liBRARY COPY 

The Catastrophic 

Late Pleistocene 

Bonneville Flood in the 

Snake River Plain, Idaho 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 596 
The Catastrophic 

Late Pleistocene 

Bonneville Flood in the 

Snake River Plain, Idaho 

By HAROLD E. MALDE 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 596 

A study of colossal features of 

erosion and deposition produced 

along the Snake River by sudden 

overflow of Lake Bonneville 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402- Price 70 cents (paper cover) 

-. 
CONTENTS 

Abstract------------------------------------------- Introduction ______________________________________ _ 

Background of study ___________________________ _ 

Acknowledgments ______________________________ _ 

Summary of flood path _________________________ _ 

Glacial and pluvial stratigraphy ________________ ~_ 

Source---------------------------------------------

Age----------------------------------------------- Magnitude of flood _________________________________ _ 

Duration _________________________________________ _ 

Comparison with last catastrophic flood from glacial Lake Missoula, Mont ____________________________ _ 

Diagnostic depositional and erosional features _________ _ 

Melon GraveL ________________________________ _ Lithology _________________________________ _ 

Physiography _____________________________ _ 

Marginal channels _____________________________ _ 

Impounded water ______________________________ _ 

Scabland _____________________________________ _ 

Path of flood above Twin Falls ______________________ _ 

Upper Snake River Plain _______________________ _ 

Rupert channeL ______________ ----------- ______ _ 

Milner reach _____________________________ ------

Erosion near Twin Falls ____________________________ _ 

The canyon below Twin Falls _______________________ _ 

Weeping Wall section (mile 175-162) _____________ _ 

Page 

1 The canyon below Twin Falls-Continued 

2 Melon Valley section (mile 162-155) _____________ _ 

2 Thousand Springs section (mile 155-151) _________ _ 

6 Hagerman Valley (mile 151-141) _________________ _ 

6 Bliss section (mile 141-121) _____________________ _ 

7 King Hill basin (mile 121-109) __________________ _ 

9 Cove section (mile 109-76) ______________________ _ 

10 Crane Falls section (mile 76-61) _________________ _ 

11 Grand View basin (mile 61-28) __________________ _ 

12 Swan Falls section (mile 28-14) __________________ _ 

Walters basin (mile 14-0)------------------------ 13 Budget of erosion and deposition ____________________ _ 

13 Discharge as indicated by size of boulders _____________ _ 

13 Comparison with historic floods ______________________ _ 

13 Reservoir failures ______________________________ _ 

17 Gohna Lake, India _________________________ _ 

18 The Great Indus Flood, India _______________ _ 

18 Flood at Gros Ventre, Wyo _________________ _ 

20 Lake Issyk, U.S.S.R _______________________ _ 

21 St. Francis Dam, Calif ______________________ _ 

21 Vaiont Reservoir, Italy _____________________ _ 

22 Floods associated with glaciers __________________ _ 

23 Jokulhlaups in Iceland _____________________ _ 

25 Various ice-dammed lakes ___________________ _ 

30 Summary of historic floods ______________________ _ 

31 References cited ___________________________________ _ 

ILLUSTRATIONS 

FIGURE 1. Map of northern part of Lake Bonneville and nearby part of Snake River Plain ___________________________ _ 

2. Map of Snake River below Twin Falls-----------------------------------------------------------------

3. Diagram showing profile of Snake River and of Bonneville Flood ________________________________________ _ 

4. Photograph showing boulders on bar of Melon Gravel near Hagerman ____________________________________ _ 

5. Photograph showing terminus of gravel bar in Melon Valley _____________________________________________ _ 

6. Sketch of pit in sandbar of Melon GraveL _________________________________________________________ -- __ 

7-9. Photographs: 

7. Festoon crossbeds in dipping layers of sand _____________________________________________________ _ 

8. Boulder train on surface of Melon GraveL _______________________________________________ - ____ -_ 

9. Minor erosional features in basalt scabland _____________________________________________________ _ 

10. Geologic sketch map of American Falls area ___________________________________________________________ _ 

11. Topographic map showing dry falls and giant potholes near Eden ________________________________________ _ 

12. Photograph showing Snake River canyon upstream from Hansen Bridge __________________________________ _ 

13. Cross sections of Snake River canyon _________________________________________________________________ _ 

14. Topographic map of Snake River canyon near Twin Falls _______________________________________________ _ 

15. Photograph showing Snake River canyon downstream from Perrine Memorial Bridge _______________________ _ 

16. Topographic map of Melon Valley area---------------------------------------------------------------- ~ 7. Geologic sketch of Melon Valley _____________________________________________________________________ _ 

18. Photograph showing boulders of Melon· Gravel near Banbury Hot Springs ________________________________ _ 

19. Topographic map of Hagerman Valley ________________________________________________________________ _ 

20. Topographic map of King Hill basin __________________________________________________________________ _ 

21. Photograph showing basalt boulders on lava flow near Bancroft Springs ___________________________________ _ 

III 

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IV CONTENTS 

Page 

FIGURE 22. Topographic map of Crane Falls area__________________________________________________________________ 41 

23. Topographic map of Walters basin____________________________________________________________________ 43 

24. Graph showing volume of Melon Gravel along Snake River______________________________________________ 46 

25. Graph showing discharge of some glacier bursts in Iceland_______________________________________________ 50 

26. Section of a supra-aquatic delta of sandur type_________________________________________________________ 50 

TABLES 

Page 

TABLE 1. Topographic relations of mariginal channels______________________________________________________________ 19 

2. Discharge capacity of Snake River canyon at selected constrictions_________________________________________ 25 

3. Computation of volume of Snake River canyon, Augur Falls (mile 17 5) to Hansen Bridge (mile 1~9) _ _ _ _ _ _ _ _ _ _ _ _ 45 

4. Volume of Melon Gravel above Grand View______________________________________________________________ 45 

5. Velocities and discharges indicated by size of boulders in Melon Gravel just below canyon constrictions__________ 46 

6. Velocities indicated by size of boulders in Melon Gravel where floodwater was not constricted__________________ 46 

7. Flood wave on the Ganges caused by rupture of landslide dam at Gohna, India______________________________ 48 

8. Progress of flood wave from St. Francis Dam, CaliL _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 49 

\., 

-' 
THE CATASTROPHIC LATE PLEISTOCENE BONNEVILLE FLOOD 

IN THE SNAKE RIVER PLAIN, IDAHO 

By HAROLD E. MALDE 

ABSTRACT 

A catastrophic flood caused by overflow and rapid lowering 

of Pleistocene Lake Bonneville at Red Rock Pass near Preston, 

Idaho, descended Marsh Creek Valley and reached the Snake 

River Plain at the site of Pocatello. Large tracts in the upper 

Snake River Plain were inundated, particularly an area near 

American Falls and a basin that surrounds Rupert. Farther 

downstream, the Snake River CtUlyon that extends 200 miles 

west of Twin Falls was flooded to a depth of 300 feet. Spectacular erosion in the form of abandoned channels, spillways, 

cataracts, nnd scabland identifies the flood path between American Falls and Twin Falls, and the cnnyon farther west is strewn 

with huge boulders-some of them more than 10 feet in diameter-which are heaped in enormous bars of boulders and sand 

that rise nearly 300 feet above the canyon floor. 

The Bonneville Flood nlmost certainly wa:s caused by capture 

of the Benr River, which resulted in greatly augmented inflow 

to the Bonneville basin. ':l'he details of this event have been 

studied by R. C. Bright, University of Minnesota, who estimates 

from several radiocarbon dates-some from the Bear River 

drainage and otl1ers from the flood path-that spillover at Red 

Rock Pass first occurred about 18,000 years ago. However, I 

believe that the history of Bear River deduced by Bright, as explained in his Ph~ D. thesis of 1963, agrees better with initial 

spillover at about 30,000 years ago, as indicated by a radiocarbon date for molluscan fossils associated with flood debris 

near American Falls and by relict soil preserved on the flood 

deposits. 

Whatever the age of the Bonneville Flood, it undoubtedly 

represents discharge from a lake level above the Bonneville 

shoreline and is not at all related to subsequent downcutting 

to the present level of Red Rock Pass-the Provo shoreline. 

Overflow could have been across weakly consolidated surficial 

material in Red Rock Pass at a height of as much as 100 feet 

above the Bonneville sho.reline. If so, the flood volume was 380 

cubic miles. A probable maximum discharge of about one-third 

of a cubic mile per hour is computed by C. T. Jenkins, U.S. 

Geological Survey, at a canyon neck near Swan Falls, south of 

Boise, which acted as a gigantic venturi flume. The flood duration at maximum discharge could therefore have been at least 

6 weeks. If peak flow lasted only a few days, as seems likely, 

voluminous discharge could have continued for at least a year. 

Debris left by the Bonneville Flood along the Snake River 

canyon is lmown by the descriptive geologic name, Melon Gravel. 

It displays an array of lithologic and physiographic features that 

demonstrate torrential origin in rapidly moving deep water. The 

Melon Gravel was derived from local basaltic lava flows and 

is otherwise distinguished lithologically by extremely poor sorting (boulders and sand), by phenomenal sets of giant crossbeds, 

and by extraordinarily abrupt changes in grain size. Rounded 

basalt boulders just below canyon constrictions are commonly 

larger than 5 feet and can be identified as having come from 

outcrops not more than a few miles upstream. The gravel occurs 

mainly in wide places along the canyon as groups of bars of 

colossal size that together resemble the greatly magnified bed 

of a braided stream. In the lower reach of wide segments along 

the canyon, bouldery flood debris gives way to sandy deposits, 

presumably because of retarded flow. Bars of gravel block the 

mouths of some tributary valleys, but at most places the bars 

stand midway between the canyon walls, from which they are 

separated by marginal channels as deep as 150 feet. The constructional form of these bars is well preserved, and they apparently have been little modified by erosion since the flood. 

Fine-grained alluvium formed broad flats in tributary valleys 

that were blocked by deposits of Melon Gravel. Other finegrained material was dropped in backwater areas of temporary 

lakes that filled broad segments of the Snake River canyon 

during passage of the flood. 

The marginal channels were formed as a consequence of the 

Melon Gravel being washed into wide places along the Snake 

River canyon and then being dumped in midstream. Some 

of these channels wind several miles along the canyon walls ; 

they are generally perched 100 feet or more above the present 

Snake River. Like the adjoining gravel bars, they demonstrate 

that the canyon flowed deep with floodwater at peak discharge. 

Distinctive scabland was formed by erosi'on during the 

Bonneville Flood, espedally in the upper Snake River Plain above 

Twin Falls. I:t forms an irregular surface of scoured basalt 

diversified by branching channels, dry falls, and rock basins. 

Lack of soil· distinguishes the scabland most obviously, but 

subtle signs of basalt erosion-such as smoothed, polished, and 

fluted surfaces, as well as stripping and plucking evident to a 

trained observer-also indicate a former forceful flow of water. 

The flood path through the Snake River Plain closely followed 

the present Snake River and is now well knOI\Vn. Floodwater 

debouching on the plain at Pocatello entered a lake that was 

held by a lava dam near American Falls, and bouldery debris 

50-80 feet thick was dumped on the lake bottom. Water escaped 

over ·and around the lava dam by several routes and then descended in a broad swath to the Rupert basin, where an area of 

300 .square miles was flooded to an average depth of 50 feet. 

Floodwater was releRsed from the Rupert basin via the Snake 

River, which even then must have occupied a canyon much like 

the present one, and it also was released by simultaneous overflow into an upland channel that rejoined the canyon near Twin 

Falls, 30 miles downstream. At this junction, an anomalously 

broad canyon segment 14 miles long wa1s apparently greatly enlarged by flood erosion. Thi,s chaotically eroded stretch of canyon now presents a weird landscape of scabland, cataracts, and 

spillway alcoves. 

1 
2 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

The Snake River canyon in the reach 105 miles below Twin 

Falls is a trench 500-600 feet deep made up of basins carved 

from ·soft deposits and of constricted segments cut into basalt. 

The narrow places impeded flow and impounded floodwater in. 

basins at Melon Valley, Hagerman, King Hill, Glenns Ferry, 

Hammett, Indian Cove, and Eagle Cove, which therefore acted 

as sediment traps for Melon Gravel. A particularly narrow constricUon near Crane Falls at the downstream end of this reach 

impounded water 300 feet deep that extended 45 miles upstream 

a·s a nearly continuous lake to the head of the King Hill basin. 

Because the King Hill 'ba·sin is situated where the Bonneville 

Flood first entered relatively slack water, it holds more than 

half of the Melon Gravel that was flushed down the canyon. 

Not all the floodwater could pass the narrow canyon at Crane 

Falls, ·and some water spilled westward along an upland channel 7 miles long that emptied into another temporary lake in the 

Grand View basin, which covered 150 square miles to a depth 

of ·as great a•s 325 feet. The lake at Grand View was the finaJ 

trap for flood debris from sources upstream. 

Floodwater in the Grand View basin was impounded by several miles of narrow canyon that includes the flumelike neck 

near Swan Falls used by Jenkins to calculate the probable 

maximum discharge. From here, the flood emerged into a wide 

basin that surrounds Walters Ferry where ano·ther enormous 

heap of Melon Gravel was deposited. Below Walters Ferry the 

Snake River canyon widens into a broad lowland that continues 

many miles downstream. Signs of the Bonneville Flood are not 

evident in this lowland, but flood debris is again found farther 

on in Hells Canyon. 

Comparison of the volume of Melon Gravel with the size of 

the Snake River canyon upstream provides a basis for estimating the probable amount of erosion caused by the Bonneville 

Flood. The volume of gravel from Twin Falls downstream to 

Grand View is about six-tenths of a cubic mile. Flood erosion 

along the 14 miles of anomalously enl•arged canyon already mentioned near Twin Falls may have alone produced from one-third 

to one-half of a cubic mile of flood debris. Thus, the canyon 

near Twin Falls was the major source of the Melon Gravel. The 

remainder was produced mostly by erosion along constricted 

canyon segments downstream, but simple arithmetic indicates 

that this downstream reach could not have been much enlarged 

by flood erosion, and its present appearance must closely resemble its aspect prior to the Bonneville Flood---except as modified 

by deposits of Melon Gravel. 

When flood velocities estimated from the size of the boulders 

in Melon Gravel along the Snake River canyon are multiplied by 

cross-sectional areas of narrow stretches of canyon inferred from 

the flood profile, the indicated discharges are comparable to 

the amount calculated at the flumelike neck near Swan Falls. 

For example, 4-foot boulders at Melon Valley indicate a velocity 

of 17 fps (feet per second) and a discharge of 17 million cfs 

(cubic feet per second)-that is, four-tenths of a cubic mile per 

hour; and 10-foot boulders between Bliss and King Hill indicate 

a velocity of 24 fps ·and a discharge of 14 million cfls---or onethird of a cubic mile per hour. Boulders in Melon Gravel may be 

rather accurate indicators of the velocity of flow. 

Although the Bonneville Flood greatly exceeded the ordinary 

behavior of rivers, its violence can be partly comprehended by 

comparison with other catastrophic floods of modern times. The 

hydrologic effects of several great historic floods are briefly reviewed: those of Gohna, India; Great Indus Flood; Gros Ventre, 

'Vyo.; Lake Issyk, U.S.S.R.; St. Francis Dam, Calif.; Vaiont 

Reservoir, Italy ; various jokulhlaups (glacier bursts) in Iceland ; and catastrophic floods from ice-dammed lakes. 

INTRODUCTION 

BACKGROUND OF STUDY 

Pleistocene Lake Bonneville, a•t one time in its early 

history, spilled northward at Red Rock Pass near Preston, Idaho, and suddenly released an enormous volume 

of water onto the Snake River Plain (fig. 1). Evidence 

of this catastrophe, known as the Bonneville Flood,1 

was discovered by Gilbert ( 1878) during his pioneer 

study of ancient Lake Bonneville, but the wide-reaching 

effects of the immense discharge downstream along the 

Snake River were not recognized until1954, when they 

were explained to me by Howard A. Powers, of the U.S. 

Geologic.al Survey. Subsequent fieldwork by several 

geologists has shown tht Marsh Creek V ailey immediately downstream from Red Rock Pass was flooded 

from wall to wall, as were large parts of the upper 

Snake River Plain-notably an area near American 

Falls and a basin thrut surrounds Rupert. Farther downstream, the 200-mile canyon of the Snake River was 

flooded to a depth of 300 feet. Spectacular erosion in the 

form of abandoned channels, spillways, and scabland 

marks the flood path downstream to Twin Falls, and 

huge bars of boulders and sand that indica.te a former 

stream of extraordinary size litter the canyon farther 

west. This report is focused on the route followed by the 

flood from the upper Snake River Plain to the canyon 

head near Twin Falls and on the stretch of canyon extending 200 miles downstream to Givens Hot Springs 

(fig. 2) 0 

Geologists first explained gigantic boulders in the 

Snake River canyon as basalt blocks worn by water 

(Russell, 1902, pl. 25B) ,

2 and then as debris eroded from 

nearby lava dams (Stearns, 1936, p. 441-442; Stearns 

and others, 1938, p. 89), but the boulders are now identified as impressive signs of havoc caused by water discharged from Lake Bonneville (Malde, 1960). 

When Gilbert ( 1890, p. 175-177) described effects of 

the voluminous outflow along the first several miles 

below Red Rock Pass, he referred to the efHuent as 

the Bonneville River, and this usage was adopted by 

Trimble and Carr ( 1961a) when they described the outflow near Pocatello in the upper Snake River Plain. 

However, to emphasize the catastrophic results of peak 

discharge, which lasted only a short time, I refer to 

the sudden outflow as the Bonneville Flood. 

1 The name "Bonneville Flood" was first used in a guidebook prepared 

for the 7th INQUA Congress (Richmond and others, 1965, p. 98), which 

included as part of the source material excerpts from a preliminary 

manuscript of this report. 

2 This photogrn.ph (I. C. Rus!"ell, 757) in the field rf>Cords of the U.S. 

Geological Survey, Denver, Colo., bears Russell's caption, '"Blocks of 

lava rounded by the river; not transported by the river or by the 

glacier. Snake River, Idaho." 
.. 

(II" 

115" 114" 

20 40 MILES 

I 

Rock Creek 

Hills 

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EXPLANATION 

D Area of Bonneville Flood 

~ 

• Melon Gravel 

114"30' 

Miles Hot 

I 

5:"" 

Canyon rim 

INTRODUCTION 

112" 

Area of Lake 

Bonneville 

4280/4200 

Altitudes, in feet, 

of flood and of valley floor 

Estimates in 

pctrentheses 

UINTA 

FIGURE I.-Northern part of Lake Bonneville, nearby part of Snake River Plain, and path of the Bonneville Flood 

from Red Rock Pass to T\vin ~ails . The area outlined at Twin Falls is shown in greater detail in the inset map ; 

that at American Falls, in figure 10. 

3 
116"45' ,.-

R. 3 W. NAMPA 

8 Ml 

116"30' 43"30' 

R. 2 W. 

T. 

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EXPLANATION 

<<<<<<<<< 

Marginal or upland channel 

Melon Gravel 

v 

Area of Bonneville Flood 

@ 

Miles upstream from 

Hot Springs Ferry 

Canyon 

~ 

rim near Twin Falls 

2460/2245 

Altitudes, in feet, of flood 

and of valley floor 

0 5 10 15 MILES 

T. 

5 

5. 

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R. 16 E. R. 17 E. R. 18 E. 

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FIGURE 2.-Snake River below Twin Falls, deposits· of Melon Gravel, marginal channels, area of Bonneville Flood, heights of floodwater, and locations 

of sections. River mileage, which is measured upstream from the Hot Springs Ferry, is based on the survey of Marshall (1914). 

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~ 
6 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

The observations reported here were made during the 

course of regional geologic studies, beginning in 1954 

(Malde and Powers, 1962). The geology of the area 

near Pocatello where the flood debouched onto the Snake 

River Plain was mapped by Trimble and Carr ( 1961b), 

and part of the flood path below Twin Falls was also 

mapped by Malde, Powers, and Marshall (1963). Practically all the flood area is covered by detailed topographic maps. 

ACKNOWLEDGMENTS 

Before my fieldwork in the Snake River Plain began, 

H. A. Powers called my attention to the significance of 

boulder deposits near Hagerman and pointed out their 

catastrophic origin. In 1955, amused by a whimsical billboard that advertised one patch of boulders as "petrified 

watermelons," we (Mal de and Powers, 1962, p. 1216) 

~pplied to them the descriptive geologic name, Melon 

Gravel, which has since become on.e of many evocative 

terms in stratigraphic nomenclature. Later, E. G. 

Crosthwaite, who was doing ground-water studies in 

the upper Snake River Plain, told me about the role 

of the Rupert basin as a "regulating reservoir" along 

the flood path, and he described important features 

along an upland channel between Rupert and Twin 

Falls. Meanwhile, D. E. Trimble and W. J. Carr were 

examining effects of the Bonneville Flood between Pocatello and American Falls, and their observations enlarged my perspective. In 1962, those who joined me on 

a field trip for the Friends of the Pleistocene (Rocky 

Mountain Section) along the flood path downstream 

from Twin Falls reviewed my analysis of the flood; 

accordingly, my understanding improved. Lastly, participants in the 1965 INQUA Field Conference E-most 

of them from foreign countries-contributed provocative ideas from their diverse experience. Of these participants, I am especially indebted to Roald Fryxell 

and G. M. Richmond, who brought to the Snake River 

Plain their intimate knowledge of Pleistocene floods 

in eastern Washington. 

SUMMARY OF FLOOD PATH 

The Bonneville Flood mostly occupied the same canyons and valleys that exist today, but it modified them 

by causing extensive erosion and by depositing great 

piles of flood debris. It left a vivid record of its cataclysmic passage. The havoc produced by the flood was 

influenced to a considerable extent by local physiographic factors and by the rocks found along the way. 

The Snake River is one of the steepest large streams 

in North America, and descends in altitude from 4,400 

feet at Pocatello to 2,200 feet at Givens Hot Springs, 

300 miles downstream. Although interspersed with reiatively tranquil stretches, this steep gradient undoubt-

. edly added to the flood force and velocity. The Bonneville Flood is best understood by a brief tracing of its 

path downstream from Red Rock Pass. 

Features along the path in Marsh Creek Valley, 

where the flood spread from wall to wall, are being 

studied by R. C. Bright and Meyer Rubin (see Richmond and others, 1965, p. 103-106). It is noteworthy that 

the flood along this stretch was impeded by the Portneuf 

Narrows, a canyon neck 45 miles downstream from 

Red Rock Pass. As indicated by the calculated discharge (see p. 12), the maximum flood probably was 

at least 400 feet deep at this place (see altitudes in 

fig. 1), and the floodwater some distance upstream was 

relatively tranquil during peak flow. Nonetheless, the 

size of boulders in flood debris along Marsh Creek Valley indicate a velocity of 25 fps (feet per second) (Richmond and others, 1965, p.106). 

From the Portneuf. Narrows the flood fell rapidly to 

the Snake River Plain at Pocatello where it entered 

shallow water of an existing lake held by a basalt dam 

near American Falls, 30 miles downstream; here it 

dumped a delta of bouldery debris on the lake bottom. 

The size of this debris ranges from boulders as large 

as 8 feet at Pocatello to- cobbles 10 miles downstream, 

owing to the spread of floodwater in the lake. (See fig. 

10.) Discharge of floodwater from this lake over and 

around the basalt dam cut several channels and spillways on the upland and along the Snake River. As 

erosion progressed, floodwater became lower, and terraces were cut on the delta. Molluscan fossils from the 

highest terrace were presumably living when the flood 

began and are dated by radiocarbon at 29,700 ± 1,000 

years (Trimble and Carr, 1961 b, p. 17 46; _ sample 

W-731). 

Beyond the maze of channels and spillways below 

American Falls, the flood descended in a broad swath 

along the Snake River to the Rupert basin, where it 

covered 300 square miles to an average depth of 50 feet. 

Floodwater escaped from this basin mostly through a 

basalt gorge aJt the head of the Snake River canyon-at 

this place a scoured trench 20 miles long of remarkably 

constant width and depth (about 1,100 by 325 ft)-but 

floodwater also escaped by simultaneous overflow into 

an upland channel that rejoined the canyon at Twin 

Falls, 30 miles downstream. (See inset map in fig. 1.) 

At this junction, apparently because of flood erosion on 

a grand scale, a canyon segment more than 10 miles long 

is anomalously broad and deep, as much as 5,000 by 500 

feet. The southern rim has a ragged edge along several 

miles, and the northern rim where flood water debouched from the upland channel is intricately dissected by numerous spillw-ay alcoves. A series of spectac-

.... 

-..: 

..._ 
i' 

.. ,. 

INTRODUCTION 7 

ular "underfit" waterfalls (for example, scenic Shoshone Falls) occurs along the canyon floor. Bordering 

scabland demonstrates that this wide stretch of canyon 

was for a time virtually brimful. 

The amount of flood erosion near Twin Falls may be 

roughly estimated by comparing the size of the enlarged part of the canyon with that of the canyon 

immediately above and below. (See p. 44.) For the 

part from mile 175 to 189, where effects of erosion are 

obvious, the flood removed fror.n one-fourth to one-third 

of a cubic mile of basalt. Considered as gravel having 

25-percent porosity, this material would have produced 

from one-third to nearly one-half a cubic mile of flood 

debris. An indeterminate mnount of basalt-but undoubtedly a much smaller quantity-was eroded from 

the 20 n1iles of canyon upstream. 

In the neX't 105 miles downstream from Twin Falls, 

the Bonneville Flood was entirely confined to the Snake 

River canyon-a trench 500-600 feet deep of variable 

width made up of basins carved from soft deposits and 

of constricted segments cut into basalt (fig. 2). The 

narrow places impeded flow and impounded floodwater 

in basins at Melon Valley, Hagerman, IGng Hill, 

Glenns Ferry, I-Iammett, Indian Cove, and Eagle Cove, 

which therefore acted as traps for basaltic flood debris 

flushed down the cm1yon . This debris-the Melon 

Gravel-was deposited in the basins as gigantic bars, 

some of them several miles long and nearly 300 feet 

high. The bars consist of chaotic mixtures of boulders 

and sand arranged in crossbeds of phenomenal.scale. 

They mostly lie midway between the canyon walls from 

which they are separated by marginal chm1nels as deep 

as 150 feet. Basalt boulders, commonly larger than 5 

feet, are conspicuous on bars just below constrictions; 

but flood debris in wider places a mile or two farther 

downstream is dominantly basaltic sand-although 

nonetheless bouldery. The boulders generally can be 

id~ntified as having come from outcrops at narrow 

places not more than a few miles upstream . 

Because the basins were effective traps for Melon 

Gravel and because subsequent erosion of this debris has 

been trivinJ, the volume of Melon Gravel roughly measures the :tmount of flood erosion. (See table 4, p. 45.) 

For the stretch between Twin Falls and Eagle Cove, 

this volume is a little more than half a cubic mile, of 

which more than one-half was deposited in the basin at 

King Hill-the farthest upstream reach of relatively 

slUJck floodwater. (See profile of Bonneville Flood, 

fig. 3.) 

A particularly narrow constriction at mile 75 near 

Crane Falls impounded water 300 feet deep upstream 

and caused some floodwater to spill across the upland 

and empty into another temporary lake in the Grand 

View basin. Deltaic debris at the spillway mouth (mile 

61 in fig. 2) shows that the lake at Grand View was as 

deep as 325 feet and therefore covered 150 square miles. 

The Grand View basin contains only scant amounts of 

Melon Gravel and must have been the final trap for 

flood debris from sources upstream. 

From the Grand View basin the Bonneville Flood 

passed through several miles of narrow canyon, including a flumelike neck near Swan Falls used by C. T. 

Jenkins to calculate a probable maximum disoharge of 

about 15 million cfs (cubic feet per second) -thrut is, 

more than one-third cubic mile per hour (seep. 12)-

and emerged into the Waltel"S basin where another 

enormous heap of Melon Gravel was deposited. 

Not far below the Walters basin, basalt rimrock 

along the Snake River disappears, the topography becomes subdued, and the canyon widens into a broad lowland that continues many miles downstream. Signs of 

the Bonneville Flood are not evident in this low land, 

but flood debris is again found farther on in Hells 

Canyon (Stearns, 1962) . 

GLACIAL AND PLUVIAL STRATIGRAPHY 

In the discussion that follows, it is necessary to refer 

to lake levels and deposits of Lake Bonneville and to 

their correlation with glacial episodes. The pertinent 

relations have been recently reviewed (Morrison, 1965; 

Morrison and Frye, 1965) and are summarized in the 

chart on page 9. The lake levels along the Wasatch 

Range at altitudes of about 4,470, 4,800, and 5,135 feet 

are referred to as the Stansbury, Provo, and Bonneville 

shorelines, respectively. Because of differential warping 

resulting from isostatic rebound after loss of lake water, these shorelines differ in altitude at Red Rock Pass. 
··' .J 

Walters basin Swan Falls section I ____ I ,---- ---------

HOT SPRINGS FERRY 

2800' / 

2600' 

2400' 

;: 

~ 

Grand View basin Crane Falls section Cove section 

Scabland at g _ t..J canyon rim :;; CO 

)( ~ U I - ~CQ Q: Boulder delta at mouth (f) 

~ of marginal spillway~ 

-j ~ I 1 Ponded water at 2740' 

lJ 

e,j 

c 

0 

·.;; 

u 

~ 

(/) 

. ~ r...., Loveridge Eagle 

Ponded water at 2625' U . . .-:-::·:._ I basin Cove I 

L __ ~citLJ]l{&;._ _ _ ~.? SNAKE RIVER 

c 

0- :;;Cl 

U I 

~Q 

22oo·l ........ ::::: ..... : .. ·:·.-:-:-:':-:::::.=:-· ........... ( ... · .. :::-·-:iiii······· J 1-- 1- - 1- I 

4000' 

3800' 

3600' 

3400' 

3200' 

3000' 

2800' 

2600' 

0 10 20 30 40 50 60 70 80 90 MILES 

EXPLANATION 

Vertical 

~ range of Melon 

Gravel deposits 

Extent of constricted canyon 

Thousand 

Springs 

Cove section King Hill basin Bliss section Hagerman Valley _section 

~ 

~ 

-q: 

Ct 

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0 

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Valley 

section 

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Weeping Wall 

section 

..... 

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..., 

~ 

c 

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(/) 

2400'r=====~~~~~~~:::J:::=:::=======~~~~~~~~~~~~~~~~~~----------~~~~~~~~~m-~~~~~~~~~~~~~~~~~~~~~~~ 100 110 120 130 140 150 160 170 180 190 MILES 

FIGURE 3.-Profile of Snake River and of Bonneville Flood at highest stage. Sections are shown in figure 13, except G-G', which is shown in figure 17. 

A ... .,I. i 14 /•. •, ) "' -- ~ I _/ 

00 

2 

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-p:· 
SOURCE 9 

Lake Bonneville 

Glacial 

stratigraphy Altitude of lake, in feet--

Sfratigraphy 

Along Wasatch Range (approximate) At Red Rock Pass 

Neoglaciation Subaerial and shallowlake sediments 4250 Not present. 

) I I I I I I 1\ II Till t \ Midvale soil It II \ I Ill l I I II I { I ! 

4410 

Upper Draper Formation 4470 Stansbury shoreline Not present. 

till 4770 

Ql c: \ I I ! l ~ ! l I I - 0 l/ l \ Graniteville soil n~nlllllll1l 1111 ~:;: 

-g .~ 4800 Provo shoreline 4780 c: u --a..t; co Middle Upper member till 5135 Bonneville shoreline 5085 

Bonneville 

Formation 

Lower White marl 

till member Not yet determined Not known. 

} ) ~ l (I HI! l ! 11!!?? l i Promontory soil 1ll ! ( ll \ l ( ~ ll \ ~ \ 

Bull Lake Glaciation Alpine Formation 

'SOURCE 

Red Rock Pass, the lowest place on the rim of the 

Bonneville basin, is the only possible source of the Bonneville Flood, as shown by numerous geologic effects 

of catastrophic discharge found downstream and by the 

absence of such effects along other tributaries of the 

Snake River. In the lower end of Marsh Creek Valley 

below Red Rock Pass, Gilbert long ago ( 1890, p. 175-

177) described the eroded surface of a lava flow that 

was scoured and cha1meled by the flood. A drainage 

connection at Red Rock Pass is also suggested by certain endemic fishes shared by the Bonneville basin and 

the Snake River above American Falls (Hubbs and 

Miller, 1948, p. 30-31; Miller, 1958, p. 211-213), but this 

common heritage may stem instead either from a connection later than the Bonneville Flood or from a more 

c01nplex route that involved a former connection between Bear Lake and .the Snake River (Miller, 1965, 

p. 577-578). 

Red Rock Pass is now graded to a level near a conspicuous shoreline of Lake Bonneville called the Provo 

shoreline ( 4,780 ft alt)-or possible to the "Provo 2" 

shoreline recognized at about this altitude farther south 

along the vVasatch Range. (See Morrison, 1965, p. 276.) 

The pass is cut in Paleozoic shale, limestone, and dolomite so as to forn1 a narrow gap 2 miles long (Williams, 

1962, p. 143-145). At one time, Red Rock Pass was several hundred feet higher, at the level of the Bonneville 

shoreline ( 5,085 ft alt, according to a spirit-level measurement by Gilbert). Jagged remnants of Paleozoic 

5100 Not known. 

rocks up to this altitude in the Pass suggest that the 

Bonneville shoreline was controlled by this sill of resistant rocks. As will be explained below, rapid discharge at a level above the Bonneville shoreline-presumably by sudden removal of unconsolidated surficial 

material-apparently accounts for the catastrophic 

character of the Bonneville Flood. When the lake level 

fell to bedrock at the Bonneville shoreline, the resistan.t 

rocks impeded erosion, and discharge dwindled. Overflow at a later time gradually eroded Red Rock Pass to 

the Provo level. 

Kelton Pass ( 5,305 ft alt), a somewhat higher drainage divide between the Bonneville basin and the Snake 

River (fig. 1), is ·a;bout 115 feet a:bove the highest nearby 

Bonneville shoreline ( 5,190 ft alt, according to Crittenden, 1963, fig. 3) . I\:: elton Pass does not appear eroded, 

and no sign of voluminous floodwater can be seen in the 

Raft River valley to the north, which would have received any spillover from this divide. 

In the present headwaters of the Snake River, Jackson Hole in Wyoming is the only basin from which a 

large discharge might account for some features attributed to the Bonneville Flood, but recent geomorphic 

studies in Jackson Hole_ yield no evidence of sudden 

release of large volumes of water during the late 

Pleistocene (John de la Montagne and J.D. Love, oral 

commun., 1958), and discharge from Jackson Holecould not account for the effects of catastrophic outflow 

found in Marsh Creek Valley or for t-he boulder deposits 

at Pocatello. 
10 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

AGE 

The Bonneville Flood was one of the most recent 

geologic events along the Snake River and is surely 

of late Pleistocene age. The only notable younger deposits along the river, other than those of the flood 

plain, are discontinuous patches of stream gravel, presumably contemporaneous with mountain glaciation. 

The flood deposits are moderately weathered at the surface and have a friable calcareous soil profile about 4-6 

feet thick, but they are otherwise rather fresh and lack 

~he deep profiles of weathering and thick layers of hard 

caliche that characterize older Pleistocene deposits in 

the uplands. Exposed boulders are stained brown ·and 

have exfoliating rinds about half an inch thick. Their 

s~rfaces a~e commonly grooved and polished by the action of windblown sand. Evidently the flood deposits 

have been exposed to weather for a considerable time. 

(See remarks on p. 11 about correlation of the soil 

profile.) 

Current ~nformation allows some latitude in dating 

the Bonneville Flood, but an estimate of age can be deduced from the flood's inferred dependence on certain 

geologic events pertaining to the Bear River. This 

stream rises in the Uinta Mountains (fig. 1) and accounts for more than 43 percent of the present inflow to 

Great Salt Lake. Diversion of the Bear River into the 

Bonneville basin, firnt discussed by Gilbert ( 1890, p. 

· 218-220, 263), was probably responsible for overflow of 

Lake Bonneville at Red Rock Pass and, hence, caused 

the flood. The details of the Bear River diversion have 

been worked out by R. C. Bright ( 1963), and they suggested to him that spillover at Red Rock Pass occurred 

during deposition of the Bonneville Formation about 

18,000 years ago. In this view he is joined by Meyer 

Rubin. (See Richmond and others, 1965, p. 105-106.) 

D. E. Trimble and i, however, as well as some of our 

colleagues, believe that the available evidence points 

more strongly to spillover in Alpine time, probably 

about 30,000 years ago. This difference in opinion arises 

partly from the complicated history of the Bear River 

revealed by Bright's study and partly from radiocarbon dates and geologic knowledge gathered by Bright 

and other workers along the flood path downstream 

from Red Rock Pass. The pertinent relations are as 

follows. 

Prior to 34,000 years ago, as indicated by radiocarbon 

dates to be mentioned shortly, the Bear River flowed 

from Bear Lake to the Snake River Plain at Pocatello. 

The river followed the present route of the lower Portneuf River. (See dotted line in fig. 1.) Subsequently, 

according to Bright, the ancestral Portneuf was 

dammed ?Y several basaltic lav·a flows (and probably 

by faulting), and the ·Portneuf and Bear Rivers 

were impounded in a basin at Thatcher. These 

events formed a lake-Lake Thatcher. Later lava flows, 

of which one or more descended the Portneuf valley 

nearly to Pocatello, returned the Portneuf River to its 

former route and at various times may have permitted 

the Bear River to discharge westward via Lake 

Thatcher. Organic material in a soil under one of these 

lava flows in lower Marsh Creek Valley, near the Portneuf, yields various radiocarbon dates-35 000 ± 3 000 I . ' ' 

years (sample W-1177); 33,000±1,600years (W-1121); 

30,000+2,000 years (W -1329) ; and 25,000+2,000 years 

(W -1334) -and the lava surfU~ce displays abundant 

signs of erosion by the Bonneville Flood. Mollusks from 

the lower part of lake beds prdbably impounded behind 

the lava in Marsh Creek Valley yield a radiocarbon date 

of 32,~00±1,500 years (W-1221). At Thatcher, several 

collectiOns of mollusks from deposits of Lake Thatcher 

range in radiocarbon age from 33,000±1,600 years 

(sample W-1128) to 27,000±900 years (W-1125). The 

similarity in age ranges indicates that eruption of the 

lava and filling of Lake Thatcher were approximately 

contemporaneous. Eventually, successive lava flows between the Bear and the Portneuf Rivers built a barrier 

higher than the south rim of Lake Thatcher (5,445 ft 

alt) ; Lake Thatcher then spilled southward across a 

quartzite divide into the Bonneville basin. Downcutting 

at tJhis spillway ultimately formed a gorge with a floor 

at an altitude of 4,600 feet, which drained Lake Thatcher 

and allowed erosion of the lake deposits. By this time, 

Lake Bonneville must have been dry or at a low level. 

A subsequent rise of Lake Bonneville deposited beds 

of the Bonneville Formation on eroded deposits of 

Lake Thatcher, and correlative beds ne.ar the Bear River 

ou~let are dated 18,900±500 years (sample W-982). 

Bright and Rubin believe that this rise of Lake Bonneville, not a rise that may have been caused by initial 

capture of the Bear River, was responsible for overflow 

at Red Rock Pass. Thus, they estimate the age of the 

Bonneville Flood at about 18,000 years. 

To me, the evidence concerning diversion of the Bear 

River points simply to its initial capture as the cause 

of spillover at Red Rock Pass. If the radiocarbon dates 

are taken literally, overflow of Lake Thatcher could 

have been as recent as 27,000 years ago, as Bright (1963) 

accepts; but if one or two anomalously young dates are 

discounted as aberrant, the dates accord with overflow 

about 30,000 years ago. (It was this suddenly increased 

inflow, of course, that accounts for spillover of Lake 

Bonneville, and not the meager water supplied by Lake 

Thatcher.) This explanation ties one geologic event (the 

Bonneville Flood) to the greatly augmented inflow 

brought by the Bear River, for which the geologic effects are otherwise either problematic or invisible (Mor-

-~ 
P.· 

.,.. 

-~ 

MAGNITUDE OF FLOOD 11 

rison, 1965, p. 276). It also lengthens the time available 

for the cutting of the outlet of Lake Thatcher-a gorge 

in quartzite more than 800 feet deep and several miles 

long. :However, if Lake Bonneville initiaily overflowed 

30,000 years ago in Alpine time, the lake level thereafter 

must have soon subsided (so as to account for downcutting at the outlet of Lake Thatcher and for the formation of the Promontory Soil at low level in the Bonneville basin). This circumstance suggests that lake 

level was influenced by other factors in addition to 

capture of the Bear River. That is, the augmented inflow brought by, the Bear River could not alone have 

accounted for both a rapid rise in lake level and a 

subsequent subsidence. The additional factors that influenced the regimen of Lake Bonneville at time of spillover were undoubtedly effects of regional climate, and 

perhaps responses to climatic change, but these factors 

are as yet unknown. Nonetheless, whatever else caused 

Lake Bonneville to overflow and then subside, a 30,000-

year age for the Bonneville Flood accords with information collected along the flood path below Red Rock 

Pass. 

At American Falls a minimum age for the Bonneville Flood is given by a radiocarbon date of 29,700± 

1,000 years (sample W-731) for mollusks from an alluvial terrace somewhat younger than the maximum 

flood, which probably formed as the flood waned and 

diminished in height (Trimble and Carr, 1961b, p. 

1746). Al-though this date stands unsupported by others, 

it pertains to the .only sample intimately tied to the 

flood deposits and must be consider~d in any reckoning 

of age. If the history of the Bear River is taken into 

account and if overflow at Red Rock Pass is assumed 

to coincide with the capture of Bear River via spillover 

of Lake Thatcher, the radiocarbon dates already mentioned for lavas in Marsh Creek Valley and for mollusks in Lak~ Thatcher do not generally contradict the 

date obtained at American Falls. 

Two other matters indicative of antiquity for the 

Bonneville Flood must be mentioned, even though difficult to evaluate. First, I judge that the calcareous soil 

profile prevalent on flood debris along the Snake River 

resembles in thickness and degree of development, as 

well as in contrast to other soils in this region, the 

Promontory Soil formed on the Alpine Formation in 

the Bonneville basin. At a depth of 3 feet below yellowish-brown and weakly calcareous material of the B 

hori~on is about llh feet of strongly calcareous soil 

that exhibits a platy structure marked by irregular 

plates of firm caliche. Visible carbonate in friable subsoil extends to a depth of more than 6 feet. Second, D. 

E. Trimble (oral commun., 1964) has observed on the 

lava devastated by the Bonneville Flood in lower Marsh 

Creek Valley a pattern of soil mounds comparable to 

paJtterned ground in the western Snake River Plain 

(Mal de, 1964; Fosberg, 1965), which I attribute to late 

Pleistocene periglacial climate. If these mounds indeed 

formed in response to cold climate, it seems likely that 

they date from the time of Pinedale Glaciation and 

that the scoured surface produced by the flood is correspondingly older. 

Whatever the age assigned to the Bonneville Flood, 

geologists now agree that it was undoubtedly older than 

the formation of the Bonneville shoreline-a level later 

controlled 'by the height of resistant bedrock in Red 

Rock Pass. Bright and Rubin (in Richmond and others, 

1965, p. 106) found wave-washed debris and evidence 

of scour 30 feet above the Bonneville shoreline at Red 

Rock Pass. These features are obviously attributable 

to initial overflow. The true height of overflow is not 

determinable, but it could have been 100 feet above the 

Bonneville shoreline. (See p. 12.) Overflow may have 

been caused by sudden failure of an alluvial barrier, as 

Gilbert (1890, p. 175-178) supposed, but the actual deposit that formerly blocked the pass has never been 

identified. Probably it was entirely removed by the 

catastrophic outflow, and Lake Bonneville was thus 

drastically lowered-presumably as far as the bedrock 

lip at the Bonneville shoreline. Erosion thereafter would 

have been at a much slower rate and not of catastrophic 

magnitude. If follows that downcutting to the Provo 

shoreline, which was accomplished by the wearing away 

of hard rocks in Red Rock Pass about 13,500 years ago, 

according to radiocarbon dates reviewed by Morrison 

and Frye (1965, p. 20-24), was in no way responsible 

for the Bonneville Flood. This conclusion differs from 

a long-standing idea of Gilbert that erosion of Red 

Rock Pass to the Provo shoreline caused the flood. Not 

surprisingly, because of the frustrating paucity of 

relevant geology at Red Rock Pass, the present understanding of the age of the flood has been largely learned 

by diligent search elsewhere-chiefly by R. C. Bright 

and D. E. Trimble. 

MAGNITUDE OF FLOOD 

The enormous size of depositional and erosional features produced by the Bonneville Flood imply a peak 

discharge of extraordinary volume. The maximum discharge can be estimated f·rom computed flow through 

certain constricted segments in the canyon of the Snake 

River, by use of standard hydraulic formulas. 

Stearns (1962) calculated a peak discharge of 10 

million cfs at Brownlee damsite near Homestead, Oreg., 

where the flood was 410 feet deep (that is, 290ft above 

present river level), and where the canyon is 1,400 feet 

wide. The method of calculation is not stated, but his 
12 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

figurre is compara:ble to that computed from the Manning equation. (See table 2, p. 25.) 

The Snake River canyon below Twin Falls has many 

constricted segments th3Jt flowed deep with water during the flood (figs. 2, 3), and any of these would give 

a measure of discharge. Canyon constrictions cut into 

basalt near Swan Falls, however, at miles 26 and 28, 

(the mileage is measured upstream from Hot Springs 

Ferry, as shownin fig. 2) give an unusual opportunity 

for calcul3Jting maximum · dischrurge by the criticaldepth method. (See fig. 13, section A-A' at mile 26.) 

As will be explained (p. 42), this series of constrictions impeded flow and impounded floodwater 325 feet 

deep upstream (2,625 ft alt). Because the canyon widens 

downstream and no futther major constrictions occur, 

there was virtually no backwater from downstream 

sources, .and conditions for flow at critical depth were 

apparently satisfied. Boulders as much as 6 feet in 

diameter at an altitude of 2,575 feet about 2 miles 

farther downstream (mile 23:1f2 at Swan Falls) establish a tailwater altitude. In brief, the Snake River 

canyon between miles 26 and 28 acted as the throat of 

a gigantic venturi flume, a device commonly used to 

measure discharge in ehannels. Assuming that the 

canyon now retains tJhe shape acquired during passage . 

of the flood and that it aoted as a ventuTi flume, Clifford 

T. Jenkins, U.S. Geological Survey (oral commun., 

1964) calculates a maximum discharge of ·from 12.2 to 

16.3 million cfs--the range being determined by the 

limiting values adopted for channel roughness and friction losses. The most likely discharge was about 15 

million cfs, or more than one-third cubic mile per hour, 

which is aibout three times the average discharge of 

the Amazon. 

The maximum historic discharge in the upper Snake 

River was recorded at Idaho Falls in early June 1894 

and amounted to 72,000 cfs (U.S. Geol. Survey, 1959, 

p. 44), or a;bout one two-hundredths of peak discharge 

during the Bonneville Flood. 'I'he destructive flood 

caused by rupture of the Gros Ventre landslide in the 

upper Snake River basin (Alden, 1928) amounted to 

60,000 cfs when it reached Idaho Falls. 

DURATION. 

The duration of the Bonneville Flood can be judged 

from the discharge capacity of canyon sections along the 

Snake River and from the probruble volume of water 

available in Lake Bonneville. Although the maximum 

proba:ble discharge was about 15 million cfs, the average 

discharge must have been much less. The probable volume of a vail able water is subject to more uncertainty. 

Radiocarbon age determinations and the geologic relations just described indicate that Red Rock Pass was 

lowered during the flood no farther than the Bonneville 

shoreline, probably by rapid removal of surficial material. The height of overflow has not been determined, 

but it could not have been more than 115 feet above the 

Bonneville level, otherwise overflow would have occurred at Kelton Pass (p. 9). According to Williams 

( 1962), a pass at 5,180 feet altitude (thus 95 ft above the 

Bonneville shoreline) a mile east of Red Rock Pass 

"shows no evidence of outflow." A duration of flow 

based on lake lowering of not more than 100 feet therefore seems reasonable. The corresponding volume is 

readily calculated from published hypsometric data 

(Eardley and others, 1957, p.1143-1145): 

Altitude of lake Surface area (feet) (square miles) (a~res) 

5,235± (spillover) 1 21,200 13.57.X l<f 2 5,135± (Bonneville level) 2 19,750 12.64 X 106 

1 Estimated from dabt of Eard.ley1

, Gvos~tsky, and l\Iarsell (1.957). 

2 Gilbert's a.verage altitude of 5,135 ft for the Bonneville shoreline is 

used although the altitude of the shoreline throughout the basin 

owing to isostatic deformation (Crittenden, 1963). At Red Rock Pass 

the Bonneville shoreline is at 5,085 ft alt, according to Gilbert. 

From the formula of Eardley, Gvosdetsky, and Marsell (1957, p.1143)-(area at level5,135 X 100ft)+ ( difference in area at levels 5,235 and 5,135X100 ft, divided 

by 21;2)-the volume of water :between the Bonneville 

shoreline and a spillway 100 feet higher would haveibeen 

1.3 billion acre-feet, or about 380 cubic miles. Thus, a 

continuous discharge of 15 million cfs-that is, a little 

more than one-third cubic mile per hour-could have 

been maintained 6 weeks. A discharge of 1.8 million 

cfs would have lasted about a year. 

An erroneously long duration of flow based on a 

misinterpretation of Eardley's data was calculated by 

Stearns (1962), who further assumed mistakenly that 

. the w3Jter was discharged between the Bonneville and 

Provo shorelines, rather than entirely above the 

Bonneville. 

Gilbert (1890, p. 177) estimated the duration by assuming that erosion features on basalt 22 miles below 

Red Rock Pass-part of the lava that diverted the Bear 

River-could be accounted for by a river approximately the size of the Niagara: 200,000 cfs. Gilbert also 

thought the discharge was from the interval between the 

Bonneville and Provo levels, and he thus estimated 

the duration of discharge as somewhat less than 25 

years. 

Lakes that discharge over earth harriers, as indicated 

by records mentioned later in this report, ordinarily 

release most of their water suddenly, a.nd the flow 

thereafter rapidly diminishes. Because a flood wave becomes attenuated during travel downstream, a constricted section of canyon like that at Swan Falls may 

not provide a reliable measure of peak discharge at the 

.. _ 
~. 

)' 

DURATION 13 

source, which could be considerably greater. Discharge 

from Lalre Bonneville therefore may have been extraordinarily large at first-perhaps exceeding the rate 

n1easured downstream-but probably soon decreased. 

For some lakes, however, discharge over an earth 

barrier is not excessively rapid, as shown by the history of Lalm Tanganyika, east Africa. From the time of 

its discovery in 1854, Lake Tanganyika rose steadily 

until 1878, when it began tJo discharge over a dam of 

earth that had blocked the exit, tho Lakuga River 

(Tison, 1949). In the next 7 years the lake dropped 9.5 

meters to a new level, at which it stabilized. In an equivalent period before overflow, the lake rose 1.5 meters; 

thus, the total outflow was equal to a lowering of 11 

meters. Because the area of the lake is 34,000 square 

kilometers (Hutchinson, 1957, p. 168), or 8.4 million 

acres, the amount discharged in the 7-year period was 

300 1nillion acre-feet, or a:bout one-fourth of the volume 

possibly discharged from Lake Bonneville during the 

Bonneville Flood. 

In the light of the above discussion, I infer that the 

major effects of the Bonneville Flood probably were 

produced in a few days (certainly in less than a month) 

and that voluminous discharge continued for at least 

a year. 

Geologic studies now in progress at Red Rock Pass 

by R. C. Bright and Meyer Rubin can be expected to 

determine the probable configuration of the spillway 

during the Bonneville Flood, and hence will allow a calculation of maximum discharge based on hydraulic 

principles. 

COMPARISON WITH LAST CATASTROPHIC FLOOD FROM 

GLACIAL LAKE MISSOULA, MONT. 

The late Pleistocene flood from glacial Lake Missoula, 

Mont., first deduced by Pardee (1910 and 1942), far 

exceeded the Bonneville Flood in violence, although it 

was only a little greater in total volume. The present 

lo10w ledge of this flood is discussed by Bretz, Smith, and 

Neff (1956, p. 1034-1042), by Bretz (1959), and by 

Richmond, Fryxell, N eft', and Weis ( 1965, p. 236, 239-

240). Lake Missoula occupied the valley of the Clark 

Fork behind a dam formed by a lobe of the Cordilleran 

Ice Sheet. At the time of its sudden release by failure 

of the ice dam, Lake Missoula held 500 cubic miles of 

water, which escaped at an estimated peak discharge of 

91h cubic miles per hour-almost 30 times the rate computed for the Bonneville Flood. Thus, Lake Missoula 

pr.obaJbly emptied even •more rapidly than Lake Bonneville. 

280-124 0-68-3 

DIAGNOSTIC DEPOSITIONAL AND EROSIONAL 

FEATURES 

MELON GRAVEL 

The Melon Gravel (Malde and Powers, 1962, p. 1216-

1217) consists of deposits of boulders and sand left by 

the Bonneville Flood during its passage down the 

Snake River. The boulders are commonly 3 feet in 

diameter, but some well-rounded boulders measure 10 

feet. Correlative deposits in the upper Snake River 

Plain attributed to the flood .are known as the Michaud 

Gravel (Trimble and Carr, 1961b, p. 1742). 

The Melon Gravel displays a multitude of lithologic 

and .physiographic features that indicate torrential 

origin in rapidly moving deep water. Consisting almost 

entirely of basalt debris derived from local lava flows 

and piled in huge bars that partly fill the canyon to 

a depth as great as 300 feet, the Melon Gravel is the 

1nost easily recognized evidence of the catastrophic 

1nagnitude of the flood. The principal lithologic features that give the gravel its character, besides its distinctive basaltic composition and gigantic rounded 

boulders, are extrmnely poor sorting (mainly boulders 

and coarse sand, but no fine sand or silt), a general lack 

of horizontal !bedding (hut common occurr:ence of thick 

sets of inclined beds), and extraordinary variation in 

size of debris in local deposits (the maximum particle 

size decreases rapidly below canyon constrictions). The 

physiographic features of the Melon Gravel are equally 

distinctive: enormous streamlined bars veneered with 

boulders, usually at midcanyon locations; gravel 

crowded in local basins just 'below canyon constrictions ; 

bars associated with marginal channels that separate the gravel from canyon walls; and tributary valleys obstructed by he.aps of gravel. In these attributes of 

lithology and physiography the Melon Gravel matches 

in considerruble detail the gravel bars in the cha1meled 

scabland of eastern Washington (Bretz and others, 

1956), which were left by glacial rivers during 

catastrophic floods. 

LIT'HO!LOGY 

Exposures in deep borrow pits and at numerous lesser 

outcrops show that the Melon Gravel is a thorough mixture of two principal components, boulders and coarse 

sand. Pebbles and finer sand are generally sparse. The 

best exposures are at excavations in gravel bars at the 

mouth of Cedar Draw (mile 168, fig. 2), Melon Valley 

(mile 161), the Union Pacific Railroad ballast pit (mile 

109) , and Walters Ferry (mile 8) . These excavations 

expose thick sections of the gravel and belie the bouldery 
14 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

aspect presented at the surface by exhibiting large 

amounts of sand. 

The boulders are almost all basalt-the only important hard material in the Snake River Plain-although 

rounded chunks of consolidated sedimentary material, 

ordinarily silt or diatomite, occur locally. Boulders of 

sedimentary material are especially evident in the rail-

. road pit at mile 109 and at the upper end of the Grand 

View basin (Stearns, 1952), where floodwater was impounded in deep temporary lakes. The basalt boulders 

are typically very well rounded and obviously have 

been thoroughly rolled, tumbled, and abraded during 

their passage downstream (fig. 4) . Rounding of 

boulders is conspicuous even though their source (determined by lithologic comparison) is only a few miles 

distant. 

FIGURE 4.-Boulders on a bar of Melon Gravel 1% miles southsoutheast of Hagerman, Idaho. They are 150 feet above Snake 

River and 2% miles east of the present c'hannel. The boulders 

were washed from a !)asalt outcrop about 3 miles upstream. 

The abundant coarse sand, consisting almost entirely 

of basalt fragments, is probably a product of abrasion 

of the boulders. This sand was evidently produced in 

great quantities in comparatively short distances of 

transport. Thus, even in the upper part of the canyon, 

at mile 168, a bar of Melon Gravel was built largely of 

basaltic sand. Basalt outcrops from which this sand was 

derived extend no farther than 40 miles upstream, and 

the sand therefore indicates rapid mechanical disaggregation of material moved by the flood. 

These attributes of the Melon Gravel-mixed particle 

sizes, well-rounded boulders from nearby outcrops, and 

large quantities of locally derived sand-differ 

markedly from the character of load transported by 

ordinary streams and suggest abnormally large flow 

Hjulstrom, 1952; Fahnestock and Haushild, 1962). 

Bedding is not conspicuous in the Melon Gravel, but 

deep excavations show crude sorting into contrasting 

layers of bouldery debris and cross-laminated sand. 

Lying generally at inclined attitudes, such layers form 

great sets of festoon crossbeds in courses 50 feet or more 

thick. At Melon Valley, in a pit at the downstream end 

of a gravel bar, beds dip downstream and toward the 

left and right banks (fig. 5) . Gi'(Yantic cross beds in sand 

are well exposed in the railroad pit (mile 109) and at 

Walters Ferry (mile 8). At the railroad pit (fig. 6), 

which is cut perpendicular to the downstream end of a 

bar, layers of sand form great sweeping sets more than 

100 feet thick that dip toward the left and right banks. 

At Walters Ferry, similar layers dip downstream. Such 

crossbeds could only have formed in very deep water, 

but they are quite unlike the regular foreset beds of 

deltas; deltaic deposits typically lie in horizontal 

courses of limited thickness. Some inclined layers several feet thick at the railroad pit display intricate 

festoons of laminated sand that demonstrate small-scale 

cut-and-fill during buildup of the dipping beds (fig. 7). 

Although the Melon Gravel is poorly sorted, the 

largest boulders at particular spots along the canyon 

are generally uniform in size. Many of them veneer the 

gravel surface, where they presumably have been concentrated by winnowing during deceleration of flow. 

They form bouldery fields littered with well-rounded 

stones ("melon patches") and suggest that the Bonneville Flood was an effective sorting agent in its power to 

move the largest pieces. (See fig. 8.) 

At wide places, or basins, along the canyon, the size 

and abundance of the boulders decrease abruptly, 

doubtless because diminishing velocities of spreading 

floodwater reduced transport capacity. These basins 

acted as sediment traps and hold most of the Melon 

Gravel, much of it being heaped in the upper reaches. 

The Melon Gravel at the downstream ends of basins is 

dominated by sand, mixed with progressively smaller 

boulders. The reduction in the size and quantity of 

boulders is especially evident at the largest basins: 

Hagerman Valley (mile 151-141); King Hill basin 

(mile 121-109); and Walters basin (mile 14-0). A 

similar abrupt decrease in size of boulders transported 

by Pleistocene floodwater is reported at Portland, Ore., 

in the broad lowland downstream from the Columbia 

River Gorge (Trimble, 1963, p. 59-68). Conversely, in 

canyon constrictions downstream from basins, the size 

of boulders again increases abruptly. 

In brief, few boulders were washed through the 

basins, and new bouldery debris was picked up by 

the flood at each canyon constriction. These discontinuities imply that the boulders were all derived locally 

and that they did not survive more than a few miles of 
DIAGNOSTIC DEPOSITIONAL AND EROSIONAL FEtATURES 15 

FIGURE 5.-View upstream of part of the terminus of 'a gravel bar in Melon Valley, opposite mile 161, showing complex dips 

of massive layers of bouldery and sandy flood debris. The exposure is 80 feet high. 

transport. This inference is confirmed by lithologic 

identification of boulders at many places along the 

canyon. A few examples will illustrate the point. 

The Snake River now flows over cataracts of silicic 

volcanic rocks near Twin Falls (mile 185-179) and must 

have done so at the time of the flood, yet identifiable 

pieces of these volcanics are extremely rare in Melon 

Gravel downstream, even short distances below their 

outcrops. At Melon Valley (mile 161), for instance, 

fragments of silicic volcanics constitute less than 1 percent of the gravel, and none larger than small pebbles 

can be found. Apparently, the silicic volcanics were 

rapidly abraded and pulverized by the flood, although 

in other stream gravel along the canyon they are one 

of the most resistant materials. 

The largest boulders in Melon Valley (diameters 

about 4 ft) consist of Thousand Springs Basalt, a distinctive unit of fresh lava that extends a few miles upstream (mile 172) along the northern canyon rim. 

Debris from weathered rocks (basalt of Glenns Ferry 

Formation and the Banbury Basalt), which crop out 

under the rimrock and which extend still farther upstream, survives as boulders of smaller size. 

Between Melon Valley and mile 155, outcrops of 

weathered lava flows (Banbury Basalt) decrease in importance, and the area of Thousand Springs Basalt 

increases correspondingly. A heap of boulders at mile 

155 consists mainly of pieces from the Thousand 

Springs. Apparently the weathered lava could not survive the few miles of transport. Still farther downstream (mile 150), below other outcrops of Banbury, 

boulders of weathered basalt again assume importance 

in the gravel. 

Locally derived basalt boulders also occur between 

Bliss and King Hill. In this reach, as far downstream 

as mile 130, the river flows in a narrow gorge cut into 

Banbury Basalt; yet boulders at mile 129 include no 

large pieces of Banbury. The boulders consist mostly 

of fresh basalt from rimrock that extends upstream to 

mile 139 (only along the north rim). The same rimrock 

supplied practically all the boulders preserved in a great 

pile of Melon Gravel in the King Hill basin. Cobbles 
,r- ·' 

---------------

~~ 

FIGURE 6.-Ballast pit of Union Pacific Railroad in downstream end of sandbar of Melon Gravel at mile 109. The cut extends 1,500 feet from left to right 

(north to south), is perpendicular to the canyon, and is 125 feet high. It exposes two sets of beds: one set with an apparent north dip of 12° is truncated 

by another set with an apparent south dip of 5°. (Sketched from a photograph.) 

): ,.( -~. -~ ·' 

1-' 

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~ > Ul 

8 

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0 

1-d 

~ 0 

~ t'j 

1-4 

Ul 

8 

0 

(") 

t'j 

z 

t'j 

ttl 

0 

~ < 1-4 

~ 

~ 

t'j 

~ 

~ 

0 

0 

~t::l 

Ul 

~ ~ t'j 

lJ:! 

1-4 

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DIAGNOSTIC D.EPOSITIONAL AND EROSIONAL FEtATURES 17 

FIGURE 7.-Festoon c ross.beds in dipping layers of sand exposed 

in Union Pa~ific Railroad pit at mile 109. 

of this basalt are identifiable in Melon Gravel as far 

downstream as Glenns Ferry (mile 107). 

The Melon Gravel near Crane Falls, from mile 74 to 

64, is entirely from local rocks. The boulders consist of 

a distinctive brownish-weathered lava (basalt of Bruneau Formation) that forms the canyon at th'is place, 

and, because a basin devoid of gravel extends some distance upstream, the finer constituents in flood debris at 

Crane Falls are also of local origin. The Melon Gravel 

at Walters similarly was derived solely from a constricted segment of canyon just upstream, mile 14-30. 

PHYSIOGRAPHY 

The Melon Gravel forms gigantic bars and enormous 

heaps of bouldery detritus at places along the Snake 

River canyon where the flood velocity was retarded, 

especially at the upper ends of local basins. Although 

FIGURE 8.-Boulder train on surface of Melon Gravell()() feet above Snake River 3 miles downstream from Bliss Dam, 

Idaho. Boulders in the foreground are from 3 to 5 feet in diameter, and some in the distance are as large a:; 10 

feet. Exposed surfaces of the boulders have been polished by the action of windblown sand. Weathered rinds 

about half an inch thick on some boulders are now exfoliating. 
18 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

the canyon at the time of the flood had its present depth 

and its major outlines, such deposits made numerous 

physiographic changes. The gravel filled the canyon 

in pl·aces to a depth of 300 feet, greatly reducing the 

former width and blocking the 1nouths of some tributary valleys. Furthermore, huge bars built in midcanyon 

determined the trend of deep marginal channels perched 

between the bars and the canyon walls. 

The bars of Melon Gravel resemble grossly magnified 

versions of ordinary bars found along braided streams. 

They have a streamlined form, which is tapered at both 

ends, and they trend downstream. In places, two or more 

gigantic bars lie close together, although not necessarily 

at equal height, and demonstrate by their parallel trends 

that they were built simultaneously. The bars generally 

lie along the axis of the canyon, where they stand isolated from the walls, but a few are along the inner 

sides of canyon bends. Some bars are tied to knobs of 

resistant bedrock that rise from the canyon floor, and 

others hang loosely from buttresses that projeot from 

canyon walls. 

The bars are of impressive size. A typical bar is 1-

1112 miles long and about half a mile broad, measured 

with respect to the lowest closed contour. The highest 

place along a bar, commonly from 50 to 150 feet above 

surrounding gravel, generally is near the middle. The 

fore and aft slopes on bars range from 1 :100 (Melon 

Valley, mile 160) to about 5:100 (Weatherby Springs, 

mile 71), and the side slo,pes are considerably steeper. 

The streamlined shapes of .the bars-looking like 

magnified channel deposits of modern streams-suggest 

that they have been preserved in virtually original 

form; that is, they are constructional shapes left during subsidence of the Bonneville Flood and have been 

scarcely modified by later erosion. Because of their 

streamlined form, discontinuous distribution, and lack 

of pairing with companion surfaces, the bars have no 

resemblance to terraces built by aggrading rivers; they 

surely represent bedload material deposited in deep 

water. The highest bars demonstrate that floodwater was 

at least 300 feet deep in some places (table 1), but even 

these bars indicate only a minimum height of floodwater at peak discharge. Like the lower bars, they have 

surfaces strewn with boulders that suggest winnowing 

during retarded flow as floodwater subsided. 

Outlets of some sm·all tributary valleys, obstructed 

by Melon Gravel, also demonstrate the height of the 

flood. One of the obstructed tributaries is spring-'fed I 

Billingsley Creek, which joins the Sn'ake River at mile 

142 after passing through a narrow orifice 200 feet 

higher between a bar of Melon Gravel and the canyon 

wall. 'V'ithin the obstructed part, Billingsley Creek 

pursues a winding meandering course in a swampy 

alluvial plain (fig. 19). Jolley Flat, a nearly level 

alluvial plain 200 feet above the Snake River 3lf2 miles 

southeast of I\:ing Hill, is another tributary valley 

blocked by Melon Gravel. Dumping of gravel at the 

valley 1nouth caused the spread of alluvium (fig. 20). 

MARGINAL CHANNELS 

The path of the Bonneville Flood from Melon Valley 

downstream is marked by numerous abandoned channels next to the canyon walls. These marginal channels 

are a direct consequence of gravel dtunped in midstream, which detoured floodwater into troughs beside 

gravel bars. Most channels are bounded by bars of 

Melon Gravel on one side and by canyon walls on the 

other, although a few abandoned channels lie wholly 

within gravel. Some channels are several miles long and 

as much as 150 feet deep rwith respect to adjacent bars 

o£ gravel. The principal cha1mels are shown in figure 

2, and their major dimensions are listed in table 1. 

Topographic details of several channels are shown in 

figures 16, 19, 20, 22, and 23. 

The mrurginal channels are perched high above the 

Snake River and do nat now carry water, although small 

ponds fed by springs and irrigation water occur in 

irregular depressions 25-50 feet deep along channel 

floors. Beside these channels the Snake River follows 

a deep trough that ·probably existed during the flooda physiographic relation that Qmplies gross discontinuities along the canyon floor. Even these irregularities 

were deeply submerged by the flood. The threshold of 

a marginal channel at Pasadena Valley, for instance, 

170 feet above the canyon floor, was covered with water 

at least 120 feet deep at peak discharge, as shown by 

the height of an adjoining gravel bar. 

IMPOUNDED WATER 

If the highest rise of floodwater is estimated from 

the height of Melon Gravel and from a few unmistakable erosion features along the canyon, the resultant profile shows that floodwater was partly impounded rut 

canyon constrictions and that basins upstream held 

temporrury lakes (figs. 2, 3). That is, the constrictions 

acted briefly as hydraulic dams. The principal lakes 

and their effect on deposition of flood debris are described in a lruter section of this report, but a brief 

sumn1·ary of their geography is pertinent to the discussion that follows. Below the canyon constriction near 

Swan Falls (mile 26), the canyon progressively widens, 

and no lakes were formed . .A:bove Swan Falls, however, 

extending 35 miles to the mouth of the Brm1eau River 

and thence into the lower Bruneau valley, there was 

a large lake. Still farther upstream, above a constriction at mile 75, a series of lakes extended past I\:ing 

)..· 
... 

DIAGNOSTIC DEPOSITIONAL AND EROSIONAL FEIATURES 19 

TABLE !.-Topographic relations of marginal channels 

Length 

Altitude dif- Minimum height Maximum height ference between of channel floor of gravel bar Location of Tog_ographic Locality of channel floor above present above present measurement qua angle map channel and adjoining Snake River Snake River that shows 

(miles) gravel bar (feet) (feet) Sec. T. R. channel' 

(feet) 

Melon Valley ____________ 2. 6 80 80 

Salmon Falls Creek ______ 2. 7 120 20 

!-!a german Valley. ________ } 4. o{ 35 140 

Hagerman towns1~e- _____ 115 100 

Pasadena Valley _________ } 5. 7{ 150 140 Frog Hollow ____________ 130 125 Jo.lley ~lat ______________ } 6. 5{ 110 145 Kmg H1ll _______________ 100 160 

Glenns Ferry ____________ 3. 0 50 40 Indian Cove ____________ 3. 0 70 25 

Op0osite Rattlesnake 1.6 50 235 

reek.2 

"Bruneau cutoff" ________ 7. 0 4 30 5 270 

Weatherby Springs _______ 1.7 140 100 C. J. Strike Dam ________ 2. 0 75 60 Walters Ferry ___________ 4. 5 80 130 

' All quadrangles arc 7~ min, except whore otherwise designated. 2 Ufsland channel. a H ghost roach of eroded bedrock knobs. 

Hill ·to mile 121, each successive lake being at slightly 

higher level because of connecting canyon necks. The 

basin n,t IGng Hill held a considerable body of water 

n,nd trapped most .of the debris flushed from the steep 

canyon upstream. This accounts for a paucity of Melon 

Gravel in the lower basins. The comparatively low 

velocities at constrictions connecting the chain of lakes 

between IGng Hill and mile 75 may explain an apparent lack of erosion at these narrow places. Floodwa'ter, however, moved more or less rapidly through 

each of these lakes, and they must be regarded as dynamic features along the flood path. 

Fine-textured surficial deposits preserved in backwater areas suggest that water impounded in the 

lakes was roily with suspended sediment, which settled 

out where currents were weak. (Suspended material 

near the center of a basin probably was flushed downstream by faster currents.) Such backwruter deposits are 

sparse ttnd isolated, and their identification must be 

hedged with doubts. During detailed mapping near 

IGng Hill, however, which was one of the most likely 

places for entrapment of suspended load, three outcrops were locruted that may represent backwater deposits left by the Bonneville Flood. 

One possible backwater deposit rests on terrace gravel 

(Crowsnest Gravel) in the NvVJ~ sec. 9, about 2 miles 

east of IGng Hill. (See fig. 20.) The deposit is 20 feet 

thick, and the top is at an altitude of 2,760 feet, the 

same ns the height of flat-topped remnants of Melon 

Gravel in the center of the basin. This level probably 

was determined by a temporary stillstand controlled by 

a canyon constriction at mile 100. The deposit is a 

160 11-12 9 s. 14 E. Thousand Springs. 

140 30 8 s. 14 E. Do. 

175 25, 26, 35, 36 7 s. 13 E. Hagerman. 

215 14 7 s. 13 E. Do. 

290 31, 32 5 s. 11 E. Pasadena Valley. 

255 19 5 s. 11 E. Do. 

255 16, 21 5 s. 11 E. Do. 

260 6 5 s. 11 E. King Hill (15 min). 

90 31 5 s. 10 E. Glenns Ferry. 

95 1, 2 6 s. 7 E. Indian Cove. 3 285 25, 26 5 s. 5 E. Bruneau. 

4 260 36 5 s. 4 E. Bruneau (15 min). 

31-36 5 s. 5 E. 

240 16, 21 5 s. 5 E. Bruneau. 

135 28, 33 5 s. 4 E. Do. 

210 15, 22 1 s. 2W. Walters Butte. 

' Height of boulder gravel at downstream end of channel. ~ Ranges from 2,711 ft alt at upper end to 2,600 ft at lower end. 

massive ·mixture of silt, some clay, and granitic sand 

such as is found in nearby bedrock (Glenns Ferry Formation). The deposit is probably not colluvium because 

it lies on a flat terrace half a mile from bedrock slopes. 

It is plausibly accounted for as a backwater deposit, 

perhaps mixed with material supplied by Clover Creek, 

a tributary that joins the Snake River at this place. 

A second possible backwater deposit is near the center of sec. 26, about 2112 1niles west of Glenns Ferry in 

the shelter of a basalt-capped spur along the canyon 

wall. The deposit rests on Crowsnest Gravel and on 

a sl~ping wall cut into the Glenns Ferry Formation. It 

rises on the slope to an altitude of from 2,750 to 2,775 

feet, about the sa1ne as that of the deposit already mentioned. This second deposit is as thick as 50 feet and 

consists of inclined beds, concordant with the slope and 

composed mainly of silt, but including some pebbles -and 

fragments of basalt that crop out on the slope above. 

The dominance of fine-grained material, the bedding, 

and the a:brupt termination upslope suggest that this is 

a backwater deposit containing debris that lay on the 

slope at the time of the flood. 

A third possible area of backwater deposits is on the 

opposite side of the valley, 21h miles southwest of Glenns 

Ferry. The deposits are exposed in several shallow gullies graded to a terrace of Crowsnest Gravel. They are 

about 25 feet thick and pinch out rapidly upslope at 

an altitude commensurate with the highest stand of 

floodwater, a little below 2,800 feet. These deposits are 

dominated by massive light-colored fine-grained material of the kind that forms the canyon in this area, but 

they probably do not represent colluvium. Near by slopes 
20 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

in analogous topographic situations, but above the flood 

height, have no such deposits. Thus these fine-grained 

materials could be some of the suspended load transported by the 'flood. 

Backwater deposits have not been found in the basins 

dpwnstream, but they would be difficult to identify in 

these places owing to extensive badlands. 

SCABLAND 

The term "scabland" was introduced by Bretz in 1923 

(see review by Bretz and others, 1956; Bretz, 1959) for 

chaotically eroded basalt areas in eastern Washington 

from which a thick mantle of loessial soil was swept by 

passage of glacial floods. The term has since become 

familiar in the vocabulary of geology. Bretz emphasized 

such incredible erosional features of scabland as colossal anastomosing channels, gigantic coulees, dry falls, 

and empty rock basins, which together make a weird 

landscape unlike any other then known on earth. Features of this kind are also found at places along the 

path of the Bonneville Flood through the Snake River 

Plain, particularly at Twin Falls, although at reduced 

scale. 

At most places in the Snake River Plain, the most obvious sign of scabland erosion is a lack of surficial sediment that is still preserved on the higher parts of a 

lava flow. The extent of the Bonneville Flood on uplands can be judged from the geographic distribution 

of such barren tracts. Moreover, even where major scabland features are lacking, surfaces of lava flows washed 

by the flood display numerous minor erosional feaJtures 

that indicate a former forceful flow of water. Smoothed, 

polished, and fluted surfaces have formed by corrasion 

of rushing sand (fig. 9), and the upper parts of lava 

flows have been stripped and washed Mvay, presumably 

by the plucking of lava blocks. Close inspection of 

scoured lava surfaces shows blunted edges on exposed 

vesicles and enlargement of pore spaces ordinarily 

crowded with fragile crystals. Removal of blocks from 

lava flows is indicated by absence of vesicular upper 

layers with ropy skins and by the consequent exposure 

of columnar lava that cooled deeper within flows. Comparison with uneroded lava, or with nearby remnants 

in the scabland, allows an estimate of the depth of 

stripping-commonly several feet. These processes of 

corrasion rmd stripping formed an intricately dissected 

lava surface of branching channels, dry falls, and rock 

basins, which are small-scale replicas of ·the gigantic 

features emblematic of scabland. 

Large erosion features of scabland 'that imitate the 

weird forms made famous by Bretz will be described in 

appropriate places in the discussion that follows. 

A. 

B 

FIGURE 9.-Minor erosion features in 'basalt scabland formed by 

the Bonneville Flood. 

A.. Upstream end of small sluiceway on basalt 4 miles southeast of King Hill ·and 175 feet a'bove present Snake River. 

Corrasion has formed rounded polished surfaces marked by 

minute fluting that indicates the direction of turbulent flow. 

Sections through exposed vesicles have ])lunted edges, and 

pore spaces ordinarily occupied by fragile crystals have 

been gutted and enlarged. Blocks of lava that formerly filled 

the sluiceway were plucked out and washed downstream. 

B. Uneroded part of the same basalt flow as shown in A. but 

2,000 feet southeast and 75 feet higher. This outcrot) was 

covered by the flood 'but lay in a '])ackwater protected from 

ra·pidly moving debris. Accordingly, the 'ba alt exhibits no 

corrasion or plucking. The scant soil cover is characteristic 

of this laya flow-but not generally of others in the path of 

the flood-and does not indicate scabland. The accumulation 

of soil dates from a time before the flood. 
PATH OF FLOOD ABOVE TWIN FALLS 21 

PATH OF FLOOD ABOVE TWIN FALLS 

UPPER SNAKE RIVER P LAIN 

The route of the Bonneville Flood to the Snake River 

Plain and westward along the Snake River was largely 

controlled by topography that exists today (fig. 1). 

From Red Rock Pass, floodwater flo,Yed no11th down the 

valley of Marsh Creek into the lower Portneuf River 

canyon ai,ld onward to the Snake River Plain at Pocatello. Here, at the canyon mouth, the flood met. slack 

\Vater of a temporary lake that was held by a resistant 

lava barrier-1the Cedar Butte Basalt at an altitude of 

4,450 feet near American Falls, 30 miles downstreamand bouldery debris 50-80 feet thick known as Michaud 

Gravel was dumped on the lake bottom. The largest 

pieces that were carried into the lake range from 8-foot 

boulders at Pocatello to cobbles at a distance of some 10 

miles (fig. 10). Discharge from the lake was by •two principal routes over and around the lava barrier, so that 

channels and cataracts were cut on the uplands and along 

the canyon wall of the Snake River (Trimble and Carr, 

1961b, fig. 2) . The largest of these was Lake Channel, 

an upland spi:llway that rejoined floodwater aJong the 

Snake River at an altitude of 4,280 feet, 10 miles below 

the lava barrier (fig. 10) . Gravel bars containing 20-

foot boulders of local basalt were built across the 

mouths of some spillways. 

In the next 30 miles downstream, the flood followed 

a broad valley carved out by the Snake River and left 

no obvious sign of its passage, before spreading widely 

over a flat alluvial plain that surrounds Rupert. This 

plain is only a few feet above present river level and 

must have been inundated throughout most of the Bonneville Flood, although tangible effects of the voluEXPLANATION 

B 

Alluvium, talus, 

and dune sand 

Terrace deposits 

~ 

Micha

~ 

ud Gravel 

~ IQaml ]: 

-' American Falls Lake Beds 

At; mapped include.<.: pal't of 

Rfl.ft F'ol'mation fW11t h teest 

of Ame,·ican Folts 

Contact, approximately located 

~ "' 

.2 Loess and a lluvium 

·;:: "' 

a:: j• Qv j 

" Cedar Butte Basalt 

!~ "' Basalt of Snake >- " a:: "' <( ~ z ~ 

a:: "" w 

I-

<( 

Ri ver Group 

Raft Formation 

D :J 

0 Unmapped areas and rocks 

mostly pre-Raft Formation ---3---

Contour, in fee t, of maximum boulder sizes 42'30'~L__Lu_ __ ~-L--------~~~----------~-ll~~------------------------------------------~ 

0 5 10 15 MILES 

FIGURE 10.-Geologic sketch map of American Falls area (slightly modified from Trimble and Carr, 1961b, pl. 1). Contours of 

boulder sizes at Pocatello added from Trimble and Carr (1961a, fig. 69.2). For area of map, see figure 1. 
22 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

minous discharge ·are hard to find. Between Rupert and 

the present Snake River, a subdued relief expressed 'by 

discontinuous west-trending swales 5-10 feet deep, several mHes long, ·and as much as a mile wide presmnahly 

was produced by passage of the flood. Stearns (1962) 

suggested that fossil mammals in gravel 6 miles northeast of Rupert may have been drowned by the flood, but 

the gravel has not been shown to be a deposit of the 

flood. . 

A spillway 13 miles west of Rupert at an altitude of 

about 4,200 feet, which proba:bly was covered with water 

at least 30 feet deep (as indicated by the height of 

erosional features), suggests that floodwater in the 

Rupert basin rose to an altitude of about 4,230 feet 

and flooded an area of 300 square miles to a depth of 

as great as 80 feet. The average depth of impounded 

floodwater would have been about 50 feet. Tranquil flow 

through such a broad lake could be the explanation for 

the lack of easily recognized flood features in the Rupert 

basin. 

RUPERT CHANNEL 

The spillway west of Rupert is a broad inconspicuous saddle in basaltic lava and lies at the head of a 

wide upland channel 30 miles long that rejoins the 

Snake River near Twin Falls. The channel is delimited 

by an eroded tract of basalt scrttbbed clean of soil ( sca)bland) and scoured locally into small rock basins and 

dryfalls. Scattered bars of boulder gravel occur along 

the route (inset map in fig. 1). This arm of the Bonneville Flood is here termed the Rupert channel. 

The slope and depth of the Rupert channel can be 

roughly estimated from physiographic relations. At the 

nearest junction of the channel with the Snake River, 22 

miles distant, the altitude of the canyon rim is 3,800 

feet-400 feet below the head. This difference in altitude indicates an average descent of 18 feet per mile. 

The maximum depth of floodwater in the channel is determined by the height of unspoiled divides along a 

rolling basalt plain between the chmmel and the Snake 

River. The lowest pass (4,140 ft alt), a swale northwest of Milner that now carries ditch water, is about 

70 feet above the nearby channel floor, but it seems 

unlikely that water in the Rupert channel was ever this. 

deep. If so, discharge computed from the Manning 

equation (see table 2, p. 25), assuming a minimum 

width of 10,000 feet and a roughness coefficient of 0.05 

(for representative values of the roughness coefficient, 

see Chow, 1959, p. 104-123, and Barnes, 1967), would 

have exceeded the canyon capacity at Swan Falls. 

Furthermore, part of the Bonneville Flood in this reach 

~vas carried thy the Snake River. The probruble depth of 

water in the channel may be estimated better by tracing 

the flood profile from unmistakable signs of catastrophic 

erosion that apparently mark the highest rise of floodwater. In this way, for the relatively narrow upper part 

of the channel, floodwater appears to have been locally 

at least 50 feet deep, but the average depth must have 

been considerably less-perhaps less than 20 feet. The 

true depth at peak discharge cannot be m~asured accurately, even from detailed knowledge of the flood 

profile, because of indeterminate erosion that undoubtedly took place during subsidence of floodwater. 

Topography within the Rupert channel, as typifies 

scabland, is extraordinarily irregular. The maximum 

local relief is at a group of dry falls and giant potholes 

between Eden and "\Vilson Lake Reservoir, which range 

in depth from 50 to 120 feet (fig. 11); but at many 

other places along the channel, a'brupt changes in altitude amount to several tens of feet. In addition, innumerruble lesser knobs and pits are displayed on topographic m·aps by a random array of deviously shaped 

closed contours crowded together. Most of this chaotic 

relief resulted from flood erosion of basalt, which 

plucked blocks here and there and unevenly stripped 

sheets of lava. Some broad relief features, however, 

appearing as ridges and swales several hundred feet 

long, may represent topography constructed by lava 

and then modified by the flood. One rather continuous 

swale forms a trough several miles long that extends 

westward along the southern limb of the channel from 

Wilson Lake Reservoir. The dry falls and potholes already mentioned are at the head of this trough. The 

trough apparently marks a route where much of the 

floodwater was concentrated-a circumstance borne out 

by some hirge bars of bouldery Melon Gravel found 

here. Scattered boulders also occur near the southern 

edge of the Rupert channel as far as 5 1niles east of 

Wilson Lake Reservoir. 

The boulders lithologically match basalt that underlies 

the channel and evidently have traveled no more than 

a few miles. 'They assume a progressively more rounded 

and better sorted appearance in successive deposits 

downstream, although these changes are not accompanied by a systematic decrease in particle size. Blocky 

subangular chunks of basalt are common a few miles 

above Wilson Lake Reservoir, but nearly aU the pieces 

farther downstream are well rounded or spherical. Such 

a high degree of rounding close to 'the source can be attributed to corrasion by abrasive detritus that was 

picked up by floodwater in the Rupert channel. 'Dhe 

boulders are mostly less than 5 feet in diameter, but an 

impressive collection of giant boulders along 'the highway 3 miles west of Eden includes rounded stones tlutt 

reach 10 feet. The maximum size of the boulders may 

have been determined not so much by the transport 

power of the flood ·rus by the spac~ng of cracks in the 

·~ 
PATH OF FLOOD ABOVE T~LN FALLS 23 

114°12'30" 114°10' 42°~~-=~~~~~~==~~~---=~--~~~~~----~-------.~~n.~--~------------------, 

37' 

30" 

0 

R. 19 E. _R. 20 E. 

1 MILE 

FIGURE 11.-Dry falls and giant potholes near Eden, Idaho, along south margin of Rupert channel. Typical scabland topography lies north of the railroad, but water of the Bonneville Flood covered the present site of Eden (3,960 ft alt) and also 

covered Melon Gravel at the 4,020-foot altitude between the potholes and ~ilson Lake Reservoir. Thus the local flood 

gradient was about 24 feet per mile, and floodwater in the area of the pot.holes was about 30 feet deep. (From U.S. Geol. 

Survey topographic map of Eden quadrangle, Id'aho, scale 1 : 24,000.) 

basalt. The lack of sorting of boulders suggests that 

basalt pieces were picked up locally at various places 

along the channel. 

MILNER REACH 

Frmn Milner (1nile 207) to the junction with the 

Rupert channel (mile 186), the Snake River ·follows a 

narrow basal1t canyon with relatively smooth, straight 

walls. Floodwruter was released from the Rupert basin 

via this canyon, as well as by simultaneous spill into the 

Rupert channel. This canyon segment~here ca;lled the 

Milner reach-appears to have formed initially by recession of falls along a course determined iby the south 

n1argin of basalt of the Snake River Group (see map by 

l\falde and others, 1963) and was then 1nodified by the 

Bonneville Flood. Stearns ( 1962) , however, inferred 

·that this narrow canyon was cut later than the flood by 

seasonal g·lacial meltwater augmented by occasional 

overflow fr01n Lake Bonneville. Although the actual 

sequence of events cannot be determined from stratigraphic evidence, the history of the Milner reach can 

be appraised from physiography and from the capacity 

of the reach to transpol'lt flood water, as are discussed 

below. I conclude that this:gorge largely antedates the 

Bonneville Flood but that it was notice.:'tbly enlarged by 

flood erosion, particularly at the downstream end. Erosion since the flood appears -to have been comparatively 

mmor. 

Floodwater was delivered to the Milner reach from 

·the Rupert basin through a gap less than a mile wide 

about 4 miles upstream. From tJhis gap the flood descended rapidly to an altitude of 4,160 feet at Milner 

and formed a belt of scabland 11;2 miles broad at ·the 

canyon head. 

In the stretch downstream to Murtaugh (mile 198) 

the canyon is rather narrow (about 600 ft) and comparatively shallow (200 ft or less). This segment is 

bordered by continuous scabland, altogether a 1nile 

across, the existence of which indicates rthat the carrying 

capacity of the canyon was at one time exceeded by the 

flood. Boulders of flood debris below rapids in the canyon at Murtaugh fonn a pHe 100 feet thick. At this 

steep place (gradient 35 ft per 1nile), the Snake River 

has cut •a narr:ow trough 50 feet below the base of the 

gravel; this depth gives a measure of erosion since the 

flood. 

In the next 12 miles below Murtaugh-that is, downstream to the junction with the Rupert channel-the 
24 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

canyon is remarkably uniform in width and depth, 

about 1,100 by 325 feet, with smooth vertical walls and 

a boxlike cross section. The canyon floor descends 

steadily at a gradient of {!!bout 24 feet per mile. The canyon wails are nearly devoid of talus, and practically no 

detr.itus occurs along the floor (fig. 12); apparently the 

canyon was flushed clean by passage of the Bonneville 

Flood. Because the floor is wider and less steep than at 

Murtaugh (where downcutting 'below flood debris 

amounts to only 50 ft), erosion along ·t'his segment since 

the flood must have been slow. Tha.Jt is, the present aspoot 

of the canyon below Murtaugh is largely a relic of the 

Bonneville Flood. But is this segment of canyon also a 

relic of a still earlier time? The distribution of scabland 

along the canyon rim provides an answer. Sca!bland 

a:long this part of the Milner reach occurs only from 

Hansen Bridge (mile 189) downstream, and its general 

ttbsence from Murtaugh to Hansen Bridge demonstra.Jtes 

that most of the canyon below Murtaugh was capable 

of carrying all floodwater delivered from the Rupert 

basin. This canyon segment is therefore more ancient 

than the flood. Only the lower part of the canyon was 

overtopped by floodwater, and at mile 181 rounded 

basalt boulders as large as 6 feet in diameter form a low 

bar on the south rim, 350 feet above the Snake River 

(fig. 14). The rather peculiar circumstance of scabland 

along the lo·wer part of the Milner reach is discussed 

below. 

The degree to which the Milner reach may have been 

enlarged by the Bonneville Flood can be surmised from 

the carrying capacity at Hansen Bridge (mile 189, fig. 

13 section J-J'), a narrow place where floodwater overflowed the canyon rim. The Manning equation indicates 

that the present capacity at brimful stage through this 

constriction would be 19 million cfs (table 2). Thecapacity below Hansen Bridge, where the canyon is wider 

(fig. 14), would be correspondingly greater. Because 

a proba:ble discharge of 15 million cfs is computed 

at Swan Falls (p. 12) and because a large amount of 

floodwater was carried simultaneously by the Rupert 

FIGURE 12.-View of Snake River canyon upstream from Hansen Bridge (mile 189), showing the near absence of talus 

and a lack of stream debris along the canyon floor. At this place the canyon is 320 feet deep and only 600 feet wide. 

A thin layer of basalt was stripped from the rim on both sides of the canyon, and this left scabland areas which 

indicate that the canyon was at one time brimful. 
EROSION NEAR TWIN FALLS 25 

TABLE 2.-p~scharge capacity of Snake River canyon a selected 

constnctwns, as computed from the Manning equation: 

Q= 1.49 AR213Sll2 

n 

[Q, discharge (cubic foot por second) at maximum high water, assuming roughness cooficlont (n) of 0.03; A, nroa of cross section of water at highest flood level and at present canyon depth and width; R, hydraulic radius (nrea divided by wetted 

perimeter); S, slope ratio; for example, 12 ft per milo equals a slope ratio of 0.00227] 

Section A R s Q 

A-A' (Swan Falls)_________ 405,000215------- t15X106 

B-B' (Crane Falls)_________ 198,000149 0. 00246 213. 7X106 

C-C' (Barber Cabin) ______ ._ 456,000 215 3. 00227 38. 8X.106 

D-D' (Indian Cove)________ 676,000 242 3. 00227 62. 2X106 

E-E' (The Narrows)_______ 651, 000 240 3. 00227 59. 4 '< 106 

F-F' (Thousands Springs) { 411, 600 191 . 00227 32. 2X 106 --- 29 400 49 . 00227 .. 9X.106 

H-H' (EllisonsSprings) _____ 487:000268 . 0038 61. 5X106 

I-I' (Blue Lakes) __________ 1, 346, 000 378 . 0038 214X 106 

J-J' (Hansen Bridge)______ 162, 000 156 . 0062 19. 1 X 106 

1 Critical-depth calculation. (Soo p. 12.) 2 Section overtopped by flood. a Assumed. 

channel, the discharge at Hansen Bridge must have 

been mqch less than 19 1nillion cfs, notwithstanding 

possible downstremn attenuation of the flood. It 'follows 

that the canyon during peak discharge, when scabland 

formed along the rim, must have been smaller than now 

and that the present augmented size resulted from flood 

erosion. If we assume that the Milner reach carried half 

the total discharge (the other half being carried by the 

Rupert channel), the canyon at Hansen Bridge could 

have been doubled in size by passage of the flood. The 

part below I-Iansen Bridge, which is twice as wide but 

which was nonetheless overtopped by floodwater, may 

have increased in size four times. 

The foregoing argument depends on the significance 

ascribed to scabland along the canyon rim and ignores 

the possible effect of the confluence of the Rupert channel 3 miles downstream from Hansen Bridge. As will 

be explained shortly, the flood at this junction rose above 

the canyon. Although the flood height at the junction 

was 120 feet below scabland at Hansen Bridge (a flood 

gradient of 40 feet per mile), it may have created some 

backwater in the canyon. If so, discharge calculated 

from the Manning equation would be misleading, because of impaired flow, and conclusions about canyon 

dilnensions inferred from discharge would be ina.ccurate. These gigantic features of the flood overwhelm the 

· magination but it seems m1likely that backwater in a 

gradient of 40 feet per mile would have greatly reduced 

the flood velocity, and I therefore believe that considerable erosion took place in the downstream end of the 

~1:ilner rench. In fact, the calculated discharge at Hansen Bridge, and hence the inferred increase in canyon 

size by flood erosion, is hased on a flood gradient of 

only 33 feet l)er mile, which is the local gradient of the 

cnnyon floor. Nonetheless, backwater is the only plansible explanation for the rise of floodwater above the 

canyon rim downstream from Hansen Bridge and for 

the consequent erosion of marginal scabland. Forceful 

flow along this segment, despite the effect of backwater, 

is demonstrated by the 6-foot boulders already mentioned on the canyon rim at 1nile 186. 

EROSION NEAR TWIN FALLS 

The most impressive erosion features produced by 

the Bonneville Flood are along the Snake River canyon near Twin Falls (mile 186-175), where the Rupert 

channel joins the Milner reach (fig. 14). This stretch 

of canyon is nearly 500 feet deep -and ranges fron1 1,300 

feet to a 1nile in width (fig. 13, section /-/'). This exceeds the size of the canyon immediately above and below. The north rim, where floodwater eritered from the 

Rupert channel, is embayed by several large alcoves 

(dry falls), and the soutJh rim from mile 186 to 180, 

where water spilled from the Milner reach, displays a 

ragged edge. The effect of spillover along the north 

and south rims apparently was to broaden the canyon 

to a width considerably more than its original size. 

These erosional features indicate that this stretch of 

canyon was virtually brilnful at peak discharge. The 

inferred flood profile (fig. 3) would have been below 

the canyon rim only from mile 180 downstream, where 

scabland is absent along the south side. But even here, 

projection of the profile suggests that floodwater was 

not far below the canyon edge. If this inference is correct, erosion along the north and south rims was not 

accomplished by water spilling as cascad~s in the ordinary sense, but was done by extraordinary turbulence 

in rapidly moving deep water. (See discussion of "kolkaction" in Bretz and others, 1956, p. 1028-1029.) This 

conclusion has significance for understanding other 

erosional features 'along the canyon floor. 

Through this broad canyon the Snake River descends 

over massive and resistant outcrops of silicic volcanic 

rocks (mile 185-179), which make up the lower story 

of the canyon. These outcrops form cataracts at Pillar 

Falls and Shoshone Falls and represent impediments 

to canyon entrenchment. Pillar Falls, although having 

a lip only 20 feet high, is surrounded by crags of 

scabland that reach 175 feet rubove the river. Shoshone 

Falls, 210 feet high, is bordered by scaJbland that extends 80 feet higher. The Twin Falls plunge 150 feet 

from basalt half a mile upstream from another outcrop 

of silicic volcanics. Between 1nile 180 and 179 an outcrop of silicic volcanics forms a jagged mass of scabLand that rises 200 feet above the ca.nyon floor (fig. 15). 

These gigantic erosional features along the canyon 

floor probably represent mainly the effect of turbulence 

in phenomenally deep water, although they must have 
W E 

3000'f\ u 11A' ~ S 

28oo· ·.·_. .- .. ·._. j 3000' E 

2625 _L 

2600' l ~ 7.. ! 2800' 

2800 

R~stored profil~>-------~ 

N 

E' 

2400' . ~- /·"'- 2600' . I _; 

..... = ------- v ~ 

2200' 2400' 2 MILES UPSTREAM FROM SWAN FALLS 

AREA 405,000 SQ FT 

W E 

B B' 

2800't: 27~ ::J 2600' " II 

2400' ----

2 MILES UPSTREAM FROM CRANE FALLS 

AREA 198,000 SQ FT 

S N 

C C' 

30oo·r 

::::~ '=--:: ____ / 1 ~ 2400' . --

2 MILES UPSTREAM FROM BARBER CABIN 

AREA 456,000 SQ FT 

S N 

D D' 

~~~}s ...... _._,., ... _~---,~~----:::71 

//~~It~ II:;: 

INDIAN COVE BRIDGE 

AREA 676,000 SQ FT 

EXPLANATION 

~~---:a:-oo ... r: ............... ~: ... ·,,~ 

. ~-----~7'····· . 

THE NARROWS 

W AREA 651,000 SQ FT E 

F r ::::·. r 3120 ~ 2800' 

3000' ~ · .. ·... •. 

. ~ .• 0 

~-.. • 

3600' 

3400' 

3200' 

3000' 

s 

H 

0.6 MILE UPSTREAM FROM THOUSAND SPRINGS 

AREA 441,000 SQ FT 

3450 

~ 

=II"' 

N 

H' 

S N 

J J' 

4000'ts:z1_ 3925 

I ~ 

3800' • •• 

- "' ! 

3600' ---

1.4 MILES DOWNSTREAM FROM ELLISONS SPRINGS HANSEN BRIDGE 

. AREA 487,000 SQ FT AREA 162,000 SQ FT 

3800' 

3400' 

s 

I 

3625 

32oo· I ~-~ · -9 ij 

2800 

0.7 MILE DOWNSTREAM FROM BLUE LAKES 

AREA 1,346,000 SQ FT 

N 

I' 

Basalt Pyroclastic and 

sedimentary deposits 

Alluvium Landslide debris Maximum height, in feet, 

of water 

0 800 1600 FEET 

li'IGURE 13.-Sections of Snake River canyon at principal constrictions, showing maximum height of Bonneville Flood. Section G-G' shown in figure 17. 

,I _,,... 

tv 

~ 

0 

~ > '(]} 

8 

~ 

0 

'"d 

~ H 

0 

~ trJ 

~ 0 

a 

trJ 

z 

trJ 

t:d 

0 

§ 

trJ 

< H 

t'4 

t'4 

trJ 

~ 

t'4 

0 

0 

~tl 

'(]} 

trJ 

~ 

~ H 

< 

trJ 

~ 

'"d 

t'4 

> z 
I. 

EROSION NEAR TWIN FALLS 27 

made a series of rapids and cataracts as the flood 

waned in volume. PiHar Falls and Shoshone Falls 

coincide in a curious way ·with aJbrupt embayments on 

both sides of the canyon, which probably were eroded 

concurrently by turbulent floodwater when the canyon 

was full. Whatever the mechanics of origin, catastrophic erosion of these silicic volcanics undoubtedly 

deepened the canyon. Because the present cataracts descend from bordering scabland, rather than from a lip 

upstream fro1n scwbland, they apparently mark the approximate limit of recessional erosion during the flood. 

That is, tJ1e topography suggests that recession of the 

falls since the flood has been comparatively negligible 

and that the present shape of the canyon floor is largely 

a relic of the Bonneville Flood. This conclusion is 

startling, especially in view of the 30,000-year age assigned to the flood, but I can interpret the ·present 

canyon near Twin Falls in no other way and only as 

impressive evidence of the magnitude of the Bonneville 

Flood. 

The alcoves along the north canyon rim; which represent places of especially turbulent flow :from the 

Rupert channel, define a spillway 10 miles wide. The 

largest are known as Blue Lakes alcove, Devils Corral, 

and Devils Wash!bowl. In describing the alcoves, Russell ( 1902, p. 127-130) noted several 1)eculiar physiographic features: absence of entering streams; headward enlargement into amphitheaters; and a general 

lack of talus, particularly at the semicircular heads of 

alcoves. Observing .also that springs emerge at several 

alcoves, he attributed the alcoves to spring action. This 

hypothesis was elaborated by Stearns ( 1936, p. 444-

447), who concluded that the alcoves were formed 

mainly by slow solution of basalt by spring water-a 

process that supposedly accounted for the perplexing 

lack of talus and for the headward enlargement of alcoves. He ( S~arns and others, 1938, p. 89) identified 

boulders at the mouths of alcoves as coarse alluvium left 

in the wake of the receding springs. 

The physiographic features that puzzled Russell and 

Stearns have long been recognized as typical of dry falls 

formed by glacial floods in the scabland of eastern 

Washington (Bretz, 1959; and many previous reports 

by Bretz dating from 1923). Phmge pools below such 

falls are flushed clean of debris by the force of turbulent 

water, and the falls characteristically erode into a semicircular form. In current understanding, the dry falls 

of eastern Washington were probably completely subtnerged by floodwater, which 1noved in giant eddies-

"subfluvial tornados" (Bretz and others, 1956, p. 1028)-

and eroded mainly by cavitation rather than solely by I 

cascading over an edge. This mechanism probrubly prevailed here, as well, at least for Devils Corral and Devils 

'Vashbowl, although the flood profile may have been 

slightly below the lip of Blue Lakes alcove. The alcoves 

also display various other physiographic features not 

explained by the spring hypothesis, but readily understood as CJonsequences of flood erosion. 

Tiered or staircased sets of a;bandoned plunge pools 

are common in alcoves that have been carved from layers of basalt. The basalt layers are from 25 to 50 feet 

thick. Erosion of these layers by turbulent water produced a random series of basins at various levels not 

connected by through drainage. Such basins are especially well displayed at Devils W ashbow I and Devils 

Corral. Some of the basins are filled with spring water, 

hut most of them are dry. 

The head of Blue Lakes alcove, a steep-walled amphitheatre 300 feet deep and 1,500 feet wide, is cut in a 

single 1nassi ve basalt flow (filling a former canyon of 

the Snake River) and exhibits no tiers of plunge pools. 

Stratified basalt flows toward the mouth of the alcove, 

however, as shown in a photograph by Russell (1902, 

pl. 24), were partly stripped so as to form benchesl 

which are further diversified by small basins. (See also 

fig. 14.) Such stripping is a common feature along the 

north edge of the canyon and evide~tly was caused by 

water spilling from the Rupert channel. 

Each major alcove is actually a group of dry falls 

that join a common stem. The plan of Devils Corral, 

for instance, resembles a misshapen glove, with several 

tributary alcoves and with plunge pools at various 

levels. The west branch of Blue Lakes aloove, perCJhed 

220 feet ~above the main stem, is cut off by a nearly 

vertical wall. The ·maze of minor alcoves between Devils 

Corral and Devils Wa8hbowl, however, is so chaotic tha;t 

it la;cks a noticeable pattern. . 

Scattered boulders are found on some benches within 

the alooves and cannot he expl·ained hy the spring 

hypothesis. (!See Russell, 1902, pl. 24.) The boulders 

are rather angular, hut the edges and corners are 

blunted. S.ome ·are more than 10 feet in diameter. They 

sit isohvted on barren basalt devoid of ·any other kind of 

gravel. Their transport, even for short distances, would 

require gre.:'tt force. 

The ragged canyon rim on the south side, where water 

spilled from the Milner reach (mile 186-180), is diversified by two waterless side canyons that form short 

gashlike alcoves at mile 185. These are minor topographic features compared to the alcoves on the north 

side but are worthy of emphasis because they suggest 

the power of the flood. The larger alcove, the east, is 

3,000 feet long, 300 feet wide, and about 100 feet deep. 

Both have scalloped east rims. The alcoves contain no 

d~bris except for some windblown sand and scattered 
28 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

FIGURE H.-Topographic map of Snake River canyon near Twin Falls, Idaho, showing erosion features produced 

by the Bonneville Flood. The limit of floodwater on the upland is determined by the extent of scabland, which 

is expressed on the map by numerous irregular contom·s and by closed basins and knobs. Gra,·eJ pits in 

scabland north of the canyon are in scattered patches of :\Ielon Gravel. The impressive erosion features along 

the canyon and on the canyon floor are described in the text. Circled numbers indicate river mileage measured 

upstream from Hot Springs Ferry. Section I-I' is shown in figure 13. (From U.S. Geol. Suryey topographic 

maps of Twin Falls and Kimberly quadrangles, Idaho, scale 1: 24,000.) 
EROSION NEAR TWIN FALLS 29 

114°22'30" 114°20' 114°17'30" rr-rr-~-.~~~~------~----.-~~r---~--~~~~--~~--r7----~~~----~--~--~~~~--~~-- 42° 

FIGURE 14.-Continued. 

37' 

30" 

T. 

9 

S. 

T. 

10 

S. 

42° 

35' 
30 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

FIGURE 15.-View downstream of Snake River canyon from Perrine Memorial Bridge (mile 180). A massive outcrop of silicic 

volcanics in the center, rising 200 feet above the canyon floor, was eroded by the Bonneville Flood to form scabland. 

Embayments in basalt along the northern (right) rim of the canyon mark the mouths of alcoves also eroded by the flood. 

The orchard at upper left is on a smoothly contoured bar of Melon Gravel that rises a!bout 100 feet above the river. The 

smooth topography at the golf course beyond the scabland is also built of )felon Gravel. At maximum discharge, thi::; 

part of the canyon-here 500 feet deep and a mile broad-was probably nearly brimful. (Photograph courtesy of Chamber 

of Commerce, Twin Falls, Idaho.) 

basalt boulders, some of which are 10 feet in diameter. 

The walls of the alcoves are parallel to weak planes 

of nearly vertical fractures that obliquely cross the 

Snake River at this place. The fractures are cleary visible in silicic volcanic rocks along the canyon floor. 

where they are emphasized by flood erosion, but they 

are less discernible in the overlying basalt. The alcoves 

are therefore diminutive copies of the joint-controlled 

channels carved by glacial floodwater in basalt at Washtucna Coulee, eastern Washington (Trimble, 1950). 

Because the heads of these alcoves are close to the highest limit of the Bonneville Flood, as determined by scrubland, they apparently were first formed by a vigorous 

crossflow of water, from east to west, which plucked 

loose blocks from the fractured rock. As erosion progressed, water could drain north into the canyon. The 

scalloped east edges of the alcoves probably indicate 

plucking by turbulent flow at eddies. These alcoves 

clearly demonstrate torrential flow of water above the 

canyon rim before the canyon had reached its present 

srze. 

THE CANYON BELOW TWIN FALLS 

Downstream from the region of alcoves near Twin 

Falls the Snake River has carved a picturesque canyon, 

generally 500-600 feet deep, which cuts through various 

sections of basalt and detrital deposits that have controlled the canyon form. In the upper part, from mile 

175 to 125, the river drops steeply over a floor of basalt 

at an average gradient of 11 feet per mile (fig. 3). The 

route is characterized by intermittent rapids. Thereafter, 

to mile 0, beyond which the canyon becomes indistinct, 

the river flows less precipitously on an average gradient 

of 3 feet per mile. The Snake is therefore one of 
THE CANYON BELOW TWIN FALLS 31 

the steepest large rivers in North America. Because the 

shape of the canyon at the time of the flood was substantially the same as now and because it greatly influenced the history of the flood, it is worthwhile to sumlnarize the canyon physiography and to indicate some 

of the flood features along the route. 

The canyon below Twin Falls consists of a series of 

wide segments (basins or coves) that include relatively 

flat bottom lands not far above the river and that are 

connected by constricted canyon segments (narrows) of 

variable length. The 'basins and narrows are outlined in 

figure 2 as the area inundated by the 1naximum rise of 

the Bonneville Flood. The basins, 20 square miles or 

n1ore in area, have irregular perimeters. The narrows 

are about 3,000-4,000 feet wide, measured at the canyon 

rnn. 

The canyon shape is a result of two factors-variable 

erodibility of rocks and enlargement of the canyon 

at tributary junctions. That is, the constrictions are 

wn.lled with basalt or are capped with basalt rimrock, 

generally on both sides; whereas the basins-which commonly coincide with tributary junctions-are carved 

from nonindurated detrital deposits that readily erode 

as badlands. 

The narrow stretches controlled the flood by impeding 

flow and thereby impounded water in basins upstream. 

Because floodwater in most of the basins was about 300 

feet deep at highest stage, the basins acted as sediment 

traps and retained debris flushed down from the canyon upstream (fig. 2). Retarded flow through the basins 

is demonstrated by a downstream decrease in height . 

of accumulated debris and by a corresponding reduction 

in size of transported material. Debris piles commonly 

decrease in height more than 100 feet along the length 

of a basin, nnd boulders in the upper reach of basins 

give way downstream to less bouldery gravel that is 

rich in sand. Some of the decrease in height of the debris 

piles and in grain size may of course have resulted from 

reworking during subsidence of floodwater, but I have 

discovered no definitive way to discriminate deposits 

of peak flow from those of possible lesser discharge. 

The deposits ~tre described in this repol't only as an 

overall effect of the flood. 

Because the basins were effective traps for sediment 

transported by the flood and because this sediment is 

almost entirely basaltic debris derived from outcrops 

along the canyon, the erosional effectiveness of the flood 

can be 'appraised in terms of the volume of debris 

trapped in 1basins. This problem is discussed later in this 

report (p. 44). For the present, I point out only that 

no material eroded from the upper part of the canym~ 

passed below 1nile 58, except possibly a little suspended 

silt, because the exceptionally narrow canyon near Swan 

Falls (mile 26) impounded a lake that extended 35 

miles upstream. This lake prevented further transport 

of flood debris. 

In the following discussion the canyon is divided into 

successive sections that have distinctive physiographic 

character and particular features caused by the Bonneville Flood. Some features that are especially indicative 

of the magnitude of the flood will be el,aborated 111 

later parts of this report. 

WEEPING WALL SECTION (MILE 175-162) 

Immediately downstream from the Twin Falls area, 

the Snake River enters a straight basalt-walled gorge 

from 1,500 to 2,500 feet wide at the rim and 450 feet 

deep, which is here named the Weeping Wall section. 

A persistent line of springs about· 150 feet below the 

north rim, between a lower unit of decomposed basalt 

(Banbury Basalt) and a rimrock of rather fresh basalt, gives this canyon segment its name. The canyon 

is even more constricted than the width and depth would 

suggest because for nearly half its length it is a twostory canyon. In the upper 6 miles the river flows in a 

trough 500 feet wide lying 125 feet below a dissected 

bench of basalt scabland (fig. 3). Melon Gravel at the 

upstream end of the bench and at the toe suggests that 

this trough could have been cut since the flood, but erosion by the Bonneville Flood cannot be ruled out. Indeed, at an earlier time H. A. Powers (in Mal de, 1960) 

thought, because of the relation of this canyon segment 

to basalt older than the flood, that all of the canyon lrad 

been cut by flood erosion, but the prior existence of a 

canyon upstream (the Milner reach) indicates that the 

Weeping Wall section antedates the flood. In the last 

7 miles of the Weeping Wall section, the lower story 

of the canyon is represented by discontinuous comparatively small bars of Melon Gravel that rise about 100 

feet above the river. The gravel lies at river level in some 

places, and there is no indication of entrenchment since 

the flood. Because the gravel occurs where the canyon 

is somewhat wider than it is upstream, deposition of 

the gravel presumably was caused by a reduced rate 

of flow. 

The Weeping Wall section is joined on the south by 

Rock Creek and Cedar Draw (miles 174 and 167), but 

these junctions are not notably enlarged and had no 

obvious effect on the flood. 

A thin mantle of talus along most of the Weeping 

"Vall section indicates that the canyon is only a little 

wider than during the flood. A projected flood profile, 

determined by scabland at an altitude of 3,600 feet on 

the north rim at mile 175 and at an altitude of 3,150 

feet at mile 156, would indicate that the canyon was 

nearly brimful at the highest flood stage. This inference 
32 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

is contradicted, however, by the carrying capacity at 

mile 172, the narrowest place (fig. 13, section H-H'), 

which is at least four times the capacity at Swan Falls 

(table 2). The discrepancy can be accounted for if one 

assumes a flattened profile or a considerable canyon 

enlargement during the flood. 

MELON VALLEY SECTION (MILE 162-155) 

Below the long and narrow Weeping Wall section, the 

river emerges into a small basin called Melon Valley, 

which is about 2 miles wide and 3 miles long (fig. 16). 

The depth of the canyon at this place, measured from 

the north (lower) rim, is from 250 to 300 feet, of which 

the lower part consists of ragged outcrops of Banbury 

Basalt that crowd the valley floor. The upper canyon 

walls consist of resistant fresh basalt on the north rim 

(Thousand Springs Basalt) and a complex array of 

interbedded basalt and clastic deposits on the south. 

Deep Creek and Mud Creek have dissected the south 

margin of Melon Valley into a rough and gullied terrain. The mouth of Salmon Falls Creek, which joins 

the canyon near mile 155, coincides with another small 

basin. The flood profile, reconstructed from scabland 

at mile 156, indicates that floodwater at peak discharge 

rose to an altitude of about 3,225 feet at the north canyon 

rim of Melon Valley (mile 161). 

The central part of Melon Valley· is covered by a 

large bar of Melon Gravel 7,500 feet long that rises at 

midsection to a height of 160 feet above the river (fig. 

17, section G-G'). Scattered on the surface of the bar, 

particularly along the crest, are well-rounded basalt 

boulders that range in maximum diameter from 4 feet 

at the head of the bar to 2 feet at the toe. A gravel pit 

at the downstream end of the bar opposite mile 161 

affords a cross-sectional view of the internal character. 

This exposure reveals that the gravel is a mixture of 

boulders and coarse basaltic sand arranged in thick, 

crudely bedded sets of inclined layers (fig. 5). The bar 

is limited on the north by the Snake River and on the 

south by a marginal channel. Several other boulder bars 

of lesser size cover much of the remainder of Melon 

Valley. 

Another interesting gravel deposit in this section is at 

mile 158, the head of an abandoned channel that carried 

a share of the flood. The surface of the gravel at this 

place is armored with a layer of 3-foot boulders that 

display an imbricate fabric in which the boulders dip 

upstream (fig. 18). 

THOUSAND SPRINGS SECTION (MILE 155-151) 

This short canyon constriction merits special mention 

because its discharge capacity indicates the maximum 

volume of the Bonneville Flood in this reach of the 

Snake River canyon. The constriction is cut into wellindurated basaltic pyroclastic material and lava flows. 

The flood height through this section is shown by scabland 240 feet above the river between miles 156 and 154 

where a square mile of rimrock on the right bank was 

scoured and strewn with basalt boulders. The canyon 

section at the lower end of the scabland (fig. 13, section 

F-F') has a discharge capacity of 33 million cfs at the 

indicated height of the flood (table 2), an amount considerably less than the capacity of any place upstream 

as far as Hansen Bridge (mile 189). Since the flood, the 

only apparent change in this canyon section has been 

deposition of pebble gravel, which drilling shows to 

extend not more than 25 feet below present river level. 

Evidently, the constricted section at Thousand 

Springs was about doubled in size during the :flood. The 

calculated flood discharge is of course limited in accuracy by several obvious uncertainties, but it demonstrates that the constriction at Thousand Springs must 

have been an important valve that limited the amount 

of floodwater delivered downstream. Because a larger 

volume of floodwater would have spread more widely, 

forming more scabland, the capacity of this constriction also determines the probable maximum discharge 

that arrived from upstream. 

HAGERMAN VALLEY (MILE 151-141) 

Hagerman Valley (fig. 19) is a broad canyon segment 

about 4 miles wide, bounded on the west by spectacular 

barren cliffs of nonindurated basin deposits (Glenns 

Ferry Formation) that rise 600 feet above the Snake 

River and bounded on the east by a lower cliff of basalt. 

An inner bench of terrace gravel along the east side, 

standing 300 feet above the river, was the effective canyon wall that limited the spread of floodwater. Scabland upstream at Thousand Springs and Melon Gravel 

below Hagerman show th~t the flood surface was at a11 

altitude of 3,030 feet in Hagerman Valley. 

Hagerman Valley, containing more than 7 billion 

cubic feet of Melon Gravel (one-twentieth of a cubic 

mile), was a major sediment trap for flood debris. (See 

table 4, p. 45.) The gravel occupies a basaltic substratum 100-150 feet above the Snake River and is 

spread over an area of about 6 square miles. The greater 

part of the Melon Gravel is in a huge bar 50~75 feet 

thick, which covers most of the central area of Hagerman Valley: The bar has a slightly curved trace 3 miles 

long and extends from mile 151 to the center of the 

valley near Hagerman. Boulders as large as 5 feet in 

diameter occur at the upstream end, and 2-foot boulders 

can be found at the toe. (See fig. 4.) A marginal channel lies along the east side of the bar. The greatest thickness of Melon Gravel in Hagerman Valley, more than 
42' R. 

4 5' 

R. 14 E. 

THE CANYON BELOW TWIN FALLS 

114 ' 50' 

~...., ---=ro~- -~~~- ~-

._:1 / 

33 

11 4 ' 4 5' 

l •• 

FIGURE 16.-Topographic map of Melon Valley and Thousand Springs area showing features produced by the Bonneville Flood. 

In Melon Valley a boulder bar extends 1% miles downstream (we~:;t) from a Rhoulder on the left bank. An adjoining marginal channel heads in a saddle 125 feet above the Snake River. On the north side, a rock basin partly blocked by boulder 

deposits is carved in Banbury Basalt. Imbricate boulders near mile 158 are illustrated in figure 18. A graYel bar at mile 

155 was deposited by floodwater that followed an ancient river channel. Basalt uplands north and east of the river generally 

stood above the flood, but the rim between miles 156 and 154 was temporarily overtopped by a rise of 240 !feet, probably 

because of the canyon constriction at section F-F' (fig. 13). (From U.S. Geol. Survey topographic map of Buhl quadrangle, 

Idaho, s~ale 1: 62,500; also published in shaded relief as Thousand Springs quadrangle, scale 1: 24,000.) (Section G- G' 

shown in fig. 17.) 
34 CATASTROPIITC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

0 2 MILES 

EXPLANATION 

Melon [ill] Gravel ~ Z 

0:: 

"",u\1 WI-- "'' Thousand Springs ~ Basalt 0 

VERTICAL EXAGGERATION X 10 D 

Older rocks 

FIGURE 17.-Geologic sketch of l\Ielon Valley, showing location 

of section through a bar of l\Ielon Gravel. 

100 £eet, is in a bar that begins at Hagerman and tapers 

out a mile downstream. The upper end o£ this bar is 

tied to a buttress o£ nonindurated lake clay capped with 

an unconsolidated terrace gravel that determines the 

right bank at this place. This circumstance suggests 

that the £orce o£ the flood at Hagerman was dispersed 

but was nonetheless strong enough to deposit gravel as 

a pendant bar. 

Flood erosion near the head o£ Hagerman Valley 

created an unusual hydrologic situation advantageous 

to wildlife. In the marginal channel between the central 

bar and the canyon wall, spring water supplies a marshy 

pond covering about hal£ a square mile. Banbury Basalt 

crops out at places around the periphery, and the 

marshy area must be held in a shallow rock basin o£ the 

basalt that was scoured out during the flood because 

the gravel bar is too permeable to act as an effective 

dam. This area is now maintained as an Idaho State 

Fish Hatchery and Game Reserve. 

BLISS SECTION (MILE 141-121) 

Downstream £rom Hagerman Valley, · the Snake 

River enters another long constriction, here named the 

FIGURE 18.-Boulders of l\Ielon Gra,·el125 feet above the Snake 

River at the head of an abandoned channel near Banbury 

Springs (mile 158). The view is upstream toward )Ielon 

Valley. Oblong boulders that dip upstream and toward the 

axis of the channel (right) express an imbricate fabric. 

Bliss section. For the most part, this constricted segment is a two-story canyon in which the Snake River 

follows a narrow inner gorge, 400-800 £eet 'Yide and 200 

£eet deep, cut mainly into Banbury Basalt. Above the 

inner gorge is a bench about a mile wide £rom which 

canyon walls rise abruptly to a height 575 £eet above 

the river. From Hagerman Valley to mile 135 the bench 

is largely built o£ Melon Gravel. From here downstream 

to Bliss Dam (mile 128), the bench is cut on Banbury 

Basalt and represents an exhumed erosion surface elsewhere overlain by nonindurated deposits o£ the Glenns 

Ferry Formation. The basalt bench carries only scattered patches o£ Melon Gravel and must ha Ye been swept 

by a £orce£ul flow o£ flood water. Gravel is again £ound 

downstream £rom Bliss Dam, where the Banbury descends below river level. This gravel chokes the canyon 

floor as £ar downstream as mile 125. It £orms a series 

o£ boulder bars that rise £rom river level to a height o£ 

275 £eet. Around the bend £rom mile 124 to 122 the 

gravel £orms a descending set o£ narrow slip-off terraces, 

each armored with a string o£ large boulders (fig. 20). 

The Bliss section is somewhat wider now than during 

the flood because talus and landslides younger than the 

flood line both sides, but the proximity o£ Melon Gravel 

to the canyon walls at several places suggests that the 

width has not increased much. The present two-story 

configuration probably also existed during the flood; 

otherwise, the carrying capacity at the inferred flood 

height would have been insufficient. Prior existence o£ 

the two-story canyon is also indicated by Melon Gravel 

at river level upstream £rom the inner gorge. 

The Bliss section has some o£ the largest boulders 

£ound anywhere along the Snake River canyon. Boulders on a bench east o£ mile 136, £or instance, are about 
THE CANYON BELOW TWIN FALLS 35 

8 feet in diameter, and a boulder train that extends from 

1nile 126 to 124 (interrupted by the inner gorge) ineludes several well-rounded stones that are as much as 

10 feet in diameter (fig. 8) . These boulders match the 

lithology of rimrock along the north canyon wa.Il and 

are probably pieces of talus that were rolled hy the 

floodwater. 

Basalt boulders 2-3 feet in diameter and lying 475 

feet above the river are found also on the south rim 

opposite mile 128. These match the lithology of rimrock 

on the north side, 1 mile distant, and seem identical to 

some boulders moved by the Bonneville Flood. However, they are 175 feet above the estimated highest stage 

of the flood and cannot be contemporaneous with Melon 

Gravel. They must represent a river deposit left before 

entrenchment of the canyon. 

KING HILL BASIN (MILE 121-109) 

The IGng Hill basin is a broad valley 600-1,000 feet 

deep covering about 35 square miles (fig. 20) . The basin 

is carved almost entirely from nonindurated detrital 

deposits (mainly Glenns Ferry Formation), but basalt 

plateaus define the north edge. Two important tributaries enter the basin on the north, Clover Creek and 

IGng Hill Creek. Because of its size and because it was 

the place where the Bonneville Flood first entered relatively slack water impounded by constrictions farther 

downstream, the IGng Hill basin trapped a large part 

of the debris flushed down by the flood. If the debris at 

Pasadena Valley is included, the basin contains 46 billion cubic feet of Melon Gravel (nearly one-third of a 

cubic mile), which is more than half the flood debris in 

the canyon below Twin Falls. (See table 4, p. 45.) 

Dumping of the gravel built impressive landforms. 

Enormous bars of Melon Gravel fill the center of the 

valley around J{ing HiH and form broad benches several hundred feet above the Snake River. Through these 

bars the river follows a winding narrow inner trough 

which reveals that Melon Gravel reaches river level, or 

lower, along most of the route. The gravel apparently 

rests on a broad floor. The gravel at Pasadena Valley 

(mile 121-119) forms a bar about 150 feet thick, which 

rises 290 feet above the river. This bar is part of a gravel 

strath that extends another 2 mHes downstream into 

the widest part of the basin. In the wide part, measured 

around the bight from mile 117 to 110, a pile of gravel 

covers 6 square miles to an average depth of 160 feet; 

the highest part is a broad strath 260 feet above the 

river, which equals the altitude of a possible backwater 

deposit 2 miles east of King Hill, previously mentioned 

on page 19. This level (about 2,760 ft alt) matches 

the top of t\ bar on the opposite (east) side of the river, 

and the level of a bar just north of King Hill is nearly 

as high. Lying between these high bars, close by the 

river channel, is another gravel surface 60 feet lower, at 

an altitude of 2,700 feet. This strath is also identifiable 

upstream between miles 118 and 120. The whole topography, however, rather than resembling ordinary river 

terraces, is irregular and dissected, like the greatly magnified bed of a braided stream. 

The benches of Melon Gravel near King Hill that rise 

to a common height 260 feet above the river (2,760 ft 

alt) might seem to represent prutches of a graded level 

surface that formerly extended over a wide area. A continuous gravel surface at this level, however, never 

could have existed. The benches are neai· the middle of 

the valley and are bounded by marginal channels that 

hug the canyon walls. They are doubtless constructional 

features left :by the flood in virtually their present form. 

The rather uniform altitude of the benches probably 

indicates a surface of relatively tranquil water impounded by a canyon constriction at The Narrows (mile 

100) .

3 A water level 40 feet higher at an altitude of 

2,800 feet, which probably was reached during peak 

discharge, is indicated by the height of graver near the 

head of the basin. 

A spectacular group of boulders is found on basalt 

240 feet above the Snake River along the right bank 

near 1nile 120 (fig. 21). This basalt was overtopped by 

floodwater 50 feet deep. Chunks of basalt from 10 to 15 

feet in long dimension were plucked from the lava and 

tumbled a few hundred feet downstream. The roBed 

pieces rest on basalt scabland devoid of any other flood 

debris. A few of them are rounded, and others have 

blunted edges, but some original pahoehoe lava skins 

suggest that corrasion during transport was moderate. 

These boulders are the largest seen anywhere along the 

route of the flood below Twin Falls and, for their size, 

lie at an exceptional height. The spacing of cracks in 

the lava indicates that these rolled pieces were the largest chunks available. Perhaps larger blocks could have 

been moved if they had been present. 

The pile of gravel in the IGng Hill basin diminishes 

in height downstream until at mile 112 the maximum 

altitude above the river is only 200 feet-a fall of 100 

feet in 41h miles. Thereafter the height drops more 

rapidly. The ma.ximmn grain size also diminishes downstream, ranging from 4-foot boulders at the head to 

1-foot boulders at the toe. At midlength the gravel surface is strewn with well-rounded basa:lt boulders 1-2 

feet in diameter. In the lower part of the basin a great 

deal of the flood debris is sand, like that displayed in a 

large sandbar 125 feet high, constructed in the lowest 

part of the basin between miles 111 and 109. As shown 

in a borrow pit at the downstream end, the bar consists 

of coarse basaltic sand in crossbeds of tremendous scale 

a The 2, 70.()-ft bench was perhaps controlled by an overflow threshhol~ 

at this height at mile 75 (fig. 22). 
36 

/ -

CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

'--

Scabland 

( 

A 

Melon 

Gravel 

L E y 

Alluvium 

.f.!: ' 

-

- ../ 

Terrace gravel 

on lake clay 

R. l3E. R. I4E. 
*'· 

-, 

THE CANYON BELOW TWIN FALLS 37 

(fig. 6). These downstream changes in height and grain 

size surely demonstrate decreased carrying capacity as 

debris-laden flood water from the Bliss section met relatively tranquil water in the I<::ing Hill basin. 

Marginal channels are important physiographic features associated w.Uth the deposits of Melon Gravel in 

the IGng Hill basin and are comparatively long and 

deep (table 1) . The channels are perched along both 

sides of the valley as continuous troughs 100-150 feet 

below neighboring gravel bars. Because they lie close 

to the canyon walls, the channels emphasize the central 

valley position of the main mass of gravel. Stearns 

(1962) identified a pond at the outlet of Pasadena Valley (opposite mile 119) as a plunge pool, but this small 

depression is only one of several minor irregularities in 

a 1narginal channel that winds along the left bank. For 

instance, Pasadena Valley, a wide place in the upper 

part of .the channel, was under water before it was 

drained for farming. Such closed depressions are typical 

of the cluinnels. The channel along the right bank, 

some 1·50 feet above the Snake River, continues past 

the mouth of Clover Creek to the basin edge north of 

l(ing I-Iill. 

Possible backwater deposits in the I<:ing Hill basin 

and the obstructed tributary at Jolley Flat are mentioned in an earlier part of this report (p. 18). 

COVE SECTION (MILE 109-76) 

The Cove section of the Snake River canyon consists 

of a chain of basins (coves) connected by narrow canyon 

necks. The IGng Hill basin can be considered as the 

uppermost cove. Below I<:ing Hill the coves are identified as follows: Glenns Ferry basin (mile 108-101), 

lfrunmett basin (mile 99-93), Indian Cove (mile 91-87), 

Eagle Cove (mile 84-81) , and Loveridge basin (mile 

81-76). All the coves are carved frmn soft detrital deposits, and most of thmn coincide with junctions of tributary streams. The connecting canyon necks are for 

the most part rimmed with basalt. 

Only sparse deposits of Melon Gravel are present 

along the Cove section, and they are mostly sand. (See 

fig. 24.) Debris flushed from rthe canyon upstre.c'tm was 

almost entirely trapped in the IGng Hill basin, and the 

short constrictions between :the coves were inadequate 

sources of additional gravel. Flood debris in the coves 

forms low bars not more than 75 feet above the river, 

which are just below constrictions and along the axis 

of the valley. The greater· part of the gravel is in the 

Glenns Ferry and Hammett basins, and this mostly 

consists of fine debris that did not get trapped at IGng 

Hill. A few basalt boulders at Glenns Ferry were 

washed through the IGng Hill basin, and spherical 

basalt cobbles are rather common. They hinder cultivation and are disposed of in picturesque fences of "petrified melons." The Melon Gravel at Hammett is almost 

all basaltic sand. Sand also predominates in flood debris 

downstream, even in Eagle Cove, which lies below a 

narrow basalt-rimmed canyon neck more than 2 miles 

long. The Loveridge basin, just beyond, contains no 

Melon Gravel. 

At the highest rise of floodwater, estimated from the 

height of scabland at an altitude of about 2,7 40 feet 

below ·mile 76 and from Melon Gravel at an altitude of 

2,800 feet in the King Hill basin, the coves were deeply 

ponded (fig. 3). Because of impeded flow through connecting constrictions, the ponded water in successive 

coves must have assumed a stepped profile. The altitude 

difference of the water surface between the upper and 

lower ends of this reach, a distance of 45 miles, amounted 

to 60 feet. Such descending levels of ponded water are 

common along streams during ordinary floods and provide a basis for calculating discharge, but this calculation has not been done for the Bonneville Flood 

because of the more favorable set of hydrologic conditions analyzed by Jenkins near Swan Falls (p. 12). 

Flow through the connecting constrictions was fast 

(at least 20 feet per second at 1niles 100 and 92), yet 

flood erosion is not apparent. No debris from the constrictions has been identified in the meager flood deposits 

immediately downstream. The constrictions are short, 

however, and are cut into beds of fine-grained silt and 

sand below the level of basalt rimrock. These soft beds 

could have been molded into smooth shapes that 

inhibited erosion. 

CRANE FALLS SECTION (MILE 76-61) 

The Crane Falls section, a deep basalt-walled gorge, 

was a major obstruction in the path of the Bonneville 

Flood (fig. 22). The narrowest place is at mile 75, a slot 

1,100 fe~t wide at the ri.m and 210 feet deep (fig. 13, 

:n•roun.E 19.-Topographic map of Hagerman Valley, showing features produced by the Bonneville Flood. Bars of l\Ielon Gravel 

disperse<! at the mouth of a constricted canyon segment occupy most of Hagerman Valley ·and reach 175 feet above the 

Snnke River. l\:faximum sizes of boulders in these bars· decrease rapidly downstream. A bar at Hagerman tie<l to a projecting 

mnss of lali:e clay rises 215 feet above the river and obstructs the valley of Billingsley Creek. A rock basin carved by the 

fioocl some 2--3 miles south-southeast of Hagerman lies near the head of n long marginal channel that bounds the gravel 

deposits. Bnsalt that underlies .the gravel of Hagerman V·alley is dissected into sca;blan<l between miles 144 and 141. 

Opposite mile 141 a. landslide crowds against a strath of Melon Gravel. (From U.S. Geol. Survey topographic maps of 

Hngermnn a.nd Tuttle quadl'angles, Idaho, scale 1 : 24,000.) 
38 

T. 

5 

S. 

T. 

6 

S. 

42" 

55' 

CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

·1 
~. 

·' 

THE CANYON BELOW TWIN FALLS 39 

section B-B'). As Stearns (1962) pointed out, this gap 

was too small to contain all the floodwater, which therefore overtopped the canyon walls and partly escaped 

through a cutoff, rejoining the Snake River at the 1nouth 

of the Bruneau River 7 miles west.· In entering the 

cutoff, the floodwater crossed a threshold about 300 feet 

above the river, as shown by scabland, and the canyon . 

rim at mile 75 was therefore submerged 90 feet. A 

shorter cutoff, between miles 76 and 73, forms a scabland 

channel 235 fe~t above the river, through wh1ch water 

flowed at least 50 feet deep. Still another upland channel 

diverted water from the Bruneau cutoff, crossed a lip 

265 feet above the river, and spilled northward into the 

canyon at 1nile 71 to form a deep plunge pool at the toe 

of a bar of Melon Gravel. Water also spilled northwa,rd 

into the canyon at mile 72 over a horseshoe bend in the 

canyon rim and carved another plunge pool in Melon 

Gravel. These plunge pools in gravel that was washed 

along the canyon demonstrate that the Bruneau cutoff 

and the canyon were flooded simultaneously. When the 

Bruneau cutoff was flooded, high water impeded flow 

through the coves as far upstream as the head of the 

IGng I-Iill basin. 

Below the narrow gap at mile 75, the canyon gradually widens and remains a gorge of solid basalt downstream to Crane Falls (1nile 73). This stretch, which 

1nust have been swept rather clean by the flood, has no 

Melon Gravel and practically no talus. Below Crane 

Falls the canyon widens abruptly to a:bout 5,000 feet, 

and the next 3 miles holds most of the gravel found 

along this part of the canyon. The gravel is bouldery 

and forms long narrow-crested bars close to the center of 

the canyon. Many of the boulders are angular. Because 

of relatively tranquil flow through ·the Cove section, this 

gravel could only have been derived from the narrow 

gorge rubove Crane Falls and from scabland along the 

upland diversion channels. This source is confirmed by 

lithologic comparison with the local basalt, a distinctive weathered-brown type. The provincial origin of the 

gravel is of further interest because none of the gravel 

was carried more than a few 1niles downstream. Deposition of the gravel implies reduced rate of flow, even 

though this part of the canyon is relatively steep (7 feet 

per mile). Buildup of gravel at this place may have been 

promoted by increased canyon width, but a con tributary 

cause could have been backwater from a temporary lake 

in the Grandview basin (p. 40). The highest gravel 

within the canyon is 240 feet above the river (2,640 ft 

alt) and therefore only 15 feet above the height of 

ponded water that existed 10 miles downstream. 

An interesting patch of gravel is formd 250 feet above 

the river at the lip of the marginal spillway at mile 71. 

The gravel includes 5-foot basalt boulders derived from 

scabland to the southeast, and it rests against 1a vertical 

face of basalt that bounds the upstream side of the spillway. This anomalous vertical face reaches a height 300 

feet a;bove the river .and can only be explained by flood 

erosion. Part of the gravel drapes over the canyon edge 

and joins gravel 60 feet lower that' 'vas washed along 

the canyon. These relations indicate that floodwater debouching into the canyon at the spillway was about 50 

feet deep and had velooity sufficient to accomplish some 

erosi'On and to 1nove 5-foot boulders. Moreover, floodwater along the canyon could not have been more than 

60·feet below floodwater on the upland. 

Owing to ponded w~ter in the Grand View basin 

downstrea·m, unusual debris deltas were built at the terminus of the Brm1eau cutoff (near 1nile 61, fig. 2). Except for basalt scaibland at the upper end of the cutoff 

and a few projecting knobs of basalt and scattered heaps 

of basalt boulders toward the lower end, most of the 

cutoff is a smooth featureless plain of Pleistocene lake 

deposits marked with no evident sign of the Bonneville 

Flood. These lake deposits (Bruneau Formation) were 

the main source of the deltaic debris, which consists 

chiefly of rounded chunks of diatomite. Two deltas are 

preserved. The smaller is at the end of a distributary 

branch terminating at mile 62. The other lies at the 

end of the main branch near the southeast corner of T. 

5 S., R. 4 E., and is well exposed along a deep longitudinal gully in sec. 35. 

Exposures along the gully (about a mile long) show 

that debris carried by the flood washed over a basalt lip 

and accumulated in layers that dip uniformly w·estward 

(downstream) at the rather low angle of 15°. Sets of 

inclined beds are continuously exposed in places along 

1l'IOURE 20.-Topographic map of King Hill basin, showing features produced by the Bonneville Flood. Slip-off terraces around 

the bend from mile 124 to 122 are armored with trains of large boulders. Pasadena Valley, which lies near the head of a 

marginal channel winding along the left bank, is held by a gravel bar that rises 290 feet above the Snake River. On the 

opposite side, where a lava cliff was overtopped by floodwater 50 feet deep, boulders torn from the lava reach maximum 

dimensions of 15 feet, as shown in figure 21. The gravel at Pasadena Valley extends downstream into the widest part of 

the valley where it forms broad straths reaching 260 feet above the river. In this wide area the Melon Gravel obliterates 

a previous river course that followed ·a shorter route between miles 118 and 111. The size of material washed into thiR 

bnsii1 decreases rapidly downstream and at mile 112 consists mostly of sand. Dumping of flood debris obstructed the outlet 

of .Tolley Flat and caused deposition of a broad plain of alluvium. Minor scabland features on basalt near the railroad 

opposite mile 119 are illustrated in figure 9A. (From U.S. Geol. Survey topographic maps of Pasadena Valley and King 

Hill quadrangles, Idaho, scale 1 : 62,500.) 
40 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

FIGURE 21.-Basalt boulders on lava flow 240 feet above right 

bank of Snake River near Bancroft Springs (mile 120). The 

man stands on the lava surface. The long diameter of the 

rolled block in front of him exceeds 15 feet. The source of 

these boulders was the lava surface that was overtopped by 

floodwater a mile or less upstream. This locality was covered 

by floodwater 50 feet deep. In the middle distance is a strath 

of Melon Gravel 100 feet below the crest of a long bar that 

lies beyond. Still farther away are badlands that surround 

the King Hill basin. 

the gully throughout a height of 100 feet. Outcrops of 

basin deposits (Glenns Ferry Formation) that m1derlie 

these beds occur along the gully floor. This pile of debris 

is therefore draped over the valley wall where floodwater from the Bruneau cutoff entered the Grand View 

basin and apparently lost velocity. (See also geology 

mapped by Mal de and others, 1963.) The deltaic beds 

consist of layers dominated by fragmented pieces of diatomite an eighth of an inch or larger in size (commonly 

pebbles) and of alternate layers rich in basaltic pebblee 

and coarse sand. These contrasting layers make a striking display of inclined white and gray foreset beds. The 

beds are as much as several feet thick, and some are 

marked by intricate festoons of laminated sand such as 

occur in bars that were deposited in tranquil water of 

coves upstream. Among the inclined layers are diatomite 

pieces the size of boulders, and one piece was found that 

was 12 feet across. Such chunks of soft diatomite could 

not have survived any great distance of transport. Indeed, their only source was along the Bruneau cutoff 

not more than 3 miles upstream. The pieces probably 

were transported as suspended load and were quickly 

dumped as the velocity slowed. The low density of this 

debris might account for the low angle of dip of the 

beds, as compared with the dips of foreset beds in orclinary deltas. 

No topset beds can be seen along the gully, but the 

truncated edges of dipping beds are armored by a level 

pavement of cobble and boulder gravel, which consists 

partly of quartzite fragments reworked from nearby 

Pleistocene gravels and partly of subangular pieces of 

basalt that must have been carried by the Bonneville 

Flood. This pavement is from 3 to 5 foot thick where 

exposed along the top of t.he gully and lies at an altitude 

of 2,600 feet. The pavement may have been built by 

waves chased up the 30-mile fetch of open water in the 

Grand View basin. E . G. Crosthwaite suggested this 

origin for the gravel pavement during a field conference in 1962.) When the delta was first built, however, 

as shown by heaps of basalt boulders dumped near 

the head, water level in the Gra.nd View basin stood at 

an altitude of about 2,625 feet. A lack of flood features 

on a divide 15 feet higher, between this delta and the 

other to the north, suggests that these boulders accurately mark the maximum level of floodwater in the 

Grand View basin. 

GRAND VIEW BASIN ( MILE 61-28 ) 

Below the mouth of the Bruneau, the Snake River 

enters a broad valley 30 miles long. The valley is bordered on the north by a precipitous wall capped with 

basalt, but the south edge is lost in badlands carved in 

basin deposits. Several tributaries join the valley from 

the south. Buttes of lava in the lower reach of the valley 

cause the river to wind between constrictions at mile 41 

and along a stretch between m.iles 34 and 28. 

At first glance the Grand View basin exhibits nothing 

at.tributahle to the Bonneville Flood, but this deficiency 

is its most significant. feature-once the magnitude of 

the flood is appreciated. Melon Gravel forms a small 

bar no more than 135 feet above the Snake River in the 

uppermost reach of the basin, between miles 61 and 58; 

otherwise no flood debris can be found. This lack of debris is a consequence of tranquil flow through a deep 

lake held by an exceptionally narrow canyon near 

Swan Falls. The lake had a surface area of 150 square 

miles, counting the part that must have flooded lower 

Bruneau Valley, and it had a maximum depth of 325 

feet. The depth is demonstrated by the height of 

boulders associated with deltaic debris at the terminus 

of the Brw1eau cutoff, as just described. (See above.) 

Ponded water surrounded the lava buttes at tihe west 

end, ''"hich now confine the Snake River to a narrow 

canyon along the north edge of the basin. 

Tranquil flow of floodwater through the Grand View 

basin is also suggested by rapid reduction of maximum 

grain size in the solitary boulder bar just mentioned. 

The boulders decrease from 4 feet to less than a foot 

in about a mile and include pieces of noninduratcd 

basin sediments (Steams, 1952) . An abrupt loss in velocity is implied. The bar begins at the mouth of the 

constricted canyon leading from Crane Falls, and is 
T. 

5 

2~ --~ 

,,.-

+ ')" 

" 

,eoO'~- -~ \';, . ~9 -

\.-, 

fis<" /\~ ·· 

..-- ... ;:;;_~--

115.50' 

'• 

w~ /\ 1 12 

\ '~ -- ' l'-.>ovn. ..,..-....... 

R. 5 E. R. 6 E. ,--- '7 T ---:=,.--,-.-1"'1'--"" 4 5' 

8 :Si);· 9 + ,___ 

30.?5 

,. . r' 

32 L ~ .• J 

s . FVi""" ......:::::: 

T . 

6 

s. 

lo:: -" q 3 

- t- _._L__ 

FIGURE 22.-Topographic map of Crane Falls area, showing features produced l.Jy the Bonneville Flood. Melon Gravel is indicated by stippel. 

A canyon constriction at mile 75 caused floodwater to rise and overflow the upland via a cutoff that leads 7 miles westward to the mouth of 

the Bruneau River. Basalt scabland at the cutoff threshold demonstrates that impounded water stood 300 feet deep upstream at an altitude 

of about 2,740 feet. Some floodwater returned to the Snake River canyon via a channel in basalt that debouches at mile 73. Water also spilled 

into the canyon at miles 72 and 71 and formed plunge pools in gravel washed along the canyon. These plunge pools demonstrate that the 

upland cutoff and the canyon were flooded simultaneously. Basalt boulders plucked from the upland are perched on the rim at the spillway at mile 71. The largest heap of Melon Gravel forms a bar 240 feet high midway between the canyon walls from mile 71 to 70. Owing to 

ponded floodwater upstream, all the gravel was derived locally. (From U.S. Geol. Survey topographic map of Bruneau quadrangle, Idaho, 

scale 1 :62,500.) (Section B-B' shown in fig. 13.) 

8 

~ 

C".l 

~ >< 0 

z 

1:1:1 

t."l 

t:"' 

0 

~ 

8 

:i1 

H 

z 

":j 

> t:"' 

t:"' 

Ul 

~ 

~ 
42 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

tied to a shoulder projecting from the north wall. From 

this shoulder the bar turns abruptly downstream along 

the interior of a river bend. A pond between the bar 

and the canyon wall lies a.t the head of a marginal channel. This bar, rather than being made up of debris 

transported along the Bruneau cutoff, as Stearns ( 1962) 

supposed, includes basalt boulders coarser than any 

fow1d in the delta at the mouth of the cutoff. It lies 

where floodwater delivered from Crane Falls would 

have form.ed an eddy in the lake. 

No flood debris fron1 the canyon upstream, other than 

a small amount of suspended material, could have 

passed through the long stretch of ponded water in the 

Gra,nd View basin. Even the patches of basaltic sand 

found in the coves upstream are absent. For the upper 

part of the Snake River, this was the final sump. If 

some silt did settle to the basin floor, it has been lost 

among the local badlands and probably has long since 

been eroded. 

The lower end of the Grand View basin is constrictecl 

by walls of basalt. The canyon between mil~ 30 and 28 

is only 1,500 feet wide at the rim and could not have 

carried the flood at the indicated height if some water 

had not passed around a canyon butte into the mouth 

of Sinker Creek (mile 28). The canyon walls present an 

uneven appearance marked by minor nooks, buttresses, 

and benches-all with smooth rounded outlines. At 

several places the walls are vertieal or overhang. Practieally no talus can be found. In short, the walls look 

to have been plucked elean of any loose blocks and then 

scrubbed. The height of floodwater through this outlet 

cannot be determined accurately from these features, 

but the scrubbed aspect is discernible to a height of at 

least 250 feet. 

SWAN ,FALLS SECTION (MILE 28-14) 

The Swan Falls section is a rock-walled gorge 

bounded by steep eliffs 600-700 feet high. The adjoining 

upland rises even higher, and all the floodwater necess~trily had to pass this gap. The naiTO\vest phtces are at 

miles 26 (fig. 13, section A-A') and 28, where the canyon is about 2,200 feet wide at the top and 650 feet wide 

a;t the bottom. These constrictions are cut into solid basalt-a series of thick lava flows .at mile 26 and the core 

of a 1nassive basalt.plug at mile 28. At a flood altitude of 

2,625 feet, the constrictions at miles 26 and 28 are, respectively, 1,700 feet and 1,900 feet wide. These constrictions acted as the hydraulic dam that i,mpounded 

floodwa.ter in the Grand View basin. The scrubbed 

appearance of this stretch of canyon and the lack of 

flood debris indicate that these constrictions probably 

a.cquired their present size during peak discharge. The 

floodwater no doubt widened the canyon at the narrowest places by removing lava blocks, but it probably did 

not appreciably deepen the canyon. Downcutting would 

be evident as a discontinuity in the profile of Sinker 

Creek where it joins the canyon at 1nile 28, but this tributary is at grade with the Snake River, and no significant knickpoint can be discerned in its profile upstream. 

(Topographic details for this part of the flood path are 

shown by the U.S. Geol. Survey map of the Oreana 

quadrangle, Idaho, scale 1 : 62,500.) 

Melon Gr~avel does not occur in important amounts 

in the Swan Falls section except downstremn from a 

point several miles below the constriction at mile 26. 

The part upstream, between miles 28 and 26, is swept 

entirely dean of gravel. The first gravel deposit below 

the gap is at the west rubutment of Swan Falls, 250 feet 

a;boYe the river (mile 23%). This deposit, which includes 

6-foot boulders, is at an altitude of 2,575 feet and was 

used by Jenkins (p. 12) to establish a tailwater altitude 

in his calculations of a probable discharge of 15 million 

cfs through the canyon neck upstream. For the next 

several miles, gravel occurs as patches hugging the 

canyon walls and distributed through a vertical range 

of 275 feet. Large 1amounts of gravel begin to appear 

only below mile 19, which marks the head of a large 

centrally located bar that extends another 4 miles downstream. This bar is built where the canyon begins to 

widen. No doubt the first several miles below Swan 

Falls, where Melon Gravel is so ineager 'and patchy, 

was a turbulent fast stretch of water. As will be seen, 

it must have also been a place of considerable erosion, 

as shown by the ainount of gravel dumped in the 

Walters basin immediately downstream. 

WALTERS BASIN (MILE 14-0) 

The wide valley at Walters Ferry was yet ·another 

trap for Melon Gravel flushed down by the Bonneville 

Flood (fig. 23). Debris trapped at Walters could have 

come only from the constricted canyon extending upstream to the outlet of the Grand View basin, and it 

therefore gives a measure of flood erosion in this reach. 

The volume of the flood debris in the '""alters basin 

cannot be measured accurately, owing to its uncertain 

thickness, but map relations suggest an amount comparable to the volume of Melon Gravel in Hagerman 

Valley-about ·one-twentieth of a cubic mile. 

The actua·l extent of floodwater in the 'Valters basin 

during the highest stage is poorly determined. The flood 

passed through a constriction a.t mile 14 tha:t is only 

half a mile wide, rose 280 feet above the Snake River 

to· an altitude of 2,535 feet, and overtopped the north 

rim. Coarse basaltic sand was spread on the upland 

along a swath half a mile broad that reaches 2 miles 

northwestward. The further extent of flooded ground 

.; 
4 3• 

20' 

Bou lders less 

than 1 ft 

18 

~~.I-.____,... ~ 

t' ,. v: .., IJ J 

r e-· -~ 

31'· ,) 

< l 

7 . ,., ;' ~ .. _,_/ 

,.,;\ ~_r -( 

; 32 ) 

.I / 

( ) 

) J·f\ 

THE CANYON BELOW TWIN FALLS 43 

35' 11 6°30' 

• T . 

1 ~4 ·~J·.~~~ - .&-"-!:"':::~:-"""· - ~ N 

'<.. 

Bad la,nds 

33 

-+ 

23 

Basa:itic 

sand 

Basa lt ib sand 

-~ spills o'v.e.r • 

C" · .. ··canyon ri·m ~ " tr 

'Icy '/)/) - ,~ N' . ~ -r·- . //) -- .. . 

, ·- ( - '•, <§-,. ,_, ) .. ··qv . '• f!!; 

26' . 

1 

19 

R. 2 W. R. 1 W. 

T. 

1 

S . 

FIGURE 23.-Topographic map of Walters basin, showing features praduced by the Bonneville Flood. Floodwater passed a 

canyon constriction at mile 14 -.Jnd spread widely, probably surrotmding 'Valter:s Butte. The canyon rim 280 feet above the 

Snake River north of mile 14 was overtopped by the flood, and the upland was covered by basaltic sand, of which some 

spilled back into the canyon opposite mile 13. Boulclery debris that washed downstream forms long linear bars separated 

by shallow channels. These constructional features trend clown the center of the valley to mile 9 and then turn to follow 

the canyon axis. Some bars accumulated in the lee of bedrock knobs that stood in the path of floocl\Yater. The highest 

boulder deposits occur several miles below the constriction at mile 14. l\Iaximum sizes of boulders decrease downstream, but 

large stones are found as far downstream as 'Valters Ferry. Below Walters Ferry, the volume of flood debris diminishes 

rapidly. (From U.S. Geol. Survey topographic map of Walters Butte quadrangle, Idaho, scale 1: 24,000.) 
44 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

in rolling basalt uplands that rise north of the river is 

not determined by known deposits, but water probably 

encircled Walters Butte, a volcanic prominence that 

stands near the north edge of the valley. In this area., 

gravel deposits opposite mile 10 demonstrate that floodwater was at an altitude of at least 2,460 feet and was 

therefore 210 feet above the river. Below Walters Ferry 

the spread of floodwater was limited on the north by a 

basalt-rimmed wall that extends 12 miles downstream. 

The south margin of the Bonneville Flood in the W alters basin is vaguely determined; a basalt \vall that held 

the flood downstream to mile 13 gives way to badlands 

in which all effects of the flood are lost. Rabbit Creek 

and Reynolds Creek join the valley here. The probable 

flood profile has been reconstructed from the height of 

water necessary to pass the flood at Givens Hot Springs 

(mile 0) and from the gradient of flood deposits above 

Walters Ferry. 

Some coarse basa:ltic sand that was carried along the 

northern upland spilled back into the canyon and is preserved in ·a small alcove opposite mile 13. Stearns ( 1962) 

commented that this deposit is foreset bedded against the 

canyon wall, but downstream dips are also present. He 

though that gravel bars immediately downstream were 

built during the waning of the flood, but a bar 21h miles 

downstream is as high as the canyon rim where the sand 

spills through. It seems likely that aU the bars were 

deposited in deep water. 

The central part of the Walters basin is covered with 

great bars of Melon Gravel that resemble the huge 

piles of flood debris at King Hill and Hagerman. Because the floodwater at Walters was not impounded by 

downstream constrictions, however, buildup of this debris was not influenced by impeded flow, and the rapidly 

moving water had a subtle modifying effeot on the character of the deposits. As at Hagerman, flood debris was 

washed relatively far into the basin so that the crests 

of bars occur several miles below the canyon mouth. 

The bars are long, narrow, and streamlined. They are 

separated by channels that trend directly down the basin 

rather than by channels that wind along the valley walls. 

Boulders 6 feet in diameter at mile 14 gradually give 

way to boulders as large as 3 feet at mile 9 and to boulders a foot in diameter •at mile 1. Basalt cobbles occur 

at Givens Hot Springs (mile 0). All these relations suggest forceful flow. Nevertheless, as at other basins along 

the canyon, the average size and the quantity of flood 

debris decrease rapidly downstream. Most of the debris 

at Walters Ferry (mile 8) is sand, and the quantity of 

debris washed beyond Walters Ferry is small. 

ICnobs of basalt in the northern part of the Walters 

basin had an important effect on the shape of the gravel 

deposits, which are strung out downstream in long 

tapered pendant bars. At least three such bars are recognizable (fig. 23). The largest, in the lee of "\Vhite Butte, 

has a closure of 60 feet at the head and a length of 2 

miles. A smaller pendant bar that hangs from a buttress 

of 1¥ alters Butte has a closure of 30 feet and is a mile 

long. The smallest bar, which has a closure of 40 feet, 

is attached to a basalt knob at Warrens. These streamlined bars differ from the rather broad, level-topped 

gravel deposits that accumulated in ponded water at 

King Hill. They are com parable to a streamlined bar 

that hangs frmn a shoulder in the canyon at mile 18. 

This pendant bar, which evidently accumulated in rapid 

'vater, has a closure of 80 feet and a length of half a 

mile. By this analogy, gravel bars in the "\Valters basin 

. indicate high flow velocities. 

Below Givens Hot Springs the valley widens into a 

broad low land that stretches many miles downstream. 

Here the Bonneville Flood must have spread widely, 

and its transport power must have been thereby greatly 

reduced. Although this segment of the Snake River has 

not been searched intensively for evidence of the flood, 

any such signs would necessarily be faint, if indeed 

recognizable, because of a lack of hard material along 

the route and because of the diminished rate of flow. 

Any debris that managed to pass Givens probably was 

carried only a few miles downstream, but even at Givens 

the amount of flood debris was small. 

BUDGET OF EROSION AND DEPOSITION 

Comparison of the volume of Melon Gravel with the 

size of the canyon upstream provides a basis for estimating the probable amount of erosion caused by the 

Bonneville Flood. Such an estimate is possible for the 

upper part of the canyon because no significant quantity of flood debris passed through the Grand View 

basin and because later erosion of the gravel seems 

i11consequential. 

The enlarged canyon segment near Twin Falls (mile 

115-189), with its spectacular alcoves, cataracts, and 

bordering scablands, is of particular interest because 

discharge from the Rupert channel and the Milner 

reach at this place evidently caused substantial erosion. 

The present volume of this segment, which must closely 

approximate the volume when the flood subsided, is 

computed to be about 60 billion cubic feet, or four-tenths 

of a cubic mile (table 3). From this volume must be 

subtracted the original canyon Yolume so as to yield an 

estimate of erosion actually accomplished by the flood. 

I assume that the original canyon size near Twin Falls 

may have been as large as an a ,·erage between the extremes represented by canyon dimensions above and 

below the enlarged segment, but I further assume that 

the canyon may have been as small as the size just suffi-

'• 
BUDGE.T OF EROSION AND DEPOSITION 45 

TABLE 3.-Computation of volume of Snake River canyon, Augur 

Falls (mile 175) to Hansen Bridge (mile 189) 

Volume (cubic feetXlOD) 

Altitude Interval (foot above sea level) 

Interval Total 

Augur Falls to Perrine Memorial Bridge (mile 175-180) 

3,100-3,200 _____________________________ _ 3,200-3,400 _____________________________ _ 3,400-3,600 _____________________________ _ 3. 0 ) 14. 2 

20. 0 

Perrine Memorial Bridge to Shoshone Falls (mile 18Q-183) 

3,150-3,300 _____________________________ _ 3,300-3,400 _____________________________ _ 3,400-3,500 _____________________________ _ 3,500-3,600 _____________________________ _ 3,600-3,700 _____________________________ _ 

Shoshone Falls to Twin Falls (mile 183-186) 

3,350-3,400 _____________________________ _ 3,400-3,500 _____________________________ _ 3,500-3,600 _____________________________ _ 3,600-3,750 _____________________________ _ 

Twin FaUsto Hansen Bridge (mile 186-189) 

3,500-3,600 _____________________________ _ 3,600-3,700 _____________________________ _ 3,700-3,800 _____________________________ _ 3,800-3,900 _____________________________ _ 

1.9} 1.9 

2. 4 

2. 8 

2. 9 

0. 1. 3 1 l 

1.8 

3. 3 

0. 6 } 1.4 

1.4 

1.5 

37.2 

11. 9 

6. 5 

4. 9 

Total______________________________________ 6Q 5 

cient to contain all the floodwater. Thus, I calculate 

lower and upper limits of flood erosion, as follows. 

An n.verage cross-sectional area of the original canyon, based on an area of 160,000 square feet at mile 189 

(fig. 13, section cl-ef') and an area of 490,000 square 

feet at mile 172 (section Il-Il'), may have been about 

320,000 square feet. Such a canyon 14 miles long would 

have had a volume of 24 billion cubic feet, and the infet-red lower limit of flood erosion would have been 36 

billion cubic feet, or one-fourth of a cubic mile. For the 

upper limit of erosion, an original canyon large enough 

to carry :the flood (probable discharge of 15 ·million cfs) 

at a. gradient~ of 27 feet per mile may have been 300 feet 

deep and would have had a trapezoidal cross-sectional 

area. of 150,000 square feet, if a. roughness coefficient of 

0.03 is assumed. This smaller canyon would have had a 

volume of 11 billion cubic feet, and flood erosion would 

have amounted to about 50 billion cubic feet, or onethird of a cubic mile. 

'Vhen solid rock is broken into gravel, a volume inerease occurs because of increased porosity. If a porosity 

of 25 percent is assumed for l\{elon Gravel (Manger, 

HW3, p. E41), 36 billion cubic feet of basalt would yield 

48 billion cubic feet of gravel, or one-third of a cubic 

mile; and 50 billion cubic feet of basalt would· yield 67 

billion cubic feet of gravel, or nearly half a cubic mile. 

The volume of gravel actually found as far downstream as Grand View is estimated to be 84 billion cubic 

feet, or nearly six-tenths of a cubic mile (table 4). Thus, 

the enlarged canyon segment near Twin Falls, whatever is assumed about its original size, appears to have 

produced the major part of the l\{elon Gravel. Furthermore, if gravel below Crane Falls is ignored-because 

it was evidently derived from nearby outcrops-the 

canyon near Twin Falls takes on added importance as 

the major source of l\{elon Gravel. The excess of gravel 

over what can be attributed to erosion near Twin Falls 

probably came partly from sources still father upstream 

but chiefly from constricted places along the canyon 

downstream, as will now be discussed. 

TABLE 4.-Volume of Melon Gravel above Grand View 

Canyon segment 

Perrine Memorial Bridge to Melon Valley_ Melon Valley ________________________ _ 

Melon Valley to Hagerman Valley _____ _ Hagerman Valley ____________________ _ Bliss section _________________________ _ 

King Hill basin: 

Pasadena Valley _________________ _ South of river_ __________________ _ East of river ____________________ _ 

North of river ___________________ _ Near Sugar BowL _______________ _ 

Glenns Ferry basin __________________ _ Hammett basin ______________________ _ Indian Cove ________________________ _ 

Eagle Cove _________________________ _ 

Crane Falls to Grand View ____________ _ 

Miles along river 

180-163 

163-159 

159-150 

150-141 

141-121 

121-118 

118-109 

118-115 

115-111 

111-109 

109-100 

99-93 

91-86 

83-80 

74-58 

Volume 

(cubic 

rcctXlOD) 

1.4 

2. 9 

1.0 

7. 3 

5.!) 

6. 3 

26. 5 

3. 9 

7. 5 

1.9 

7. 2 

4. 1 

1.4 

.2 

6. 4 

TotaL___________________________________ 83. 9 

The distribution of Melon Gravel below Twin Falls, 

represented graphically in figure 24, together with the 

occurrence of boulders, gives clues for evaluating the 

canyon as a source of flood debris. Although some gravel 

was lodged in upstream basins such as Melon Valley 

and Hagerman Valley and a little was distributed along 

constricted sections, major amounts were not deposited 

until the debris reached the first stretch of ponded 

water, namely the J(ing Hill basin. Because large 

boulders were rapidly reduced during passage downstream and because the existing boulders came fronf 

nearby outcrops, 1nuch of the material eroded at Twin 

Falls must be represented by sand. Most of the sand 

probably was swept into the IGng Hill basin, and diminishing amounts settled out in the string of coves downstream. The bouldery component of the gravel, which 

is conspicuous in the upper part of the canyon and 

along certain constricted places, therefore can be attributed mainly to local erosion along the canyon. Boulders 

constitute about 20 percent of the flood debris and imply 
CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

:a 

llll 

= 

~ 

I 

= 

~ 

~ llll 

~ 

:r: 

I 

80 110 140 170 180 

MILES UPSTREAM FROM HOT SPRINGS FERRY 

FIGURE 24.-Graph showing volume of Melon Gravel along Snake 

River from Twin Falls to Grand View. 

an equivalent volume of canyon erosion. Moreover, because the Twin Falls segment fails to account for all 

the remaining nonbouldery component of Melon 

Gravel-unless the calculated upper limit of erosion at 

Twin Falls applies- some additional canyon erosion beyond that indicated by the boulders must have occurred. 

Despite the amount of inferred canyon erosion, however, simple arithmetic shows that enlargement at any 

particular segment of the many miles of canyon below 

Twin Falls must have been small. During passage of the 

Bonneville Flood, the canyon physiography probably 

was modified more importantly by deposition of Melon 

Gravel. In short, even though the canyon below Twin 

Falls was somewhat enlarged by flood erosion, it probably closely resembles its aspect just prior to the Bonneville Flood. 

DISCHARGE AS INDICATED BY SIZE OF BOULDERS 

Hydraulic engineers distinguish between two kinds of 

sediment mov~ment by streams: the "bedload" consisting of larger particles that slide and roll along the bed 

and the "suspended load" consisting of smaller particles 

transported in suspension. The proportion between .these 

sediment components depends on the velocity of flow 

and on grain size. If the velocity does not exceed a certain small value, all the sediment moves in contact with 

the bed, whereas a sufficiently high velocity may cause 

even the largest particles to move in suspension. If 

sediment of mixed sizes is supplied to a stream, the 

coarse particles ordinarily will move along the bed, 

whereas the finer particles flow past; below a critical 

traction velocity, the coarse particles cease to move. The 

larger particles along the bed of a stream are therefore 

indicators of former maximum velocities of flow. Bruun 

(1962, p. 43-67) summarized what is known about sediment transport in alluvial channels. 

Observations of traction velocities generally have depended on measurement of sand and fine gravel in laboratory flumes and in some large waterways. No experiments have been made with material as large as some 

of the boulders in the Melon Gravel nor have there been 

any experiments made with material as poorly sorted. 

T~e largest pieces moved by streams for which data are 

available measure less than 2 feet. (For data on weight 

of stones used in constructing coffer dams in flowing 

water, see Torpen, 1956, p. 61.) Any calculation of velocities based on larger particles is therefore a considerable extrapolation. For example, Trimble and Carr 

(196la, p. B166) estimated that 8-foot boulders found at 

Pocatello indicate a velocity for the Bonneville Flood 

of from 16 to 48 mph (miles per hour) or 24-70 fps. 

Velocities necessary to move boulders found in the 

Melon Gravel have been computed in tables 5 and 6 by 

use of two formulas that give somewhat different values. 

The U.S. Bureau of Reclamation (USBR) formula, 

which relates bedload velocity to grain size (Peterka 

and others, 1956) is based on a study by Berry (1D48), 

who summarized numerous reports of the transport 

TABLE 5.-Velocities and discharges indicated by size of boulders in 

Melon Gravel just below canyon constrictions 

s ~ 

5 ~ 8 .... 

·~ § "' '5 0,-... 

~~ ~ - ~ "''"' d:;;' .... ax oo Cl) 

Location :9 ~ ~ -=-~ -c'"' ::s ~ 't;: "'x ~g gj.!!l .... 0 en Cl) 

Cl) .t:J e. e 0'-" .t:J~ I> s ~d Clll>. ·;::: 

~ I» uo ~:.:: d ::s ~~ 

.§ ~~ ~u 

0 ·s ~-~:: ..=o 

~ ~ ..9 ~§ 8~ .~~ ~ ~ Cl) 

::s > > C,) ~ 

----------

Upper Melon Valley __________ 162 4 18 17 1.0 17 

Banbury Hot Springs _________ 158 3 15 15 .8 12 

Upper Hagerman Valley ______ 150 5 20 19 1.0 19 

Bliss __________ - - - - - - - - - - - - - - 136 8 25 22 .6 13 

Swiss Valley _________________ 124 10 28 24 .6 14 

1 U.S. Bureau of Reclamation. 

TABLE 6.-V elocities indicated by size of boulders in Melon Gravel 

where floodwater was not constricted 

Maximum Velocity Velocity 

Location Mile on boulder (U.S. Bur. (Nevinriver size (ft) Reclama- Hjulstrom) 

Eden ________________________ _ 

Bancroft Springs_______ 120 

King Hill basin_ _ _ _ _ _ _ _ 117 

Glenns Ferry__________ 107 

C.J.StrikeDam_______ 60 

Walters Ferry__________ 9 

Givens Hot Springs_____ 0 

10 

12 

2 

1 

4 

3 

Yz 

tion) (fps) (fps) 

28 

31 

13 

9 

18 

1.5 

6 

24 

26 

13 

10 

17 

15 

8 

_. 
COMP.ARISON WITH HISTORIC FLOODS 47 

power of streams, including one on the movement of 

particles as much as 6 inches in diameter. The formula 

can be expressed as V b = 9 X d1

12 , where V b is the bedload velocity in feet per second and the diameter, d, is in 

feet. The Nevin-l-Ijulstrom formula is derived from a 

plot of data by these authors, reviewed by Fa,hnestock 

(1963, fig. 30) and expressed as Vt=10X d112 '

6 , where Vt 

is the traction velocity in feet per second and the diameter, d, is in feet. The particle size in the USBR for-

. mula depends on the square of the velocity (the "sixth 

power law"), but the particle size in the Nevin-Hjulstrom formula varies as the 2.6 power of the velocity. 

Thus, the Nevin-Hjulstrom formula attributes a greater 

competency to streams than the sixth-power law would 

predict. 

The velocities c-alculated by both these formulas may 

be larger than was actually required to move boulders 

found in the Melon Gravel. For instance, in a bouldery 

glacial stream, Fahnestock ( 1963, p. A30) caught some 

stones 1.8 feet in diameter . that were moved hy water 

flowing 7 fps. Also, in flume experiments, a flow of 4 fps . 

was sufficient to move stones about half a foot in diameter a.long a bed of sand (Fahnestock and Haushild, 

1962). 

·Movement of large pieces at unexpectedly low velocities may be a consequence of mixed sizes in the stream 

load. The water of flooded streams heavily laden with 

sand is relatively dense, and its transport capacity is 

greatly increased (Segerstrom, 1950, p.108-113; fig. 54). 

The Snake River during the Bonneville Flood evidently 

was loaded with sand, and this burden may have increased its capacity to move boulders. In less turgid 

streams, fine particles fill irregularities on the bed and 

pennit larger particles to move 1nore readily. For s~wh 

streams, Fahnestock ( 1963, p. A30) points out that "one 

can expect * * * erosion [that is, movement] of particles larger than those predicted by formulas based on 

data for uniform materials." However, stones move 

along rocky glacial streams as easily as along sandy 

streams (Fahnestock and Haushild, 1962). 

If the height of floodwater is known, flow velocities 

indicated by hu·ge boulders deposited just below canyon 

constrictions should give a measure of discharge. At 

places in the upper part of the Snake River canyon 

where the flood profile can be reconstructed from geologic evidence and constricted sections can be measured, 

the flow velocities indicated by boulders suggest discharges comparable to the canyon capacity computed 

near Swan Falls (table 5). Boulders in Melon Gravel 

therefore seem to be rather accurate indicators of velocity of flow. This result is of interest because unexpectedly large boulders are found at some wide places 

along the canyon (table 6). If these boulders indicated 

average velocities through the wide sections during 

highest flood stage, very large discharges would be 

demonstrated. Such aberrant boulders occur in isolated 

groups strung out along bars and evidently represent 

zones of fast currents that swept large stones into basins-not high average velocities. The rapid reduction 

in particle size commonly observed elsewhere in such 

basins, as at King Hill and in the upper reach of the 

Grand View basin, probably can be considered as an 

accurate 1nodel of retarded average flow . 

Finally, the structural habit of the local basalt imposes restrictions on inferences drawn from sizes of 

transported debris. If the dimensions of basalt columns 

and the size of talus blocks limited the size of flood 

debris, no boulders la.rger than about 5 feet would be 

found, regardless of the power of the flood to 1nove 

bigger pieces. However, the surfaces of lava flows, 

heaved locally into pressure ridges of coherent blocks 

about 15 feet across, also supplied material for transport. Such lava flows were washed by the flood in the 

Rupert channel (near Eden) and on the canyon rim 

above Bancroft Springs (mile 120), and they exhibit a 

few rolled boulders nea.rly as large as the available 

blocks. vVithin the canyon, on the other hand, close 

spacing of columnar joints mainly determined the size 

of boulders-except for a few large talus blocks dislodged from lava surfaces. Thus, the size of boulders 

may indicate only a minimum velocity of flow. For example, tremendous average flood velocities at Thousand 

Springs and Swan Falls-which are respectively calculated as 34 and 37 fps from the cross-sectional areasimpinged on columnar-jointed basalt and are underrated by the size of debris found immediately 

downstream. 

COMPARISON WITH HISTORIC FLOODS 

The Bonneville Flood so greatly exceeded the 

ordinary experience of river behavior that it can be 

compared only with catastrophic floods, particularly 

those caused by failure of reservoirs or by rupture of 

natural dams of earth and ice. Such floods have produced· 

brief discharges about as large as the Bonneville Flood, 

but none have lasted as long. A few historic floods of 

this kind are summarized here. (See also, IIutchinson, 

1957, p. 42-56.; and Barrows, 1948, p. 136-137.) 

RESERVOIR F AlLURES 

I 

GOHNA LAKE, INDJ!A 

In early September 1893, landslides at Gohna, India, 

blocked the river Bireh Ganga, a tributary of the 

Ganges, and formed a lake more than 800 feet deep 

(Holland, 1894). The Indian Corps of Engineers took 

charge, predicted the probable time of overflow, and 
48 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

established a warning system to prevent loss of life 

(Strachey, 1894). Rupture occurred about a year later, 

August 25, 1894, when the reservoir contained 161;2 

billion cubic feet of water (one-ninth of a cubic mile). 

Observers trying to witness the actual burst were frustrated by rain and darkness (Lubbock, 1894), but 41;2 

hours later the water level had fallen 390 feet and the 

lake had discharged more than 10 billion cubic feet. 

Contemporary accounts (Glass, 1896; Frizell, 1901) 

accurately recorded the progress of the flood wave as 

given in table 7. It appears that, although the average 

discharge during the first 41;2-hour period was 620,000 

·cfs, the initial flow must have been much greater, 

undoubtedly several million cubic feet per second. The 

computed flow velocity, even 150 miles downstream, was 

c01nparable to estimated velocities of the Bonneville 

Flood. No information is available concerning the size 

of material moved by the Gohna flood. 

TABLE 7.-Flood wave on the Ganges caused by rupture of landslide 

dam at Gohna, India 

[Modified from Barrows, 1948, p. 7] 

Maxi- Beginning of rise Maximum rise mum ---------- rise 

Distance from above Feet Feet 

reservoir (miles) normal Elapsed Miles per Elapsed Miles per river time per sec- time per sec- (feet) hour ond hour ond 

--------- -- Just below 

reservior ______ 260 13 _____________ 160 ------- ---- ---- 25m 31 45 20 _____________ ------ 30 _____________ 47m 25 37 -------· ----- ---- 130 lh20m 22 32 2hl5m 13 19 51 _____________ 140 2h30m 20 29 4h15m 12 18 72 _____________ 42 3h45m 19 28 5hlOm 14 20 150 ____________ 11 9h15m 16 23 12h 12Yz 18 

THE GREAT INDUS FLOOD~ INDIA. 

In the winter of 1840-41, part of the Lechar spur of 

Nanga Parbat collapsed into the Indus River. This dam 

impounded a temporary lake 1,000 feet deep and nearly 

40 miles long. Early in June the barrier was overtopped, 

and in 24 hours the lake emptied, sweeping everything 

before it. The meager records of this great flood have 

been compiled by Mason (1929, p. 15-17). At Attock, 

260 miles downstream, the water rose about 100 feet, 

with an immediate rise of nearly 80 feet. A Sikh army 

encamped close to the river near Attock was overwhelmed by an "absolute wall of mud." Mason quotes 

from an eyewitness account, 

It was a horrible mess of foul water, carcasses of soldiers, 

peasants, war-steeds, camels, prostitutes, tents, mules, asses, 

trees, and household furniture, in short every item of existence 

jumbled together in one flood of ruin * "' *. As a woman with 

a wet towel sweeps away a legion of ants, so the river blotted out 

the army of the Raja. 

The actual volume of water discharged by the Great 

Indus Flood is not known, but Mason gives dimensions 

of the barrier that would indicate a volume of at least 

40 billion cubic feet (more than one-fourth cubic mile). 

The release of this volume within 24 hours would indicate an average discharge of 500,000 cfs, but the height 

of the flood wave at Attock suggests an initial discharge 

several times greater. 

FLOOD AT GROS VENTRE, WYOMING 

On June 23, 1925, a mass of rock loosened by heavy 

rains slid into the Gros Ventre River valley, "\Vyoming; 

and formed a dam from 225 to 250 feet high , (Alden, 

1928). A lake grew rapidly behind the barrier, but overflow did not occur until May 18, 1927. Erosion at the 

dam rapidly lowered the lake level 50 feet and released 

1.9 billion cubic feet of water. Limestone blocks 15-20 

feet in diameter were moved some distance below the 

dam. The village of J{elly, 4 miles downstream, was 

overrun by a wall of water 15 feet high, and the flats 

below J{elly were flooded to a depth of 10-14 feet. Five 

hours after overflow the flood had passed, and the stream 

had receded within its ba.nks. The flood was felt the next 

day as far downstream a.s Idaho Fa.lls, about 150 ·miles 

away, where the discharge was 60,000 cfs (U.S. Geological Survey, 1959, p. 31, data at Heise). 

LAKE IssryK, U.S.S.R. 

On July 8, 1963, a Inudflow rushed down a precipitous valley on the north side of the Transilian Alatau 

and entered Lake Issyk (lat 43° N., long 77° E.), a body 

of water 1.5 kilometers wide, 1.8 kilometers long, and 

50 meters deep ( Gerasimov, 1965). High waves produced by the violent mudflow surged over the lake's barrier, a landslide dam, and caused rapid erosion. Huge 

blocks of rock began to move, and material from the 

dam poured down the Issyk valley. The lowland at the 

foothills was transformed beyond recognition by deposition of a wide uneven boulder-covered field, and a 

terrace 20 meters above the original valley floor was 

destroyed. Passage of this debris was accompanied by 

erosion along the valley walls; bedrock formerly concealed by alluvium was thereby e:x;posed. The catastrophic outflo'w continued more than 5 hours, and the 

lake drained completely in 24 hours. At the end, an outlet channel 70 meters deep, 100 meters wide, and 800 

meters 'long had been cut through the landslide dam. 

ST. FR.ANOLS D-AM, C~IFORNIA. 

St. Francis Dam, a structure 205 feet high about 45 

miles north of Los Angeles, failed suddenly about midnight March 12-13, 1928, and released 1.6 billion cubic 

feet of water at a maximum rate of from 400,000 to 
COMP.A-RISON WITH HISTORIC FLOODS 49 

500,000 cfs. (For a popular account of this disaster, see 

Outland, 1963.) The failure of the dam prompted 

intensive studies by geologists and engineers. The 

flood wa.ve moved rapidly downstream (table 8) 

at n, speed comparable to the inferred velocity 

of the Bonneville Flood. Exceptionally large velocities just below the broken dam are suggested 

by transport:. of colossal concrete blocks s01ne 

distance downstream. For example, a block weighing 

10,000 tons was washed 3,000 feet below the dam, and 

smaller pieces of the dan1 were carried almost a mile. 

Bowers ( 1928, fig. 4) shows a map of the dislodged 

blocks and reconstructs their original position in the 

dismembered structure. To a person standing among 

these blocks, the awesome power of such rapidly moving 

water would seem invincible. 

TADIJE B.-Progress of flood wave from St. Francis Dam, Calif. 

[Modified from Barrows, 1948, p. 138] 

Velocity Elapsed 

time from Distance 

Location Time preceding (miles) Miles Feet 

station per per 

(minutes) hour second 

- ----- At dam _____________ 11:58 p.m. ------- ------ ------ ---- City Power House 2 __ 12:03 n.m. 5 1.5 18. 0 26 

Southern California 

Edison Co. sub- station ____________ 12:38 a.m. 35 7. 5 12. 9 19 

Southern California 

lTidison Co. con- struction camp _____ 1:20 a.m. 42 7. 5 10. 7 16 

Fillmore Bridge ______ 2:25a.m. 65 12. 7 11. 7 17 

VAYONT RESERVOIR, ITALY 

On October 10, 1963, after 2 weeks of heavy rain, a 

mass of rock exceeding 240 million cubic meters (8.5 

billion cu ft) slid violently into the reservoir of Vaiont 

Dam, Italy, which then held 135 million cubic meters 

of water-that is, 4.2 billion cubic feet (!Gersch, 1964). 

The displaced water swept over both abutments to a 

height of about 100 meters (without important damage 

to the actual structure) and claimed nearly 2,600 lives 

in its disastrous passage downstream.l\foving at a speed 

of about 60 miles per hour, the flood wave was more than 

70 meters high when it reached the Piave VaHey, 1 mile 

away, and it struck the village of Longarone head on. 

Floodwn.ter that continued down the Piave Valley destroyed other villages, and .the wave that surged up the 

valley caused further wreckage and loss of life. Moments later-less than 15 minutes-"the flood waters 

had receded and n.U was quiet in the valley." The rueful 

lesson taught by this tragedy, the worst dam disaster 

in history, is that the threat of landslide was brought 

about by several adverse geologic factors, in part augmented by hydrologic effects induced by the impounded 

water, and that these unfavorable conditions could have 

been predicted by prior geologic study. Even more 

tragic, movement of rock was detected 3 weeks before 

the collapse, but no one anticipated its ultimate 

magnitude. 

FLOODS ASSOCIATED WITH GLACIERS 

JOKULH!LAUPS IN IC:aLAND 

The subglacial volcanic eruptions of Iceland are accompanied by floods of large magnitude called glacier 

bursts (Icelandic jokulhlaup). Similar floods also occur 

by sudden release of subglacial lakes, which store water 

generated slowly by volcanic heat. The most active subglacial volcanoes are in southern Iceland, and their intennittent jokulhlaups release enonnous floods onto 

boulder-strewn outwash plains ( sandnrs) tha.t lie almig 

the coast. (In a lucid display of arn1chair sleuthing, 

Warren ( 1965) attributes an anomalous scabland tract 

described by Smith (1965) in Wright Dry Valley, 

Antarctica, to a catastrophic flood produced by an ancient volcanic eruption under thick ice, but scabland on 

this scale has not yet been found in areas of jokulhlaups 

in Iceland.) 

Discharges and volumes of jokulhlaups have been 

measured accurately during the last few decades (fig. 

25), and records by eyewitnesses extend back several 

centuries. None of the measured discharges has yet 

matched the size of some jokulhlaups estimated from 

early observations. Thus, Grimsvotn, a caldera depression under western Vatnajokull, has discharged at a 

maximum recorded rate of 50,000 cubic meters per second (134 1nillion cfs), but a jokulhlaup caused by the 

1918 eruption of l(atla, a volcano buried by the Myrdalsjokull, reached a maximum discharge of at least 

100,000 cubic meters per second (31h million cfs) 

(Thorarinsson, 1957). The 1918 jokulhlaup from Katla 

has been estimated as high as from 300,000 to 400,000 

cubic meters per second (10-14 million cfs), an amount 

comparable to the computed peak discharge of the Bonneville Flood. 

The large differences in volumes of jokulhlaups 

shown in figure 25 range from 28 million cuhic 1neters 

(one billion cu ft) to about 7 billion cubic meters (250 

billion cu ft) and are due to various capacities of the 

subglacial reservoirs. The Grimsvotn caldera is a large 

depression capable of holding most of the water released 

during eruptions. It currently fills at the rate of 0.6 

cubic kilometers per year, and its recent glacier bursts 

have been at intervals of 4-6 years (Thorarinsson, 1960, 

p. 43) . Lake Graenalon, a deep basin dammed by ice at 

the southeast margin of Vatnajokull, also has a large 
50 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

FIGURE 25.-Discharge graph of some glacier bursts recorded 

in Iceland. (Modified from Thorarinsson, 1957, fig. 2.) A, 

Grimsvotn, March 22-April 1, 1934, total di1scharge --7 X lOb 

m3 ; B, .Graenal6n, J.uly 23-August 2, 1939, 1.5Xl09 m3 ; 0, 

Grfmsvotn, July 4-22, 1954, 3.5Xl09 m3 ; D, Katla, June 25, 

1955, 28 X 106 m3

• 

capacity. The subglacial reservoir of Katla, however, is 

small (Thorarinsson, 1957). 

The different rates of discharge shown in figure 25 

are related to constrasting slopes down which the jokullhlaups must descend while forcing a way under the ice. 

Thus, the slope from the storage area to the outlet is 

1: 38 for Grimsvotn, 1:40 for Graenalon, and 1: 16 for 

l{atla. Discharge rates for Grimsvotn and Graenal6n, 

which have similar slopes, are therefore nearly alike; 

whereas the maximum discharge for Katla, which has 

a steeper slope, is reached very quickly, within an hour. 

Jokulhlaups carry rocks of extraordinary size far 

onto the sandur plains. Published photographs of the 

sandurs show a gravelly plain strewn with subrounded 

boulders several feet in diameter (Thorarinsson, 1958, 

figs. 5, 13). Blocks from Oraefajokull, southern Vatnajokull, which were carried by a jokulhlaup in 1362~ 

project 4-5 meters above a sandur plain 4 kilometers 

from the point of discharge ( Thorarinsson, 1958, p. 34). 

The largest piece found in this area has a base measuring 50 square meters and weighs more than 5,000 

tons. Thorarinsson's map (1958, pl. 1) shows that the 

gradient on the plain at this place is about 7 percent. 

A tuff-breccia block of about 400 cubic meters lying on 

the lVI~rrdalssandur 14 kilometers from the glacier margin was washed out by the 1918 jokulhlaup from Katla 

( Thorarinsson, 1960, p. 43). 

Sedimentation on a sandur plain in the Hornafjordur 

area at the east margin of Vatnajokull has been studied 

in some detail by Hjulstrom (1952). Near the glacier 

margin, where the gradient is 13-14 percent, the sediment consists of poorly sorted rounded boulders as 

much as 2 feet in diameter. Large pieces are found as 

far as 3 kilometers from the glacier, where the gradient 

is reduced to 4-5 percent, although the average size of 

particles decreases rapidly from head to toe. The particle size also diminishes from the surface downward. 

Hjulstrom attributes the wide range of sizes partly to 

deposition by jokulhlaups and describes the sandur 

plain as a "supra-aquatic delta" in which material is 

deposited simultaneously everywhere on the plain (fig. 

26). From a quantitative study of discharge, however, 

Krigstrom (1962, p. 341-342) concludes that the Hoffellssandur to the east of Vatnajokull is never completely submerged, even during glacier bursts. Some 

idea of the rapidity of accumulation of such sandur 

deposits is given by Thorarinsson (1958, p. 26) from an 

eyewitness account of the 1362 eruption of Oraefajokull : a glacier burst carried into the sea "such quantities of rocks, gravel and mud as to form a sandur plain 

where there had previously been thirty fathoms of 

water." Such descriptions evoke an image of the Melon 

Gravel. 

VA-RIOUS :teE-DAMMED LAKES 

Of the many glacier-dammed lakes that occasionally 

discharge catastrophic floods (Hutchii1son, 1957, p. 

50-5·6), the Marj elensee, a lake dammed by a branch of 

the Aletsch Glacier in Switzerland, is a classic example. 

In July 1872, this lake catastrophically released 10.7 

million cubic meters of water; in July 1892, 7.5 million; and in July 1913, 3.1 million (that is, 380 million cu ft, 260 million, and 110 million, respectively) .. 

These floods transported great amounts of sediment 

(Liitschg, 1915). 

In western North America the more than 50 icedammed lakes in Alaska and British Columbia are remote and difficult to study, but catastrophic outbursts 

from some of them are known to have had disastrous 

consequences for life and property. Marcus (1960) reviews current knowledge of these glacial floods and 

gives a detailed history of one ice-dammed lake, Tulsequah, which lies on the east edge of the Juneau Icefield. Almost annually, during at least the past 50 years, 

Lake Tulsequah has drained suddenly by failure of its 

FIGURE 26.-Section of a supra-aquatic delta of sandur type 

compared with section of ordinary subaquatic delta. (Reproduced from Hjulstrom, 1952, fig. 8.) 
REFERENCES CITED 51 

glacier dam. In 1958, the lake released 230 million cubic 

meters of water (8 billion cu ft.), most of it in 2 days, 

which discharged at a maximum rate of more than 

1,500 m3 per second ( 55,000 cfs). The periodic glacial 

bursts from Lake Tulsequah are trivial in comparison 

with the Bonneville Flood, but they are nonetheless of 

grn ve concern to the imperiled residents downstream. 

SUMMARY OF HISTORIC FLOODS 

The foregoing exan1ples of catastrophic floods dra~ 

matically illustrate the havoc cnused by large volumes 

of water released suddenly. Some of these historic floods 

1nay have discharged for short periods at rates com~ 

paraJble to ~the sustained discharge of the Bonneville 

Flood, and all of them attained high velocities. They 

therefore transported debris of impressive size and 

quantity. None of them, however, was nearly as large in 

volume as the Bonneville Flood. The largest, the 1934 

jokulhlaup from Gr:hnsvotn in Iceland (volume about 

1.7 cubic 1niles) was less than one two~ hundredth of the 

possible volume of the Bonneville Flood; namely, 380 

cttbic miles. These floods m1doubtedly would have 

caused spectacular erosion if the avn.ilable volumes of 

floodwater had been larger and if the floods had corre~ 

spondingly lasted longer. 

These historic floods also provide obvious lessons for 

evaluating the hydraulic behavior of large volumes of 

rapidly n1oving water. Such infonnation is of value to 

engineers. Conversely, the hydraulic behavior of the 

Bonneville Flood, as displayed by its geologic record, 

tells 1nu.ch ·about conditions required to produce cata~ 

strophic geologic effects of lesser magnitude. The sandur 

streams of Iceland, for instance, flow in a braided net~ 

work ·runong bars that resenrble diminutive versions of 

ba1~ of Melon Gravel. The discharge and water depth 

inferred for the Bonneville Flood, if appropriately reduced in scale, therefore can give some idea about floods 

responsible ·for the sandur plains. Lastly, the Bonneville 

Flood caused substantial erosion, notably where floodwater ·that de;bouched over jointed rimrock produced 

turbulent eddies. Such erosion, and the resulting piles of 

deJbris downstream, can be anticipated wherever floodwater is deep enough to submerge large irregularities in 

the flood path. 

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52 CATASTROPHIC PLEISTOCENE BONNEVILLE FLOOD, SNAKE RIVER PLAIN 

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