Fork.
Additional features such as strandlines and fine grained deposits, typical of lacustrine environments, are not readily apparent, although these features would not necessarily be expected in narrow, deep, short-lived lakes.
Maximum level for Glacial Lake East Fork was determined by surveying fifteen of the ice-rafted erratics along the valley. The potential for down-slope movement of the erratics is considerable, and those surveyed show a vertical spread of 44 m, defining a high stand (based on the highest erratic) of 2280 m. A hypsometeric curve was constructed for the lake using U.S.G.S. maps (7.5' and 15') and a planimeter. Total impounded volume for Glacial Lake East Fork is estimated to have been 6.3 km3 , with a mean depth in excess of 130 m.
Regression equations relating lake volume to maximum discharge for historic jokulhlaups (dague and Mathews, 1973; Beget, 1986; Costa, 1988) provide an outburst discharge estimate ranging from 26,000 to 38,000 m 3sec-1 for the glacial lake.
Unrealistic extrapolation of these regression equations may be expected for some
Pleistocene floods, however, the small size of Glacial Lake East Fork approaches that
from which the equations were derived. Disparities between discharge estimates for
Box Canyon and Glacial Lake East Fork are addressed in a subsequent discussion.
Evidence suggestive of outburst floods from Glacial Lake East Fork is somewhat 31 scarce. Down-valley from the confluence of Wildhorse Creek and East Fork Big Lost
River, large, well rounded boulders exist singularly, or less commonly as accumulations of boulders. Boulder and cobble deposition can be traced as far downstream as Barton
Flats (Fig. 2), and a brief inspection revealed decreasing particle size with increasing distance downstream. The largest erratic identified anywhere along the entire Big Lost
River is located approximately 18 km from the confluence of Wildhorse Creek and East
Fork Big Lost River. This gneissic boulder with distinctive migmatitic fabric measures
5.2 x 3.3 x 2.6 m, and was probably fluvially transported (perhaps encased in ice) because no evidence of glaciation has been found that far down-valley. Also, thick colluvial mantles may be covering other potential flood features such as trimmed alluvial fan fronts, flood deltas, and distinctive ice dam break-out features. The scarcity of flood evidence within the Big Lost River Valley is somewhat enigmatic, although the lithologies of erratics along Box Canyon compare favorably with clasts on the hillslopes of the East Fork Valley. The catastrophic emptying of the glacial lake may have emplaced these erratics some 100 km from source terrains in the Pioneer
Mountains.
Back-of-the-Envelope Flood Routing
The disparity of peak flow estimates from the hydraulic modeling of Box Canyon
(60,000 m3sec-1 ) and outburst discharge estimates for Glacial Lake East Fork (26,000-
38,000 m3sec-1 ) is even more profound considering the distance downstream the flood travelled before encountering Box Canyon. As a simple, first approach to evaluate the
effect distance may have had on the flood peak, an equation was used which relates
peak discharge from a dam failure flood to distance downstream.
Costa (1988) presents an envelope curve equation for constructed dams which have
failed and flooded broad, open valleys similar to the Big Lost River Valley: 32
Q= 100 (2) 10(0.0052x)
where Qx is the percentage of the peak discharge 'x' kilometers downstream from the site of peak discharge determination. For example, assuming a maximum discharge of
38,000 m3sec-1 from Glacial Lake East Fork, 100 km downstream at Box Canyon the discharge would have attenuated to 30% of the peak flow, or 11,500 m3sec-1 . A discharge of 200,000 m 3sec- I is required from the lake to yield the 60,000 m3sec -1 required by the hydraulic modeling of Box Canyon, implying a lake volume at least ten times greater than the estimated 6.3 km3. Peak discharges from jokulhlaups are typically smaller than those from constructed dams, for which equation (2) was developed, and probably overestimates the percentage of peak discharge. Users are cautioned (Costa, 1988) that wide floodplains, such as the one in the Big Lost River
Valley, may lead to more rapid attentuation than the equation predicts.
Assuming a steady discharge of 60,000 m3sec-1 , the duration of the flood flow would be approximately 30 hours for a lake volume of 6.3 km3, and 292 hours for a lake with volume ten times that of Glacial Lake East Fork. The overall scarcity of large-scale flood features along the flood path may indicate a relatively short flow duration. The available information regarding Glacial Lake East Fork suggests the lake was not large enough, and at too great a distance to account for the flood features along Box Canyon. Evidently, either additional source waters are required to account for the discharge disparity, or inadequacies in the above procedure (equation 2) and inherent limitations in the data set, preclude a positive identification of Glacial Lake East Fork as the source of Box Canyon flooding. Full-scale hydraulic routing down the Big Lost
River Valley would add important information on storage and attenuation of flow within 33 the wide, alluvial valley. A flood hydrograph could then be constructed, offering a better understanding of the nature of the flooding and its duration.
Hydraulic Ponding and Other Flood Sources
Hydraulic ponding, and/or water contributions from a pluvial lake may help
resolve the discharge disparity between Box Canyon and Glacial Lake East Fork.
Although field work testing these hypotheses has yet to be conducted, two plausible
sites can be identified.
Mackay Reservoir (Fig. 2) is dammed at the only constriction within the entire
Big Lost River Valley. The constriction is formed by steep cliffs of Paleozoic
limestones, which are mantled by a layer of colluvium west of the river, and by
Pliocene to early Pleistocene alluvial fan deposits to the east (Scott, 1982). Outburst
flows from Glacial Lake East Fork may have ponded behind this constriction, and again
abruptly released due to incision or erosional enlargement of the constriction. The
presence of flood-trimmed (?) alluvial fan fronts downstream of the reservoir may
further support intravalley ponding and subsequent cataclysmic release. The flood
hydrograph for the Big Lost River was probably fairly peaked, and ponding with
subsequent erosion of the constriction could further sharpen this peak.
Thousand Springs Valley, downstream of the major southeastern bend in the Big
Lost River (Fig. 2), is a perennial marsh fed by a series of springs issuing from the
fronts of local alluvial fans. This nearly closed basin is probably subsiding tectonically
relative to Barton Flats (K. Vincent, personal communication, 1989), and may have been
the site of a pluvial lake formed by streams draining the high peaks of the Lost River
Range. Pierce and Scott (1982) suggest these streams in southeastern Idaho during the Pleistocene had "sustained seasonal flows probably at least ten times larger than
discharges of present streams." Outburst flooding upstream may have been discharged 34 into this pluvial lake, dammed at the constriction near Mackay Reservoir (?), initiating another catastrophic release due to erosion of the valley constriction.
CONCLUSIONS
The suite of distinctive landforms along Box Canyon, lower Big Lost River are the product of torrential discharge(s) during the Pleistocene. Preservation is enhanced by the inability of Holocene processes to greatly modify these features. Glacial-lake
outbursts along the East Fork Big Lost River may have played a role in the
paleoflooding, though only a portion of the modeled discharge can be attributed to the lake. This discrepancy may be further resolved by locating sites of additional sources of water and temporary hydraulic ponding of flood waters within the Big Lost River Valley. Hydrodynamic conditions during the Big Lost River paleoflooding were apparently ripe for modification of the landscape, evidenced by maximum channel unit stream
power estimates. However, judging by the relatively few, somewhat poorly developed,
and in cases incipient flood landforms along Box Canyon, this 60,000 m3sec-1 flood was
probably a short-lived event. The lack of temporally-sustained, high-magnitude flows prevented further geomorphic changes.
The areal distribution of flood power during peak Box Canyon flow corresponds with the gross characteristics of flood-induced sediment entrainment, transport, and deposition. Local topographic relief, however, appears to be the dominant control on
the removal and accumulation of flood boulders along Box Canyon. The irregular volcanic rift topography initiated random flow perturbations which locally accelerated
and retarded flow velocities and transport capabilities. Results from the hydraulic modeling of Box Canyon suggest that flow may have 35 entered the Snake River drainage. Flood water from the Big Lost River eventually entered former Lake Terreton, which at maximum extent included the Big Lost River Sinks (Pierce and Scott, 1982), and may have overflowed, draining eastward into the
Snake River near Idaho Falls, Idaho. Additional flow modeling may define the actual flood path beyond what is already known. 36
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227-243. 41
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Geology of Central and Southern Idaho: Idaho Geological Survey Bulletin 27, p.
43-54. Appendix 1. Topographic composite map showing channel cross sections used for hydraulic modeling. Includes Arco South, Butte City, Arco Hills SE, Quaking Aspen Butte, and Big Southern Butte U.S.G.S. 7.5' quadrangles.
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