Author(s): Shan Zheng, Colin R. Thorne, Shasha Han, Baosheng Wu
Linked Author(s): Shan Zheng
Keywords: Evolution model, evolution stage, degradation and aggradation, lateral adjustment, North Fork Toutle River
Abstract: Channel evolution models (CEMs) are frequently used to explain and predict channel changes in incised streams but they have not yet been applied to volcanically disturbed rivers. The North Fork Toutle River (NFTR) provides excellent opportunity to examine the applicability of the CEM approach to rivers disturbed by volcanic eruptions that introduce heavy large inputs of sediment. In this study, vertical and lateral adjustments in the ~30-km of river channel disrupted by a 2. 5 km3 debris-avalanche deposition during the catastrophic eruption of Mount St. Helens in 1980 are investigated. A recent revised channel evolution model named the Stream Evolution Model (SEM) is applied to the NFTR. The results show that the upstream channel reaches generally degrade in the first few years following the eruption, evolving in SEM Stage 4 (i. e. degradation and widening), while the downstream reaches evolve in Stage 5 (i. e. aggradation and widening). Starting in the late-1980s, this simple pattern is disrupted by renewed incision and secondary adjustments. Upstream reaches evolve to stages 5 (aggradation and widening) and 7 (laterally active) as rates of vertical adjustment slow and river valley top widths relax to asymptotic values. Channel evolution at individual cross-sections tends to follow the sequence of stages expected according to the SEM, indicating the applicability of the model. However, stages 4 and 5 are sometimes repeated and stage 6 is sometimes omitted. Stage 8 (anastomosing) only occurs in the reach furthest downstream, where the valley floor is very wide. As expected from the SEM, lateral adjustment has become predominant since 1990s and now seems largely responsible for persistently high sediment yields from the upper NFTR. According to the SEM, degradation, widening and lateral activity are forecast to continue until stable transverse valley profiles and channel planforms are formed and floodplain and terrace surfaces are stabilized by vegetation
Year: 2017