Controls on fluvial morphology in Olympic National Park, Washington, USA: physical factors or wolf-driven trophic cascades?
Abstract
Identifying the relative contributions of physical and ecological processes to channel evolution remains a substantial challenge in fluvial geomorphology. Some researchers propose that a trophic cascade consisting of wolf eradication, increased elk abundance, and herbivory impacts on 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 abundance, suggesting that in this system effects of the wolf-driven trophic cascade are subsidiary to physical controls on channel morphology. Our examinations of stage-discharge history, historical maps, photographs, and descriptions, and empirical geomorphic thresholds do not support a previous assertion that these rivers underwent a fundamental geomorphic transition (widening, and a shift from single-thread to braided) resulting from large elk populations that followed wolf extirpation in the early 1900s; we infer that these rivers have long been dynamic braided channels.
Vegetation can modulate geomorphic change, and vegetation changes along Olympic National Park rivers have been consistent in the past with intensive elk use. However, 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 influence of trophic cascades.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP33B..05E
- Keywords:
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- 0481 Restoration;
- BIOGEOSCIENCESDE: 1813 Eco-hydrology;
- HYDROLOGYDE: 1825 Geomorphology: fluvial;
- HYDROLOGYDE: 1834 Human impacts;
- HYDROLOGY