THE 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 feet wide at the rim and 210 feet deep (fig. 16, 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 mouth 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 feet above the river, through which 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 mile 71 to form a deep plunge pool at the toe of a bar of Melon Gravel. Water also spilled northward 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 King Hill basin.

Figure 22 . – Topographic map of the Crane Falls area, showing features produced by 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 scrubland 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 of 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 water 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.)

Below the narrow gap at mile 75, the canyon gradually widens and remains a gorge of solid basalt downstream to Crane Falls (mile 73). This stretch, which must have been swept rather clean by the flood, has no Melon Gravel and practically no talus. Below Crane Falls the canyon widens abruptly to about 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 above 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 miles 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 contributory cause could have been backwater from a temporary lake in the Grandview basin. 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 found 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 a vertical face of basalt that bounds the upstream side of the spillway. This anomalous vertical face reaches a height 300 feet above 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 was washed along the canyon. These relations indicate that floodwater debouching into the canyon at the spillway was about 50 feet deep and had velocity sufficient to accomplish some erosion and to move 5-foot boulders. Moreover, floodwater along the canyon could not have been more than 60 feet below floodwater on the upland.

Owing to ponded water in the Grand View basin downstream, unusual debris deltas were built at the terminus of the Bruneau cutoff (near mile 61, fig. 2). Except for basalt scabland 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.

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).

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 westward (downstream) at the rather low angle of 15'. Sets of inclined beds are continuously exposed in places along the gully throughout a height of 100 feet. Outcrops of basin deposits (Glenns Ferry Formation) that underlie 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 Malde 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 pebbles 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 ordinary 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 feet thick where exposed along the top of the 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 Grand 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.