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Storage and response to prior disturbance as primary controls on coarse sediment dynamics in a large glacially-fed river

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Author(s): Scott W. Anderson, Kristin L. Jaeger

Document Type: Presentation #EP53G-2265
Publisher: American Geophysical Union Fall Meeting, San Francisco, CA
Published Year: 2019
DOI Identifier:
ISBN Identifier:

Links between geomorphic processes and flood hazards are well understood in the White River, Washington State, where chronic riverbed aggradation in an alluvial fan reach has created year-round flood risks. There are concerns that forecast glacial retreat and increases in flood frequency and magnitude may increase coarse sediment delivery from glaciated headwaters on Mount Rainier, accelerating downstream deposition rates. Motivated by these concerns, we use a large set of repeat high-resolution topographic surveys, supplemented by sediment transport measurements and historic channel maps, to better understand coarse sediment delivery and routing through the 1,279 km2 watershed. We find that, over the past century, erosion of the lower-watershed valley floor has been the dominant source of coarse sediment deposited in the fan reach. Lower-watershed erosion represents the river response to an early-20th century drop in local base-level, caused by a major avulsion across the alluvial fan. That avulsion was, in turn, part of the on-going watershed response to a massive mid-Holocene lahar. Local base-level was then dropped further by dredging along the new channel alignment. In the proglacial headwaters, coarse sediment export has been dominated by infrequent large pulses that deposit material across proximal valley floors; downstream responses to such pulses appear muted by the slow release of material from those valley floor deposits. Our observations suggest that deposition rates in the alluvial fan are not likely to be sensitive to short-term variations in headwater sediment delivery, and that the disequilibrium geologic context of the lower valley may be the larger flood management issue. More generally, results highlight that local watershed histories of natural and human disturbance and the internal dynamics of storage may often be central to understanding contemporary sediment and channel dynamics, including how rivers may respond to further disturbance.

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Suggested Citations:
In Text Citation:
Anderson and Jaeger (2019) or (Anderson and Jaeger, 2019)

References Citation:
Anderson, S.W. and K.L. Jaeger, 2019, Storage and response to prior disturbance as primary controls on coarse sediment dynamics in a large glacially-fed river: Presentation #EP53G-2265, American Geophysical Union Fall Meeting, San Francisco, CA,