Application of the Stream Evolution Model to a Volcanically Disturbed River: The North Fork Toutle River, Washington State, USA

Abstract

In this study, a recently revised version of the channel evolution model, named the Stream Evolution Model (SEM), was applied to the upper North Fork Toutle River disrupted by the deposition of a 2.5‐km3 debris avalanche during the catastrophic eruption of Mount St. Helens in 1980. The results show that, in the first few years following the eruption, upstream channel reaches generally incised, evolving in SEM Stage 4 (i.e. degradation and widening), while downstream reaches aggraded, evolving in Stage 5 (i.e. aggradation and widening). However, starting in the late‐1980s, this simple pattern was disrupted by incision in the downstream reaches, which seemed to propagate upstream. Since the 1990s, lateral channel adjustments have become predominant as rates of vertical adjustment have slowed and river valley top widths relaxed to asymptotic values. Spatial and temporal sequences of channel evolution have tended to follow the sequences of stages expected according to the SEM, although these sequences have been frequently disrupted by renewed incision, secondary cycles of adjustment and the impacts of local geologic, geomorphic and hydraulic conditions. Within the quasi‐full SEM cycles, stages 4 and 5 were sometimes repeated, while stage 6 (quasi‐equilibrium) was sometimes omitted, and stage 8 (anabranching) only occurred in the downstream braided/anabranching reaches. According to the SEM, degradation, widening and lateral activity (stages 4 and 7) are forecast to continue until transverse valley profiles and channel planforms stabilize and floodplain and terrace surfaces are fully colonized by vegetation. Copyright © 2017 John Wiley & Sons, Ltd.

Multidecadal Geomorphic Evolution of a Profoundly Disturbed Gravel Bed River System—A Complex, Nonlinear Response and Its Impact on Sediment Delivery

“…Portraying changes in bed elevation and cross‐section area between resurveys of NFTR in an alluvial phase space diagram reveals that during the last two decades, vertical channel adjustments, although remaining locally important, have largely diminished in magnitude, while lateral adjustments, especially through channel migration and widening, have persisted. This finding supports concordance with the SEM as reported by Zheng et al ().…”

“…In both our populated phase space diagrams and in the SEM proposed by Cluer and Thorne (), vertical and lateral adjustments can, and do, occur cyclically, with multiple episodes of channel degradation, aggradation, widening, and narrowing manifesting through disturbance responses that are complex in both space and time. In these respects, the geomorphic behavior of upper NFTR has shown some consistency with the SEM (Zheng et al, ).…”

“…Nevertheless, to develop an integrated surface-water drainage network on the debris avalanche deposit, it took nearly 3 years of storm and snowmelt runoff, breaching of ponds and valley margin lakes, pumping of water from Spirit Lake (its outlet blocked by the debris avalanche in the basin headwaters; Grant et al, 2017), and another moderately large (10 7 m 3 ) lahar (Janda et al, 1984;Meyer & Martinson, 1989;Pierson, 1999;Rosenfeld & Beach, 1983;Simon, 1999). In the nearly four decades since the eruption, the geometry and texture of that channel network, and upper NFTR channel to which it drains, have evolved continuously and considerably (Major , 2018;Meyer & Martinson, 1989;Mosbrucker et al, 2015;Simon, 1999;Simon & Thorne, 1996;Zheng et al, 2014Zheng et al, , 2017.…”

Applying geomorphological principles and engineering science to develop a phased Sediment Management Plan for Mount St Helens, Washington

“…We now know that elevated sediment yields in the NFTR have indeed persisted, primarily due to post‐disturbance channel evolution characterized by geomorphically complex response (Schumm, ). Such response involves cycles of incision, aggradation and widening, with channel changes becoming increasingly dependent on the occurrence of floods due to the combined effects of slope adjustments, bed armouring/fining, bank instability, and lateral channel erosion (Simon and Thorne, ; Major et al ., ; Zheng et al ., , Zheng et al ., ). In this context, Major et al .…”

“…But the District understands and takes seriously cautions from sediment experts that elevated sediment yields from the debris avalanche may persist for many decades (e.g. Major et al, 2000;Meadows, 2014;Zheng et al, 2017). The phased SMP therefore includes provision for further re-assessment in 2025 that will draw on the results of on-going monitoring to elucidate the processes responsible for trends in annual sediment yields, identify long-term trends in those yields, and re-evaluate the hazard posed by sedimentation rates in the lower Cowlitz River in light of the remaining sediment storage potential of the SRS.…”