From the Ice Harbor Dam visitor center, drive southwest on the paved road toward Pasco.

Intersection with State Route 124 (elevation 530 ft). Turn west (right) toward Pasco. To the south and west there is a good view of the hills and escarpment that form the Olympic-Wallowa lineament. The anticlinal hills along the lineament include Jumpoff Joe (to the southwest), Badger Mountain (to the west), and Rattlesnake Mountain (to the west-northwest). Rattlesnake Mountain lies at the south edge of the Hanford Site managed by the U.S. Department of Energy. Formerly the emphasis of work on the site was on nuclear weapons and nuclear energy, but today work there centers on nuclear waste storage and disposal. Considerable funds are being spent for cleaning up the radioactive wastes.

Enter Burbank Heights (elevation 411 ft).

Turn left (southwest) on U.S. 12 toward Walla Walla. The mouth of the Snake River is 2 km to the southwest. Just northwest of here is Hood Park (U.S. Army Corps of Engineers). On the other side of the Snake River, where it joins the Columbia River, is Sacajawea State Park. The Lewis and Clark Expedition camped at the mouth of the Snake River in October 1805 (Majors, 1975).

McNary National Wildlife Refuge. On the northeast side of U.S. Highway 12 is the northwest end of Burbank Slough, an old meander scar that was flooded in 1953 when McNary Dam was constructed 50 km down the Columbia River.

Cross the southeast part of Burbank Slough. (You are now in the McNary State Wildlife Recreation Area.) Wallula Gap is visible 12 km to the south.

On the west side of the highway is Boise Cascade's Wallula mill. The chief products are corrugating medium and paper. Fiber sources are mainly wood chips and sawdust, but soon will include recycled paper and trees from the huge cottonwood plantation east of the highway. The genetically identical cottonwood trees, which are fertilized and irrigated, grow as much as 1 inch in diameter and 10 feet in height per year.

To the southwest, you can see the delta of the Walla Walla River (Fig. 52) extending into Lake Wallula along the Columbia River. When McNary Dam was completed, the reservoir extended up the Columbia River to Richland and several miles up the Walla Walla River. A yacht club was established in the 'harbor' along the lower Walla Walla River. The severe erosion of the Palouse loess and the Touchet Beds loads the Walla Walla River with a large amount of suspended sediment; within years the lower Walla Walla River 'silted up', and the yacht club was moved to Wallula Gap. Now the sediment carried by the Walla Walla River is deposited in a delta growing out into Lake Wallula (elevation 340 ft).

Entrance to Madame Dorion Park on the left (east). (There are rest rooms here.) Madame Dorion was an Iowa Indian who left Missouri in 1811 and arrived at Wallula in January 1812 with the Wilson Price Hunt party of the Pacific Fur Company (information from sign at park).

Cross the bridge over the mouth of the Walla Walla River. At the 'Y' turn west (right) on U.S. Highway 730 toward Umatilla. This is Wallula Junction (elevation 400 ft).

STOP 10: Olympic-Wallowa lineament. Park on the north (right) side of U.S. 930 and be careful of traffic. Raisz (1945) proposed the term Olympic-Wallowa lineament (OWL) for a northwest-striking feature stretching 600 km from Cape Flattery in northwestern Washington to the Wallowa Mountains in northeastern Oregon (Fig. 2). A segment of the OWL called the CleElum-Wallula deformed zone (CLEW) is marked by anticlines that trend N50°W. The Rattlesnake-Wallula alignment (RAW) is the part of the OWL and CLEW that forms the southwestern boundary of the Pasco Basin (Reidel and Lindsey, 1991).

The hillside to the south of this stop is the northern escarpment of the Horse Heaven Hills, a long, broad anticlinal ridge that stretches across southern Washington from the Cascades to Wallula Gap. From Wallula Gap, the ridge trends east-southeast and intersects the Blue Mountains near Milton Freewater, Oregon (Fig. 52).

West of Wallula Gap, the OWL is expressed as northeast-facing fault scarps that extend northwestward along the flank of the Rattlesnake Hills. East of the gap, faceted spurs marking the northern boundary of the Wallula fault zone constitute the OWL (Fig. 53). The OWL is the topographic expression of a zone of high-angle faults along which Miocene basalts are in the upthrown block to the south and Quaternary sediments are in the downthrown block to the north.

In Washington, the maximum relief across the OWL occurs about 65 km to the northwest of here. On the northeast side of Rattlesnake Mountain (elevation 3,524 ft) there is about 900 m of relief, but much of that relief is due to the anticlinal fold and only in part to faulting. To the southeast of Wallula Gap, there is a fault scarp in the southeastern corner of the Zanger Junction quadrangle (1:24,000) that exceeds 100 m. Farther southeast in Oregon, the relief along the OWL along the northeast side of the Wallowa Mountains is more than 1,500 m.

Examine the subparallel faults (part of the much wider Wallula fault zone) in the basalt in the roadcut on the south side of U.S. Highway 12 (Fig. 54). The lava is the Two Sisters unit of the Frenchman Springs Member of the Wanapum Basalt (Gardner and others, 1981). The Two Sisters unit is equivalent to the Sand Hollow unit of south-central Washington (S. P. Reidel, Westinghouse Hanford Co., written commun., 1995). Note the fault breccia and yellowish fault gouge along each of the near-vertical faults. Along the western fault are subhorizontal slickensides that indicate that the last movement was strike-slip (Fig. 55). About 8 km to the east, a fault on the southwest side of Vansycle Canyon appears to deflect intermittent streams as much as 250 m; the offset drainage suggests right-lateral strike-slip movement (Gardner and others, 1981). However, nearby drainages and ridges are not as deflected, so the apparent offset drainage may be due to local erosion along the fault plane (Reidel and Tolan, 1994, p. 1638).

The age of the most recent movement along the faults that form the OWL is not known with certainty. Carson and others (1989) found evidence for deformation along the OWL on the northeast side of the Wallowa Mountains; the Enterprise Gravels (about 2 Ma) have been tilted. Mann and Meyer (1993) argue that there has been some right-oblique slip displacement along the OWL in the Holocene. Their assessment is based in part on what they interpret as a 5-m offset of a Mount St. Helens ash (dated at 10.7 ka) between here and Milton-Freewater, Oregon. Also near Milton-Freewater there was an earthquake of approximately magnitude 6 in 1936 (Brown, 1937), but that earthquake may have been associated with the Hite fault (Fig. 2) rather than the OWL (Reidel and Tolan, 1994, p. 1636).

The delta of the Walla Walla River, visible less than a kilometer to the northeast, is advancing into Lake Wallula.

On the floor of Lake Wallula is the April 1806 campsite of Lewis and Clark. They described the "Wallah wallah River" as "a handsom stream about 4-1/2 feet deep and 50 yards wide." When members of the Lewis and Clark expedition danced to a fiddle, the "chimnahpoms" (Yakima Indians) and "wollahwollahs" sang and joined the dance (Majors, 1975).

A bit farther north, and also now submerged, Fort Nez Perce (also known as Fort Walla Walla) was established in 1818. The log fort burned in 1841 and was replaced by an adobe fort that washed away during the Columbia River flood of 1894 (Majors, 1975). That flood had a recorded discharge of 34,000 m³/sec (1,200,000 ft³/s). The mean discharge of the Columbia River (at Bonneville) is 5,266 m³/sec (185,900 ft³/sec). Of this discharge, 1,446 m³/sec (51,060 ft³/sec) is contributed by the Snake River (Bonneville Power Administration, 1993).

Contrast the 1894 floodflow with Baker's (1973b) estimate for a typical Missoula flood of 21,000,000 m³/sec (740,000,000 ft³/s). (This is about 18 mi³ of water per hour!) These floods poured into the Pasco Basin from the northeast, north, and northwest, but the only outlet was Wallula Gap. At less than 2 km wide, Wallula Gap could accommodate only about half of the peak discharge of a Missoula flood. Therefore, the water level in the Pasco Basin rose rapidly to 1,200 ft. This formed Lake Lewis, which lasted about a week during each Missoula flood (Allen and others, 1986). Lake Lewis backed east and west up the Walla Walla and Yakima Valleys, respectively; in these arms of the lake accumulated thick slackwater sediments, or the Touchet Beds. The bottom of the Columbia River has an elevation here of about 300 ft. Imagine 275 m (900 ft) of water rushing through Wallula Gap at velocities of as much as 80 km/hr (50 mph) (Allen and others, 1986).

Continue southwest on U.S. Highway 730 through Wallula Gap (Fig. 56).

The Two Sisters stand above the east side of U.S. Highway 730 (Figs. 57 and 58). These buttes are scabs or erosional remnants of basalt (Frenchman Springs Member) left after passage of the Missoula floods. Each butte is part of the entablature of a basalt flow whose colonnade is at the base of the buttes.

The uppermost limit of flood scour at Wallula Gap is about 1,200 ft (Waitt and others, 1994). The largest flood channel adjacent to Wallula Gap is east of the Two Sisters (Fig. 57). Huge volumes of water rushed south there, creating an area of 'channeled scabland' with a floor at 500 ft. At the south end of that channel, from the Two Sisters to the Columbia River and on both sides of U.S. Highway 730, is a small dune field most of which is in active because it has been stabilized by vegetation.

The basalt exposure on the east (left) side of U.S. Highway 730 is followed by a roadcut that exposes a lens of Mazama ash about 1 m thick. This lava flow is near the bottom of the Frenchman Springs Member of the Wanapum Basalt. To the southwest, between the road and the river, is the uppermost flow of Grande Ronde Basalt (Gardner and others, 1981).

Notice the huge talus slopes on both sides of the Columbia River. It is likely that the high-discharge, high-velocity Missoula floods removed all unconsolidated debris from Wallula Gap, so all this talus has accumulated in the last 12,700 years.

Washington-Oregon state line. Continue southwest on U.S. Highway 730.

STOP 11: View of Wallula Gap (Fig. 56). Park on the left (southeast) side of U.S. 730. Be careful of traffic!

Parts of the Grande Ronde, Wanapum, and Saddle Mountains Basalts are exposed at Wallula Gap (Swanson and others, 1980). A thick section of the Frenchman Springs Member of the Wanapum Basalt is overlain by the Umatilla Member of Saddle Mountains Basalt. A Martindale flow of the Ice Harbor Member (about 8.5 m.y. old) of the Saddle Mountains Basalt caps the highest point visible on the northwest side of the Columbia River. This does not mean that there is a complete section of the Columbia River Basalt Group exposed here. The cliffs at Wallula Gap are a maximum of 300 m high, but the total thickness of the Columbia River basalts is thousands of meters. More than 300 individual flows are known (Tolan and others, 1989), but only about 20 flows have been recognized in the vicinity of Wallula Gap (Gardner and others, 1981, p. 6).

The Martindale flow overlies gravel compositionally similar to that at Stop 7. Therefore, we can conclude that the Clearwater-Salmon River was cutting across the Horse Heaven Hills here by 8.5 m.y. ago (Swanson and Wright, 1981). At that time the course of the Columbia River was about 80 km (50 mi) northwest of Wallula Gap. There has been considerable debate about why the Columbia River shifted east to Wallula Gap.

Fecht and others (1987, p. 238) "...believe that continued subsidence of the central Columbia Plateau centered in the Pasco Basin and uplift of the Yakima folds, specifically the Horse Heaven Hills, were important factors in the diversion of the Columbia River" and its capture by the Clearwater-Salmon River. Fecht and others (1987) described the paleodrainage of the Columbia River system and summarized the history of the theories about why the Columbia River shifted eastward from the central Pasco Basin to Wallula Gap.

At road level are lava flows of the Frenchman Springs Member (about 15.5 Ma) of the Wanapum Basalt. Note the following: the flow contact with flow-top breccia and ropy texture, which is called pahoehoe; the difference between polygonal contraction columns in the entablature and the basal colonnade; and the presence of phenocrysts, vesicles, and amygdules. The pahoehoe flows of the Columbia River basalt were very fluid; rapid cooling of the flow surface produced a smooth or ropy texture. As the molten interior continued to move, the surface was cracked and broken, and solid fragments became incorporated in the molten part of the flow. The upper part of a flow that consists of these fragments is called a flow-top breccia. The top of the flow and (or) some fragments may be arranged so that they resemble ropes laid side by side or mixed together. The geological contact where a younger flow overrides the older flow-top breccia may be a reddish zone, due to oxidation. Phenocrysts are large crystals (commonly the mineral plagioclase) that crystallized in the lava before it erupted. Vesicles are cavities that formed by bubbles of volcanic gases; amygdules are vesicles that were filled by minerals such as calcite, quartz (or opal), or zeolites.

Return to the Oregon-Washington state line by driving northeast on U.S. Highway 730. We now start the last leg of the field trip.