Downloaded from gsabulletin.gsapubs.org on March 27, 2010 Geological Society of America Bulletin EVIDENCE OF LAKE BONNEVILLE FLOOD ALONG SNAKE RIVER BELOW KING HILL, IDAHO HAROLD T STEARNS Geological Society of America Bulletin 1962;73;385-388 doi: 10.1130/0016-7606(1962)73[385:EOLBFA]2.0.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. Downloaded from gsabulletin.gsapubs.org on March 27, 2010 HAROLD T. STEARNS EVIDENCE OF LAKE BONNEVILLE FLOOD ALONG SNAKE RIVER BELOW KING HILL, IDAHO Abstract: A catastrophic flood, probably exceeding flowed banks at bends and constrictions, scoured 10 million cfs, was released down the Portneuf deep channels (many of which are now abandoned), River at Red Rock Pass, Idaho, when ancient Lake and left extensive high bars of sand, gravel, and Bonneville broke its natural dam in Wisconsin boulders. The flood was at least 410 feet deep at time. The flood entered the Snake River and over- Brownlee damsite near Homestead, Oregon. Pleistocene Lake Bonneville had its only out- a hypothetical lava dam upstream. Subse- let through Red Rock Pass near Preston, Idaho. quently, he searched for the lava dam and Gilbert (1890, p. 177) described the effects of never found it; hence he concluded that the the resulting great flood near McCammon, gravel must have been laid down by the Idaho. Malde (1960, p. B295) has described Bonneville flood. evidence of the flood between Pocatello and A spectacular deposit of huge boulders and Twin Falls. The flood was of such great thick loose sand and gravel lies near the Guffey volume that, at bends and restrictions in the bridge on the road to Murphy and is well ex- Snake River Canyon, it would overflow the posed in borrow pits. The sand is foreset- bank and shortcut across the plain creating bedded against the north canyon wall and spectacular bars and plunge pool holes where much higher than the boulder bar. This is a the overflow waters returned to the canyon. puzzling relationship until it is realized that the Opposite King Hill is a large bar, and another bar was reworked in the channel as the flood remnant of a bar lies upstream at the bend in receded while the sand remained as a high the river. The flood overtopped the canyon terrace in a re-entrant where the receding rim at the start of the bend and gouged Pasa- water could not reach it. Subsequent floods dena Valley in the weak Pliocene Hagerman also probably reworked the boulder bar. Formation. An undrained depression contain- During excavation for the Brownlee Dam ing a pond occupies an abandoned plunge pool 16 miles above Homestead, Oregon, loose in Pasadena Valley, and downstream from the water-laid sand was found in a small re-entrant valley is a voluminous deposit of sand and 290 feet above the river at an altitude of 2090 gravel, one of the many large bars left in the feet. The canyon is 1400 feet wide at this ele- wake of the flood. This deposit, like many of vation, and bedrock lies at 1680 feet. Assuming the others, is a valuable source of sand and the flood scoured to bedrock, the water was at gravel. least 410 feet deep and must have been of the The flood exceeded the capacity of the big order of 10 million cubic feet per second. bend in Snake River canyon south of Moun- Where the canyon widened at the mouth of tain Home even though the canyon is 210 feet Brownlee Creek, 2 miles upstream from the deep and 500 feet wide at the lava-rock con- dam, a large bar of loose sand was deposited by striction 2 miles downstream of the Loveridge the flood. The sand was used as a borrow pit Bridge on Snake River. The excess water by- for the dam, and a few bones of Pleistocene passed the bend, flowed across the desert under- animals were found in the bar. Near Twin lain by the weak Hagerman Formation, and Falls, Minidoka, and American Falls bones of cascaded back into the Snake River canyon at extinct elephants, bison, horses, etc., are com- the mouth of the Bruneau River. Here it over- mon in the older alluvium (Stearns et al., 1938, topped the opposite bank of Snake River valley p. 92). It is possible that many animals were and dug a plunge-pool hole and built a vol- drowned in the flood. uminous bar of unusual gravel. The writer de- Near the Oxbow Dam, where Snake River scribed this deposit (Stearns, 1952) and errone- has an altitude of 1700 feet, a terrace on the ously attributed its origin to the breaking of Idaho bank stands 150 feet above the river. Geological Society of America Bulletin, v. 73, p. 385-388, March 1962 385 Downloaded from gsabulletin.gsapubs.org on March 27, 2010 386 H. T. STEARNS—EVIDENCE OF LAKE BONNEVILLE FLOOD, IDAHO The terrace was covered with large boulders quartz sand which dominated the river-bed but, upon excavation, was found to contain deposits below Weiser. only fine sand and small gravel in foreset The bar remnants indicate that vast quanti- bedding dipping toward the Idaho bank, ap- ties of sand and gravel have been carried to parently all that remains of a former vol- the Columbia Basin. Large boulders were con- uminous bar. No positive evidence of over- centrated, while fines were washed away. How- flow was found in the road-pass notch at an ever, the narrow canyon above Twin Falls is 9 altitude of 1969 feet in the Oxbow Ridge al- miles long, and some of the longer coves prob- though a deep plunge-pool hole was found by ably were not cut entirely by the great flood. drilling directly below the notch on the down- Doubtless each spring meltwaters from the stream side at the site of the former power wasting glaciers in the Centennial and Teton house. If the flood was as high at this point as ranges, plus the overflow from Lake Bonne- at Brownlee Dam, water may have spilled ville, increased Snake River to many times its through the notch. However, the plunge-pool present peak flood runoff which was 100,000 hole lies at the intersection of two major faults, c.f.s. on March 3, 1910, at Weiser, Idaho. It hence the hole could have been cut into the was higher in 1894, but the flow was not weak fault breccia by the flood in the main measured. canyon. During excavation for the Oxbow Thus, each summer, floods of great magni- surge tanks, a fault crack was encountered tude carried away more and more of the loose which contained 2 feet of bedded loose sand debris from the original flood bars and channel and gravel in its bottom between elevation fills and rapidly cut the narrow gorge which 1828 and 1830. The top of the crack was 195 now runs through miles of hard but highly feet above the river and indicates that the fractured basalt from Shoshone Falls to Milner flood reached this height at this point. Drill Dam (Malde, p. B296). holes indicated bedrock to be 90 feet below Bonneville Lake had at least two high stages, Snake River near the terrace described above. and probably three. The first, or Alpine Stage, It is assumed that a flood of such magnitude is pre-Wisconsin in age. It rose to elevation scoured to bedrock. If so, the water was at least 5050 but did not reach the Red Rock Pass 285 feet deep at this place at the time of the level. The second lake, in the Wisconsin flood. Epoch, reached elevation 5135 and spilled The next extensive deposit downstream, of through the pass and cut it down about 315 lag boulders and sand, lies at Big Bar about 15 feet. The reduction of the spillway caused the miles below Homestead where the canyon lake to fall to elevation 4820, known as the widened and the flood lost velocity. The bar is Provo I Stage (Eardley et al., 1957, p. 1166). a mile long, nearly a mile wide, and rises about Williams (1952, p. 1375) states that the spill- 170 feet above the river. While mapping the way is 400 feet wide at the top and 500 feet geology of the proposed Hells Canyon dam deep.