Rattlesnake Hills Landslide Information
What is known and what is being done, from the Washington Department of Natural Resources.
Approximately 20 acres in the Rattlesnake Hills, 3 miles south of Yakima near Union Gap, WA is currently moving at a rate of ~1.5 ft/week in a southward direction. The type of movement is a translational landslide composed of blocks of basalt sliding on a weaker sedimentary layer. The geologists and engineers monitoring the landslide suggest that the most probable scenario is that the landslide will continue to move south and accumulate into the quarry. In this scenario, rockfall is expected within the quarry and will accumulate along the toe of the landslide, potentially slowing and eventually halting landslide movement. Additional rockfall to the west and south may impact Thorp Rd., which has been closed since late December by Yakima County as a precaution. There are other very low-probability scenarios where the landslide could mobilize quickly and reach I-82 and impact homes south of the quarry, or potentially reach the Yakima River. These scenarios are less likely and will likely have precursor signs that current instrumentation will register, permitting planning by emergency managers. Precautions and plans are in place to take action if monitoring data suggests that this may happen.