WALTERS BUTTE SECTION

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 Walters 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 actual extent of floodwater in the Walters basin during the highest stage is poorly determined. The flood passed through a constriction at mile 14 that 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 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 attitude 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 Walters basin is vaguely determined; a basalt wall 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 basaltic 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 21/2 miles downstream is as high as the canyon rim where the sand spills through. It seems likely that all 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 effect 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 7. 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.

Figure 23. – Topographic map of Walters basin, showing features produced by the Bonneville Flood. Floodwater passed a canyon constriction at mile 14 and spread widely, probably surrounding Walters 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. Bouldery debris that washed downstream forms long linear bars separated by shallow channels. These constrictional features trend down 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 the floodwater. The highest boulder deposits occur several miles below the constriction at mile 14. Maximum sizes of boulders decrease downstream, but large stones are found as far downstream as Walters 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.)

Knobs 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 White 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 Walters 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 comparable to a streamlined bar that hangs from a shoulder in the canyon at mile 18. This pendant bar, which evidently accumulated in rapid water, has a closure of 80 feet and a length of half a mile. By this analogy, gravel bars in the Walters basin indicate high flow velocities.

Below Givens Hot Springs the valley widens into a broad lowland 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.