EVIDENCE FOR A LARGE LANDSLIDE-DAMMED PALEOLAKE ON THE EEL RIVER, CALIFORNIA. IMPLICATIONS FOR ANADROMOUS FISH MIGRATION, OFFSHORE SEDIMENTATION, AND LANDSCAPE EVOLUTION

We present evidence for a landslide-dammed paleolake along the Eel River, northern California. Large, long-lived landslide dams are unexpected in this fine-grained, low-gradient, yet highly erosive landscape.

We discovered paleolake shore features from LiDAR-derived digital topography. Along >28 km of the Eel River, subtle topographic benches (10²–10⁴ m²), generally rare in our study site, cluster at 240–243 m, up to 120 m above the present river thalweg. The discontinuous benches are both depositional (delta-like) and degradational (cut into hillslopes). In the valley wall of a tributary, we found a 3 m thick section of finely-laminated lacustrine deposits, suggesting a minimum lake lifespan of decades. 14C dating of detrital charcoal within the deposits returned a calendar age of ~22.5 ka. The damming landslide originates from Neafus Peak, a high-relief (>1000 m) resistant greenstone block. The >40 x 10⁶ m³ catastrophic failure created a 130 m high hard-rock dam. The resulting lake stretched 50 km upstream, flooding 30 km², and held 1.3 km³ of water. The paleolake had several secondary effects on Eel River biology and geomorphology.

First, Nielsen and Fountain (1999) analyzed Pacific steelhead (Oncorhynchus mykiss) upstream of the dam, and document a late Pleistocene genetic divergence between summer and winter run steelhead. The landslide would have forced the reproductive ecotypes to spawn and interbreed over the same stretch of river. The genetic divergence potentially dates to dam failure, whereupon steelhead ecotypes reoccupied their distinct spawning grounds.

Second, paleolake shore features are preserved in the toes of two large, dormant earthflows, and provide a rare relative constraint on the duration of earthflow dormancy.

Third, the dam arrested the prodigious flux of sediment down the Eel River. VanLaningham et al. (2008) quantified changing provenance-specific sources of terrigenous sedimentation off the southern Oregon coast. They discussed that a hitherto unlikely shutdown of Eel River sediment yield could explain the provenance ratios of sediment packages from 25–22 ka.

And fourth, an unusual, organic-rich layer (~22 ka) in the Escabana Trough (Zuffa et al. (2000)) is potentially derived from outburst flood generated turbidity currents, suggesting the dam failed catastrophically.