Geologic Publications for Mount Rainier
Coarse sediment dynamics in the White River watershed
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Scott W. Anderson
, Kristin L. Jaeger
Report for King County Water and Land Resources Division, Department of Natural Resources and Parks
United States Geological Survey, Washington Water Science Center
Changes in upstream sediment delivery or downstream base level can cause propagating geomorphic responses in alluvial river systems. Understanding if or how these changing boundary conditions propagate through a watershed is central to understanding changes in channel morphology, flood conveyance and river habitat suitability. Here, we use a large set of high-resolution topographic surveys to assess coarse sediment delivery and routing in the 1,279 km2
glaciated White River, Washington State, USA. This study was motivated by the concern that changes in climate may increase coarse sediment delivery from the watershed's volcanic and glaciated headwaters, potentially accelerating chronic deposition in a populated alluvial fan near the river's mouth. However, we find that most of the coarse sediment load in the lower river is derived from erosion of the lower-watershed valley floor, as the river continues to respond to an early-20th century drop in local base-level. Base-level initially declined following a major avulsion across the fan in 1906, an event conditioned by the watershed history of continental glaciation and a massive mid-Holocene lahar, but was then further lowered by dredging along the new channel alignment. In headwater proglacial areas, coarse sediment export has been dominated by infrequent large pulses that blanket downstream valley floors with material. In the periods between these pulses, most bed material transport in the proglacial rivers is sourced from erosion of previously-emplaced valley floor deposits. We suggest coarse sediment fluxes in the lower White River are unlikely to be sensitive to short-term changes in headwater delivery rates, both because punctuated proglacial sediment pulses are attenuated by storage and because headwater delivery is an overall modest component of lower river sediment load. More generally, the introduction of bed material into the White River appears dominated by extreme hydrologic or extra-fluvial (glacial, volcanic) events with 102
year recurrence intervals, while more typical floods primarily redistribute existing stored sediment.
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In Text Citation:
Anderson and Jaeger (2019) or (Anderson and Jaeger, 2019)
Anderson, S.W. and K.L. Jaeger, 2019, Coarse sediment dynamics in the White River watershed: Report for King County Water and Land Resources Division, Department of Natural Resources and Parks, United States Geological Survey, Washington Water Science Center, 50 p..