Northwest end of Lower Monumental Dam. Turn southwest toward Windust.

Entrance to Lower Monumental Dam visitor parking lot. The visitor area is open April 1 through October 31 for self-guided tours of the fish-viewing window and the powerhouse. (The visitor area has rest rooms.)

Continue southwest along the northwest edge of Lake Sacajawea, the reservoir impounded by Ice Harbor Dam, 50 km farther down the Snake River. Across the Snake River is a gravel bar deposited by the Missoula floods.

East entrance to Windust Park (rest rooms). Continue west along the north side of the Snake River.

The Lewis and Clark Expedition camped just down-river of Windust Park in October 1805 (Majors, 1975).

On the opposite side of the Snake River is the northeast end of the Scott gravel bar (Fig. 39), which extends 9 km along the southeast side of the river. As much as 6 m of gravel (largest boulder 1 m in diameter) are exposed in railroad cuts in the giant ripple marks on the Scott bar. In places there are 2 m of slackwater sediments overlying the gravels, with perhaps a dozen Touchet Beds exposed. This stratigraphic relation suggests that the last dozen Missoula floods were going up the Snake River at this location; if true, the last Missoula floods did not descend the Cheney-Palouse tract, but reached the Pasco Basin only by other routes. The flood gravels and Touchet Beds are capped by 1 m of loess. The ripple marks have a wave length of approximately 150 m; their amplitude of 6-8 m has been accentuated by gullying of the troughs.

Leave the Snake River. Turn right (northwest) under a railroad bridge.

On the southwest side of the road are sediments deposited by the Missoula floods in the eddy that existed at the mouth of Burr Canyon.

Exposures of Touchet Beds, the slackwater sand and silt deposited by the Missoula floods.

Turn left (southwest) onto the Kahlotus-Pasco highway (elevation 1,215 ft). Drive southwest through a portion of the Palouse Hills, which are underlain by Quaternary loess as much as 75 m thick. Kahlotus is about 20 km to the right (northeast).

Crests of hills (elevations about 1,420 ft and 1,195 ft, respectively). Visible in the distance to the southeast is the crest of the Blue Mountains anticline, a northeast-trending uplift of Miocene basalt flows. To the south and southwest are hills along the Olympic-Wallowa lineament (OWL).

Also visible to the southwest from mile 107.2, and present only 2 km west of mile 113.8, are the Juniper Dunes (Figs. 40-43). The Juniper Dunes Wilderness is administered by the Spokane District of the Bureau of Land Management. Information about the Juniper Dunes and a permit to visit this area can be obtained from the Bureau (East 4217 Main Ave., Spokane, WA 99202, phone (509) 353-2570). The wilderness area is 'landlocked' in that access is across private lands. Permission to cross private land must be obtained in advance from the landowner.

Much of the dune field was made a wilderness in 1984; land uses (sometimes in conflict) of the Juniper Dunes include wildlife habitat, agriculture, nonmotorized recreation, and off-road vehicle activity. The dunes support grasses, shrubs, and western juniper trees (Juniperus occidentalis).

Much of the Juniper Dunes is characterized by transverse dunes (Figs. 41 and 42), which form where there are fairly large supplies of sand. Slip faces of these dunes are oriented northwest and are a maximum of 40 m high. The sources of the sand are in the Pasco Basin: the Pliocene Ringold Formation (Newcomb, 1958), Pleistocene glaciofluvial sediments (including the Touchet Beds), and Holocene alluvium.

At the leeward (north-northeast portion) of the Juniper Dunes are overlapping parabolic dunes (Fig. 43). One advancing parabolic dune has buried an old fence. Parabolic dunes have horns that point upwind and form on either side of a blowout or hollow created when an unstable portion of the dune migrates downwind (Cooke and Warren, 1973). In general there is more vegetation stabilizing the parabolic dunes; large parts of the transverse dunes are completely free of vegetation.

The orientation and asymmetry of the transverse and parabolic dunes reflect the prevailing southwesterly winds. During occasional periods of sustained northerly winds, the transverse dunes become reversing dunes—sand blown up the large slip faces forms crests with south-facing slip faces. The ripple mark asymmetry reveals the direction of the most recent wind strong enough to move the sand. Occasionally the right wind and moisture conditions reveal the giant cross beds inside the dunes.

Along the leading (northeast) edge of the Juniper Dunes, intermittent streams flowing southwest from the Palouse Hills have been blocked by the advancing dunes (Fig. 40). Fine sediment underlies the flat floors of the intermittent lakes.

Roadcuts in Quaternary loess. Our route is southwesterly into the Pasco Basin, which is structurally and topographically lower than the Palouse Hills to the northeast and the anticlinal ridges to the south. The Missoula floods were hydraulically ponded by the constriction of Wallula Gap where the Columbia River leaves the Pasco Basin. The highest hydraulic lake surface was at an elevation of about 366 m (1,200 ft) (O'Connor and Waitt, 1994, p. 38). Because the elevations here are only 1,100 to 900 ft. we are lower than the maximum elevations of the floods. There has been reworking of the top of the loess, and some erratics are present.

Intersection with Murphy Road (elevation 797 ft). Continue south on the Kahlotus-Pasco highway. Vegetation subdues the dunes just to the northwest. This is the southeast edge of the Juniper Dunes. The dune field has an area of about 130 km².

Intersection with Levey Road (elevation 700 ft). Continue west on the Kahlotus-Pasco highway. The geological materials underlying this 'terrace' between the Juniper Dunes and the Snake River can be viewed by turning left (southeast) on Levey Road. Nearby is a gravel pit with excavations in Missoula flood deposits. Beneath the late Pleistocene sediments, basalt forms a cliff facing the Snake River. The flows here belong to the Frenchman Springs Member of the Wanapum Basalt and the Pomona and Elephant Mountain Members of the Saddle Mountains Basalt (Swanson and others, 1980). Touchet Beds are exposed in railroad cuts between the cliff and the river.

Levee Park, which has rest rooms, is 1.5 mi from the Kahlotus-Pasco highway.

Turn left (south) toward Ice Harbor Dam.

Railroad underpass.

Roadcut on the north reveals late Pleistocene Touchet Beds with clastic dikes and a thin cap of Holocene loess. From north to south across Ice Harbor Dam are the lock, a fish ladder, the spillway, the powerhouse, and another fish ladder (Figs. 44 and 45). Miklancic (1989a) summarized the engineering geology of Ice Harbor Dam.

Turn left toward boat ramp.

Continue east on the primitive dirt road from the northeast corner of the parking area for the boat ramp. (An alternative is to park here and walk 0.6 mi to the end of the dirt road.) At the two junctions on the dirt road, go either way, proceeding eastward along the north side of the Snake River.

Stop 8: Park here at the end of the dirt road. (The small parking/turn-around area is between an abandoned railroad grade and the Snake River.) Walk east about 300 m along the railroad grade.

Beware of rattlesnakes and ground hornets.

The geology here is described by Swanson and Wright (1976, p. 8-9; 1981, p. 22). At this stop are some of the youngest lava flows of the Columbia River Basalt Group. In railroad cuts are exposed flow contacts, dikes, and an invasive flow.

Stop 8A: Western railroad cut (Fig. 46, section A—A'). In the western railroad cut, a flow of the Elephant Mountain Member is cut by the Ice Harbor dikes. The dikes, and tuff cone at Stop 9, are part of an 80-km-long linear vent system for the Ice Harbor flows (Swanson and others, 1975). Above the railroad cut is an Ice Harbor flow.

In the railroad cut, the Elephant Mountain flow has large columns, a vesicular top, and a thick breccia at the bottom (Fig. 47). The breccia, which exhibits ropy texture in places, was described by S. P. Reidel and Karl Fecht (Westinghouse Hanford Co., unpub. data., 1987): "The base is, in part, a 'rubble' zone with chilled blocks of basalt mixed with sediment."

Stop 8B: From the east end of this railroad cut, walk approximately 250 m east along the railroad grade to the next railroad cut (Fig. 46, section A—A'). Here the Pomona flow has invaded a tuff that is exposed at the top of the northern side of the middle of the railroad cut (Fig. 48). An invasive flow occurs where the advancing, more dense lava 'submarines' through less dense sediment that is likely water saturated and of fine grain size. The basalt pillows here have glassy rims and are surrounded by white vitric tuff. The tuff is consolidated ash that contains mostly glass particles. The Pomona flow has large vesicles, some of which are called amygdules because the gas bubble cavities are filled with minerals such as calcite, quartz, and zeolites.

Walk back to the end of the primitive road and drive west toward Ice Harbor Dam.

The dirt road becomes paved at the parking lot for the boat ramp.

Turn left toward Ice Harbor Dam.

Begin to cross the dam (daylight hours only).

Leave dam; turn right (west).

Turn right (north) toward visitor center.

Near the bottom of the hill, turn left (west) toward Stop 9. (The visitor center is to the east.) Proceed downriver on the gravel road.

End of the road, Stop 9. Park here and beware of rattlesnakes and poison oak. Swanson and Wright (1976, p. 9-10; 1981, p. 23) described the geology of the exposures of basalt.

Stop 9A (Figs. 49 and 50) is the bluff adjacent to the end of the road (Fig. 46, section B—B'). This rock is described by Swanson and others (1975, p. 893) as "crudely bedded, poorly sorted, palagonitized sideromelane breccia and tuff forming remnant of tuff cone." (In other words, the fragmental volcanic rock is composed of altered basaltic glass.) Different dips of the tuff breccia indicate minor shifting of the vent during construction of the tuff cone, which is overlain by a slightly younger Ice Harbor flow.

Walk about 700 m west along the south bank of the Snake River. Along the trail are exposures of tuff, a basalt flow, and the Pasco gravels.

Stop 9B is the cliff where the trail rises (Fig. 46, section B—B'). The lower Martindale flow is cut by a Goose Island dike, which feeds the upper flow (Fig. 51) (S. P. Reidel, Westinghouse Hanford Co. written commun., 1995). The flows and dike are part of the Ice Harbor Member of the Saddle Mountains Basalt, dated at about 8.6 Ma. The upper part of the dike is breccia interpreted by Swanson and Wright (1976, 1981) as drainback rubble.

Walk east to the tuff cone and drive toward Ice Harbor Dam.

Where the gravel road meets the paved road, continue east to the visitor center.

Ice Harbor Dam Visitor Center (open all year). Inside the visitor center are a view of the powerhouse, a window to watch migrating fish, and rest rooms.