Ellensburg Chapter, Ice Age Floods Institute Landslides and Related Mass Wasting Features in the Yakima River Corridor Field Trip Karl Lillquist Geography Department Central Washington University 1 April 2018 Field Trip Overview Field Trip Description: Central Washington includes a variety of landslides and related “mass wasting” features. Why? What causes these? How active are they? What have they impacted? On this trip, we will explore the evidence of various types of mass wasting along the Yakima River corridor from east of Cle Elum to south of Union Gap. Examples will include landslides, debris flows, rockfall, dry ravel, and creep. Along the way, we will develop a sense of what indicates past mass wasting, what caused it, what are its implications on the surroundings, and where it might occur again in the future. Our stops will range from ancient landslides to recent debris flows to the currently active Rattlesnake slide. Tentative Schedule: 10:00am Depart CWU 10:30 Stop 1— Lambert Road, East of Cle Elum 11:15pm Depart 11:45 Stop 2—Near MP 101, WA 10, North of Thorp 12:45 Depart 1:30 Stop 3—MP 20, WA 821, Yakima River Canyon 2:30 Depart 3:00 Stop 4—Between MP 3 and 4, WA 821, Yakima River Canyon 3:45 Depart 4:15 Stop 5—North of MP 74, US 97, South of Union Gap 5:00 Depart 6:00 Arrive at CWU 2 Our Route & Stops 1 2 3 4 5 Figure 1. General route map of field trip. Source: Google Maps. 3 Ellensburg to Lambert Road Route. From Ellensburg, head west on I-90 toward Cle Elum. Take Exit 85 (the first Cle Elum exit when westbound). At the yield sign on the off ramp, turn right onto WA 970 toward Wenatchee. Follow this approximately 3 miles east just past the junction of WA 970 and WA 10. Turn right (east) onto Lambert Road and follow this to its crossing of the Teanaway River. East of the bridge, pull off to the right side of Lambert Road and park. Please do not park on the bridge! This is Stop 1. Kittitas Basin geology. Our drive from Ellensburg begins on the floor of the Kittitas Basin syncline with downfolded Columbia River Basalts ~4000 feet below us (Figures 2, 3, & 4). Mantling the Columbia River Basalts are volcanic lahar sediments of the Ellensburg Formation, alluvial fan sediments from streams exiting the surrounding mountains, and windblown loess (Waitt, 1979; Tabor and others, 1982). Mass wasting at 70 mph. Just west of Thorp, look north and see a steep escarpment in the Ellensburg Formation and overlying alluvial Thorp Gravels. At the toe of this escarpment, you will a “hummocky” surface. This escarpment and the hummocky deposits represent huge, mostly ancient landslides. We will discuss these landslides enroute to Stop 2. Past glaciers. West of Thorp, we ascend from the Kittitas Basin onto Elk Heights (or “Thorp Prairie”). This surface consists of Columbia River Basalts overlain by older glacial end moraine and more recent loess. The end moraine mantling Thorp Prairie represents the farthest downvalley extent of the Upper Yakima River Watershed glaciers during the Kittitas-Swauk Prairie glaciation (Porter, 1976; Waitt, 1979) (Figure 5). These are also apparently the farthest downvalley extents of any glaciers in the Upper Yakima River Watershed. Thorp Prairie moraines are thought to be the same age as those of Swauk Prairie to the north (Figures 5 & 7). Therefore, the Kittitas-age glacier that deposited these features had split into two lobes near its terminus because of prominent Lookout Mountain. The glacier that deposited these moraines formed from coalescence of the Cle Elum, Kachess, and Keechelus valley glaciers (Figure 8). At its maximum extent, >500,000 years before present, this glacier was >40 miles long (Swanson and Porter, 1997). Kittitas Basin climate. The wind towers of the Kittitas Valley Wind Farm remind us of the regularity and strength of winds in the Kittitas Basin. The thick, fine textured deposits of loess that blanket the Thorp Prairie area are a reminder of the importance of wind over time as well. 4 Ellensburg to Lambert Road Figure 2. Location Naneum Manastash of Kittitas Basin Ridge Ridge syncline between Kittitas Naneum Ridge Valley and Manastash Ridge anticlines. Source: Jack Powell. 1 Figure 3. The Columbia Plateau and the spatial extent of the Columbia River Basalt Group, the four major structural-tectonic subprovinces (the Yakima Fold Belt, Palouse, Blue Mountains, and Clearwater-Weiser embayments), the Pasco Basin, the Olympic-Wallowa lineament. Number 1 indicates approximate location of Stop 1. Source: Reidel & Campbell (1989, p. 281). 5 Ellensburg to Lambert Road 1 Figure 4. Generalized map of major faults and folds along the western margin of the Columbia Plateau and Yakima Fold Belt. Bold number 1 indicates approximate location of Stop 1. Source: Reidel & Campbell (1989, p. 281). 6 Ellensburg to Lambert Road Lambert to Ellensburg 1 7 Figure 5. Glacial-geologic map of the Upper Yakima River drainage basin. Bold number 1 indicates approximate location of Stop 1. Source: Porter (1976, p. 62). Stop 1—Lambert Road Location. We are located on the floodplain of the Teanaway River near its junction with the Yakima River. Lookout Mountain to our east represents the westernmost extent of Columbia River Basalts in the area. Mass wasting. Mass wasting includes all types of gravity-driven slope movements including: - slide - flow - creep - fall - dry ravel Often these movements may occur together in the same area. Central Washington’s Yakima River corridor is characterized by all of these forms of mass wasting Our focus at Stop 1 is on slides and falls. Landslides & their evidence. Landslides are just that—sliding masses of land that may also include surficial features such as trees and various human structures. Evidence for landslides includes: - tilted features—land, trees, fences, etc. - escarpments (often arcuate & vegetation free) - tension cracks - displaced blocks (often as steps on slope) - hummocky terrain - ponds/wetlands - sharp bends in roads & streams - fan-shaped deposits - pressure ridges - deranged drainage patterns Which of those pieces of evidence do you see here? Types of landslides. Two varieties of landslides exist—translational (like a sled on hill) and rotational (like sliding or slumping down in a chair or couch). For this reason, rotational slides are also referred to as slumps. The recent Oso Landslide that resulted in the deaths of 44 Stillaguamish River residents was a rotational slide with a flow at the toe. Translational and rotational slides occur on steep slopes. Translational slides typically follow geological structures such as layers of bedrock or sediment. Rotational slides typically cut across bedding. The easiest way to differentiate the two is based on the presence or absence of of backtilted blocks in the upper portions of the slide—i.e, if present, the feature is a rotational slide (Figure 6). 8 Stop 1—Lambert Road Figure 6. Model of a rotational slide. Source: Highland and Bobrowsky, 2008, p. 5) Lookout Mountain rotational landslide and rockfall. The Lookout Mountain landslide was first recognized by Russell (1900) and Smith (1904). It is a large, rotational landslide that occurred in prehistoric times. Evidence for this rotational slide includes: 1) the prominent escarpment (i.e., main scarp) high on Lookout Mountain; 2) the large, backtilted blocks and associated minor scarps and ponds just downslope of the escarpment; and 3) the hummocky toe that extends to the Teanaway River near here (Figures 8, 9, & 10). At least four ponds are impounded within the large slide blocks. Subsequent rockfall occurred from the near vertical, bare, basalt face of the main scarp. Age. How old is the feature? First, we don’t know if the slide occurred all at once or piecemeal over time. Second, we can only address its age in a relative sense. Smith (1904) recognized a river terrace atop the middle portion of the slide. Waitt (in Tabor and others, 1982) attributes that terrace to the Swauk Prairie-age glacial outwash deposits. Recall that Swanson and Porter (1997) estimate that Swauk Prairie deposits are >500,000 years old so this slide is potentially more than one-half million years old! This is hard to believe given that the feature is so well-preserved. Perhaps this preservation is due to the semiarid climate and the hard basalts so little degradation has occurred over time. 9 Stop 1—Lambert Road Figure 7. Middle Pleistocene glacial features in the vicinity of Lookout Mountain, Upper Yakima River 1 Watershed. Bold number 1 indicates field trip stop. Source: Porter (1965, p. 37). Figure 8. Geologic map of Stop 1 and vicinity. Bold number 1 1 indicates field trip stop location. Purple = Columbia River Basalt, pale yellow with arrows = landslides, and bright yellow = alluvium. From Tabor & others (1982) 10 Stop 1 Stop Sag Ponds — Hummocky Road Lambert Terrain 1 Main scarp Figure 9. Lookout Mountain rotational landslide enclosed within red lines. Large arrows indicate landslide movement direction. Number indicates field trip stop. Source: Google Maps. 11 Stop 1—Lambert Road Slide Hummocky Teanaway Blocks Terrain River Main Scarp 1 Figure 10. Oblique view of Lookout Mountain landslide. Red lines outline extent of landslide. Heavy red arrows indicates rotational nature of slide and direction of movement. Bold number 1 is field trip stop. View toward north. Source: Google Earth. Causes. What caused this rotational slide? Weak interbeds between the Columbia River Basalts likely played a role as did a wetter climate. Perhaps a large earthquake triggered such a large slide. Waitt (in Tabor and others, 1982) speculates that a large lake impounded in the lower Teanaway River Valley by glaciers or moraines may have saturated substrates causing the failure of this slope.