Turn northwest on State Route 261. Elongate loess 'islands' can be seen to the southwest.

HU Ranch. Roadcuts to the east and west expose the Roza and Frenchman Springs Members of the Wanapum Basalt (Swanson and Wright, 1981).

To the south is a cataract at the head of HU or Davin Canyon (Figs. 6 and 30). Before the Missoula floods, there was probably a small stream here flowing south to the Snake River. When the Missoula floods overtopped the south valley wall of the ancestral Palouse River and crossed the divide to the Snake River, they excavated HU or Davin Canyon. It is likely that during each flood the waterfall at the head of the coulee retreated farther north. Today the cataract is a 'dry falls' (like the classic one between Upper and Lower Grand Coulee) except for a tiny waterfall on the northeast rim. Cataract retreat was a common result of erosion of the Columbia River basalts by the Missoula floods. Each flood washed basalt columns and blocks bounded by joints away from the lava flows, causing the waterfalls to 'retreat' upstream. This migration resulted in lengthening of the coulees below the falls.

Cross the west margin of the Cheney-Palouse tract of the Channeled Scabland. Enter a portion of the Palouse Hills bounded by the Cheney-Palouse tract on the east, Washtucna Coulee on the north, Devils Canyon on the west, and the Snake River on the south. The Palouse Hills are underlain by thick loess deposits. This part of the Palouse Hills is at about the same elevation as the maximum water levels of the Missoula floods. Although most of the floodwater went around this area, some of the water probably crossed it.

Mazama ash is exposed in cuts on the southwest side of the road. A thin blanket of ash fell in this area during the catastrophic eruption of Mount Mazama volcano at what is now Crater Lake about 6,845 years ago (Bacon, 1983). The ash washed off the Palouse Hills and filled gullies, where it is preserved.

Junction of State Routes 260 and 261 (elevation 964 ft). The abandoned railroad stops and (or) towns called Sperry and McAdam are at this location.

Turn left (southwest) down Washtucna Coulee (Fig. 6). This dry valley was the course of the ancestral Palouse River to the Pasco Basin. Between here and Kahlotus, the highest continuous basalt flow along the coulee walls is the Roza Member of the Wanapum Basalt. Above it are discontinuous exposures of the Priest Rapids Member, the youngest unit of the Wanapum Basalt (Swanson and Wright, 1981). In places loess is visible at the top of the coulee walls.

Near Harder and Wacota, the ancestral Palouse River made a meander, swinging first northwest and then southeast. The two meander loops became eddies when Missoula floods rushed southwest along Washtucna Coulee. Whereas many giant gravel bars are on the insides of bends, gravel bars here are on the outsides of the meander loops (Figs. 31 and 32).

Lake Kahlotus, also called Washtucna Lake, is a scour depression on the floor of Washtucna Coulee. Note that the Missoula floods left a scab or erosional remnant of basalt; this is an island in the spring when the water table is high enough for the intermittent lake to exist.

Enter Kahlotus, a boom town while Lower Monumental Dam was under construction in the 1960s.

In Kahlotus (elevation 901 ft), turn left (south) toward Windust, Pasco, and Lower Monumental Dam.

Leave Kahlotus and Washtucna Coulee.

Part way up the hill, turn left (south) toward Lower Monumental Dam and enter Devils Canyon (Figs. 6 and 33). This is the third (and westernmost) place where the Missoula floods jumped the divide between the ancestral Palouse River and the Snake River. As in the modern canyon of the Palouse River and dry HU Coulee, there most likely was a small intermittent stream flowing south to the Snake River. The Missoula floods turned it into a northward-retreating waterfall or cataract as the coulee of Devils Canyon was eroded. The sides of Devils Canyon are composed almost entirely of lava flows of the Frenchman Springs Member of the Wanapum Basalt. However, at the top of the east side is the Roza Member, and at the top of the west side are both the Roza and Priest Rapids Members of the Wanapum Basalt (Swanson and others, 1980).

STOP 6: Superimposed intracanyon basalt flows in Devils Canyon (Figs. 34 and 35). This was the course of the ancestral Clearwater-Salmon River from more than 12.5 m.y. ago until about 10.5 m.y. ago. Notice the Frenchman Springs flows (averaging about 15 m thick, with mostly vertical columnar joints) exposed to north and south along the canyon walls. Contrast them with thicker (varied but on the order of 50 m thick) intracanyon flows that have columnar joints oriented in many directions.

The ancestral Clearwater-Salmon River flowed from central Idaho to south-central Washington where it joined the ancestral Columbia River (Fecht and others, 1987).

"From this vantage point, we can see an impressive natural cross-section through portions of three different Saddle Mountains Basalt flows that flowed down the canyon cut by the ancestral Salmon-Clearwater River between 14 and 10.5 Ma. . . .The earliest two flows (Esquatzel Member, massive entablature, northern-third of the exposure; Pomona Member, curved columns, southern two-thirds of the exposure) did not fill the canyon and allowed the river to reoccupy it after these flows were emplaced. At about 10.5 Ma, the Elephant Mountain Member was emplaced (uppermost entablature/colonnade that unconformably lies upon the Esquatzel and Pomona flows) and was voluminous enough to fill and obliterate this canyon of the ancestral Salmon-Clearwater River..." (Reidel and others, 1994, p. 14).

After the Elephant Mountain intracanyon flowfilled the west-trending canyon 10.5 m.y. ago, the ancestral Clearwater-Salmon River cut a new course 3 km to the south. At Stop 7A on the south side of the Snake River is exposed a younger intracanyon flow. All intracanyon flows here belong to the Saddle Mountains Basalt.

Continue south toward Lower Monumental Dam.

The basalt scabs on the floor of Devils Canyon are remnants of the Frenchman Springs Member.

A gravel bar deposited by the Missoula floods blocks the mouth of Devils Canyon (Fig. 33). Bretz (1928b, p. 662-663) stated that:

"a large deposit of poorly worn basaltic material was built a quarter of a mile out into Snake River canyon from the mouth of this trench. The deposit has no really definitive shape and no significant altitude, but it constitutes (or did constitute) a complete barrier 50 feet high in the middle of Devils Canyon, and its form, position, and composition indicate its origin in a flooded Snake River Valley by a large and vigorous stream through Devils Canyon."

Junction of the northwest end of Lower Monumental Dam with the road along northwest side of Snake River.

Exposed just north of this junction at about eye level is the unconformity between the Grande Ronde Basalt and the Frenchman Springs Member of the Wanapum Basalt (Fig. 36). There are radiometric age estimates of 15.6 Ma for upper flows of the Grande Ronde Basalt and 15.5 Ma for the Ginkgo flow of the Frenchman Springs Member (Campbell and Reidel, 1991), so this disconformity represents only a short time hiatus. The oxidized top of the Grande Ronde Basalt is the same incipient residual soil that we saw at Stop 2 (Swanson and Wright, 1981).

Turn left (southeast) and cross the dam (daylight hours only). From north to south, on Lower Monumental Dam (Fig. 37) note the fishway bypass, the fish ladder, the powerhouse, the spillways, another fish ladder, and the lock. The normal pool elevations of the reservoirs below and above the dam are 440 ft and 540 ft. respectively. (Miklancic, 1989c, summarized the geological and engineering aspects of this dam.)

Southeast end of Lower Monumental Dam (rest rooms). Turn right (southwest) on Lower Monumental Road and proceed under the railroad tracks.

STOP 7A: Lower Monumental intracanyon flow on the southeast side of the Snake River (Figs. 36-38). At 6 Ma, this is the youngest lava flow in the Columbia River Basalt Group. It can be traced as far east as Asotin (Swanson and Wright, 1981), 167 km up the Snake River from here. The intracanyon flow overlies fluvial gravels consisting of basalt, quartzite, metavolcanic rocks, and a few plutonic rocks (Swanson and Wright, 1976) (Fig. 38). Cross-beds dip to the southwest. After its channel to the northwest (Stop 6) was filled by flows of the Elephant Mountain Member of the Saddle Mountains Basalt, the ancestral Salmon-Clearwater river shifted south to this position and cut a new canyon in the Wanapum Basalt. The river deposited these gravels before the channel was filled by the intracanyon flow 6 m.y. ago. Subsequently, the Salmon-Clearwater river cut the channel now occupied by the Snake River. About 2 m.y. ago (in the late Pliocene or early Pleistocene), a tributary to the Salmon River captured the Snake River (Fecht and others, 1987); one result was the carving of Hells Canyon by the Snake River.

Continue south along Lower Monumental Road, which winds up the hill. Above the Lower Monumental intracanyon flow is a large eddy bar deposited by the Missoula floods.

Road intersection (elevation 808 ft). To the left (northeast) is the Lower Monumental airstrip. To the right (southwest) is a gravel pit in the lower part of the Missoula floods eddy bar. Continue uphill on Lower Monumental Road.

STOP 7B: Missoula floods eddy bar at the intersection of Lower Monumental Road and Matthew Road. Park at this intersection. To the east is a gravel pit in the eddy bar (Fig. 37). There are giant cross-beds composed mostly of sand and pebbles, with some silty layers; the trough cross-bedding dips 10° to 35° with the trough axes plunging easterly (Ryan Ott and Pat Spencer, Whitman College, written commun., 1995). In the west wall of the gravel pit Holocene fluvial (and eolian?) sediments, including Mazama ash, overlie the deposits of the Missoula floods. The Mazama ash is about 6,845 years old (Bacon, 1983). (See Stop 12 for details.)

Walk west along Matthew Road though giant ripple marks (Fig. 37). Roadcuts at the crest of each ripple reveal that the 'gravel' is mostly coarse sand, granules, and pebbles. The 'gravel' is overlain by approximately 1 m of loess.

The giant ripple marks are arcuate, convex toward the northeast (Fig. 37). The eddy bar was deposited by an upstream current of the Missoula floods. The fact that this eddy bar consists of fairly fine sediment can be explained in two ways: (1) the upstream eddy had a lower velocity than the main current of the Missoula floods, and (2) the eddy bar is higher in elevation than nearby gravel bars.

Baker (1978, p. 95-96) summarized the characteristics of eddy bars deposited along the routes of Missoula floods. They occur in alcoves along the sides of valleys and in the mouths of tributary valleys. Grain sizes and sedimentary structures within eddy bars are varied. The amount of dip of the beds and the dip direction are also varied. The variation is due to the velocity of the currents that deposited the bars. "The stronger currents carried the coarsest flood debris up the tributary valley. Weaker back-flow currents then deposited the finer granule gravels" (Baker, 1978, p. 86).

Baker (1978, p. 86) goes on to state "that giant current ripples are never associated with eddy bars." This eddy bar is an exception. These giant ripple marks have amplitudes of approximately 1-4 m; however, in places the troughs have been accentuated by gullying. At about 50-60 m, their wave lengths are shorter than the wave lengths of the giant current ripples on most Missoula floods gravel bars.

North of Matthew Road is another gravel pit (approximately 1 km west of the eastern gravel pit) (Fig. 37). The gravel is mostly basaltic pebbles and cobbles, with one lens of silt. There are large cross-beds dipping in different directions. This western gravel pit is about 50 m lower in elevation than the eastern gravel pit and the eastern end of Matthew Road. These coarser gravels could be part of a different Missoula-floods gravel bar.

Return to and cross Lower Monumental Dam and begin the third leg of the trip.