Graduate Thesis Or Dissertation
 

Sediment routing in a small watershed in the blast zone at Mount St. Helens, Washington

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  • A descriptive model of the routing of sediment that was delivered to the study area by the 1980 eruptions of Mount St. Helens has been developed. On hillslopes this sediment was distributed among three major storage compartments: 1) the tephra profile, 2) primary storage, and 3) secondary storage. The most important sites of hillslope-sediment storage were colluvial wedges and sites upslope of logs or other organic debris. Hillslope-erosion processes redistributed sediment on hillslopes, and delivered sediment to the channel system. These processes included sliding from slopes steeper than 0.70, inter-rill erosion (including sheet erosion), and rill erosion. Sediment from the 1980 eruptions was delivered to the channel system by direct deposition and by hillslope-erosion processes. Much of this sediment was then transported by fluvial erosion, and either redeposited in channels or exported from the watershed. Important sites of sediment storage in channels included alluvial fans, sites related to logs, channel beds and alluvial terraces. Twelve percent (49,000 t/km²) of the total sediment delivered to the study area (400,000 t/km²) was received by the channel system in the first post-eruption year. Six percent (24,000 t/km²) remained in storage in the channel system, and six percent (25,000 t/km²) was exported from the watershed.
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