Earthflows are large, slow-moving landslides common in northern California’s coast ranges. Picture a local hillside dotted with oak woodlands and small groves of fir and redwood among grasslands hiding tiny ponds and wetlands. A road across the hillside is continually in need of repair as the asphalt is buckled and cracked each winter or material bulges across the road. Sound familiar?
Earthflows are characterized by irregular or “hummocky” topography. The surface is undulating, with numerous closed depressions that harbor tiny ponds during the wet season. Large, forbidding networks of gullies might appear as gashes on the hillside. Up close, cracks and “slickensides” (the scratch marks left on the surfaces of two materials grinding past one another) might be seen. However, earthflows can appear more subtle particularly when they sit inactive for long periods of time. They can lie quiet and hidden under a mantle of forested stands. Over time, the sharp-crested topography that was evidence of recent movement becomes smoothed and rounded.
Earthflows occur in many shapes and sizes. The upslope “head” of the earthflow often occupies a broad, amphitheater-like bowl where ongoing land sliding provides a constant source of material (much like accumulating snow and ice feeding a glacier). The middle section functions as a transport zone. Large, fixed bedrock masses may protrude out of the earthflow - deflecting moving materials to either side. In many instances, though, these seemingly stable rocky protrusions are actually floating along in the mix. The route from Garberville over to Alderpoint and beyond offers spectacular earthflow vistas.
The “toe” of an earthflow often terminates along a river where frequent landslides heave enormous quantities of debris into the river channel. Many sections of our local rivers depart from their normal habit of broad, sweeping gravel bends where earthflows enter. Instead, the suddenly narrow river channel is littered with house-sized boulders too large for the river to move. The Donniker Earthflow across from Goat Rock on the Van Duzen River is a good example. In similar fashion, Truttman Sink, at the north end of Big Lagoon, is an example of an earthflow creeping into the ocean. The massive landslide that recently closed Highway 101 north of Redway originated from a broad expanse of earthflow terrain along the South Fork Eel River.
To better visualize earthflow movements, imagine that we have a camera focused on an earthflow, taking one photograph each week for 1,000 years. After 52,000 frames, our time-lapse movie is complete. Played fast enough, we can see where the ground stretches and cracks as it pours through a steeper segment. In flatter areas, material buckles and folds like taffy as it accumulates. Other areas lie motionless until some unseen nudge sends it on its way. Tributary earthflows feed into this jumble. Entire stands of trees raft along for a while before they gradually topple. At the toe, the river is pushed aside, eroding into the opposite hillside and reactivating a dormant earthflow. Trees, roads and structures appear as unwitting passengers on this grand conveyor belt.
Sam Flanagan is a geologist who works with the Bureau of Land Management out of the Arcata Field Office.
This article is part of a series about natural life on the Lost Coast, sponsored by the Lost Coast Interpretive Association, which may be contacted at email@example.com. For photos, educational information and news about the Lost Coast, please visit the Lost Coast Interpretive Association’s facebook page. _https://www.facebook.com/pages/Lost-Coast-Interpretive-Association/241253955966296.
Upper portions of the Minor Creek earthflow in the Redwood Creek watershed of northern Humboldt County. Note the overall gentle hill slope with numerous irregularities and small closed depressions marked by darker, wet vegetation.