DRIVERS OF FLUVIAL MORPHOLOGY IN OLYMPIC NATIONAL PARK, WASHINGTON, USA: PHYSICAL FACTORS OR TROPHIC CASCADES? (Invited Presentation)

Identifying the relative contributions of physical and ecological processes to channel evolution remains a substantial challenge in fluvial geomorphology. It has been proposed previously that a trophic cascade consisting of apex-predator eradication, increased herbivore abundance, and consequent reduction of riparian vegetation can be a primary driver of river-channel morphology in western U.S. landscapes. We use a 74-year aerial photographic record, historical stream-gage records, and elk-population estimates to investigate whether physical or trophic-cascade-driven ecological factors are the dominant controls on channel planform in the Hoh, Queets, Quinault, and Elwha Rivers of Olympic National Park, Washington. Channel width and braiding show strong relationships with recent flood history; all four rivers have widened concurrently with increased flood activity since the 1970s. Channel planform also reflects sediment-supply changes, shown, e.g., by the response of the Elwha River to a large landslide. We surmise that the Hoh River, which shows a strong multi-decadal trend toward greater braiding, is adjusting to increased sediment supply associated with rapid glacial retreat. We infer no correspondence between channel evolution and elk (herbivore) abundance after wolves were eradicated in the early 1900s, suggesting that in this system effects of the wolf-driven trophic cascade are subsidiary to physical controls on channel morphology. Our data do not support a previous assertion that these rivers underwent a fundamental geomorphic transition (a shift from single-thread to braided) resulting from large elk populations that followed wolf extirpation; we infer that these rivers have long been dynamic braided channels.

Although many factors have likely shaped river planform, the weight of currently available evidence indicates that floods and other physical factors have been the dominant controls on geomorphic change. We see no convincing evidence that the dominance of physical drivers occurs under an overarching umbrella of trophic-cascades influence.