A Decision Framework for Managing the Spirit Lake and Toutle River System at Mount St. Helens (2018) / Chapter Skim
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2 Regional Setting
2 Regional Setting
Pages 43-82
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From page 43... ...
Alternatives for controlling water levels in Spirit Lake are also discussed in Chapter 5. A major point emphasized in the concluding section of this chapter is that regional characteristics must not be considered in isolation but instead must be analyzed as interrelated elements within a broader system.
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From page 44... ...
From the SRS, water and untrapped sentiments are transported into the main channel of the Toutle (approximately 33 miles [53 km] downstream of Spirit Lake)
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From page 45... ...
Helens National Volcanic Monument (white) within the Gifford Pinchot National Forest (green)
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From page 46... ...
Because the current outflow pathway for Spirit Lake is through the North Fork Toutle River system (which was the river most heavily impacted by the eruption) , discussions in this report will emphasize that drainage system, but there are management issues related to the decision framework that may involve other river systems in the region.
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From page 47... ...
SOURCE: USACE, 2016a. The northward-directed blast and debris avalanche from the 1980 eruption temporarily displaced all the preexisting Spirit Lake from its bed (see Figure 2.3)
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From page 48... ...
Helens, the North Fork Toutle River originated at Spirit Lake, where the debris blockage currently exists. Discharge from Spirit Lake now reaches the North Fork Toutle River via the Spirit Lake tunnel and South Coldwater Creek (see Figures 1.2 and 1.3 for locations)
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From page 49... ...
Because it emanates from Spirit Lake and traverses Coldwater Lake on its way to the Toutle River, the lake outflow contributes no sediment until it flows across the sediment plain behind the SRS, picking up small amounts of sediment, approximately in proportion to its flow contribution. The rest of the upper Toutle River valley, outside of the Spirit Lake basin, is the source of approximately half of the mean annual flood discharge and of all the sediment arriving at the SRS and being transported to the lower Cowlitz River.
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From page 50... ...
GEOLOGIC SETTING The Spirit Lake and Toutle River region is in an active geologic setting. Understanding its geology and associated hazards is important to understanding the risks associated with management decisions for the region.
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From page 51... ...
Regional Setting FIGURE 2.4 Block diagram showing sources of seismicity in the region. The Juan de Fuca Plate is shown being subducted under the North American Plate.
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From page 52... ...
The combination of contrasting materials, abundant loose debris, and steep slopes contributes to additional hazardous processes. The volcanoes themselves are very unstable; large sectors may collapse and spread over surrounding valleys as debris avalanches -- mixtures of loose clastic debris and intact blocks of volcanic rock.
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From page 53... ...
The lahars can grow as they flow and pick up more debris, extending tens of kilometers or more downstream and inundating rivers and valleys. Ancient Lahars Geologic evidence interpreted from the extent, height, and sedimentological characteristics of deposits along the Toutle River and its confluence with the Cowlitz River indicates that catastrophic breakouts of a lake in the general vicinity of Spirit Lake have occurred repeatedly during past eruptions (Scott, 1988a,b)
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From page 54... ...
Hausback and Swanson (1990) identified two large debris avalanche deposits on the northern flank of the volcano that occurred within the same 2,500-3,200 years before present period, and it is likely that these deposits dammed the predecessors of Spirit Lake and provided easily erodible material that could be entrained into water during subsequent catastrophic breaching of the debris blockages.
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From page 55... ...
Helens created three debris dams that reshaped the Toutle River watershed: the Spirit Lake blockage, the Castle Lake blockage, and the Coldwater Lake blockage. The Castle Lake and Coldwater Lake debris blockages have received less attention to date.
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From page 56... ...
. Figure 2.6 is an idealized geotechnical engineering cross-section of the debris blockage based on the original subsurface investigation conducted shortly after the 1980 eruption of Mount St.
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From page 57... ...
FIGURE 2.6 Idealized cross-section of the Spirit Lake blockage. The debris blockage also includes pits resulting from the melting of blocks of ice from glaciers located on Mount St.
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From page 58... ...
Harry's Ridge is located immediately to the right (west) of Spirit Lake.
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Regional Setting FIGURE 2.8 Photograph of erosion of the Spirit Lake debris blockage, erosion pipe within the ash deposit. SOURCE: Glicken et al., 1989.
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From page 60... ...
They also receive infiltrated water from a number of sources, including streams draining the volcano and Johnston Ridge. Given that the materials of the debris blockage are heterogeneous and some have low density, preferential flow paths could well develop within the debris blockage, creating a risk for the development of piping failure if the water level in Spirit Lake rises sufficiently.
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From page 61... ...
, which documents the presence of a shallow groundwater body in the debris avalanche deposit. Groundwater Flow in the Spirit Lake Debris Blockage Basin-scale surface and subsurface hydrologic processes, surface erosion rates, and subsurface stability of the debris blockage are governed by precipitation patterns (rain and snow)
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From page 62... ...
. Data collected prior to 1992 indicate the water table within the debris blockage "tends to lower between the edge of Spirit Lake and areas downstream" (USACE, 2016a)
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From page 63... ...
. Groundwater flows into Spirit Lake on the east of the divide and discharges at the North Fork Toutle River on the west of the divide.
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From page 64... ...
Lower-density materials could allow preferential flow paths to develop within the blockage (such as the piping shown in Figure 2.8) , elevating a risk of failure associated with piping if water levels in Spirit Lake rise sufficiently.
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From page 65... ...
Helens landscape was poorly documented at the time of the eruption, it is thought to have com prised thousands of species. The Upper Toutle River watershed contains suitable habitat for several species that are listed as threat ened or endangered or as USFS sensitive species.
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From page 66... ...
While many lakes contained no fish until active management and fish stocking began, Spirit Lake was connected to the Toutle River system without barriers, allowing the presence of coastal cutthroat trout, winter steelhead, and coho salmon. However, stocking was common to maintain levels of these species sufficient to sup port a thriving sports fishery industry in the Toutle River system ( wanson et al., 2005: 20-26)
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From page 67... ...
. Management decisions made regarding the Spirit Lake tunnel and the SRS may affect a range of ecological components, a number of which were identified as important by interested and affected parties during the committee's open session meetings in Kelso, Washington.
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From page 68... ...
The hatchery is operated to produce adult fall Chinook and coho salmon for commercial and sport fisheries in the northeast Pacific and Columbia River basin (see http://docs.streamnetlibrary.org/IHOT/NorthToutle-Cohos1997.pdf )
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From page 69... ...
, and access to Spirit Lake from the Toutle River system remains blocked by the debris avalanche. The issue of restoring anadromous fish to the upper North Fork Toutle River is one with not only biophysical challenges related to erosion and sediment movement into the North Fork Toutle River drainage but also 69
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From page 70... ...
, they face issues of finding suitable habitat and have no access to Spirit Lake itself because of the debris blockage and the design of the Spirit Lake tunnel. Beyond anadromous fish species, there may be other management and legal considerations associated with the ESA.
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From page 71... ...
Tunnel As described in Chapter 1, a 1.56-mile (2.6-km) tunnel was constructed in 1984-1985 through Harry's Ridge, west of Spirit Lake, to regulate lake levels and prevent breaching of the debris blockage (see Figure 1.3)
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From page 72... ...
There are five mapped shear zones shown. The location of the 2015 repair area is shown.
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From page 73... ...
water. Engineering solutions for the control of water levels in Spirit Lake are discussed in Chapter 5.
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From page 74... ...
Leveed areas along the lower Cowlitz River and property in leveed areas are valued at about $3.65 billion (USACE, 2017) ; they include portions of Longview, Kelso, Lexington, and Castle Rock (population of approximately 50,000)
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From page 75... ...
FIGURE 2.12 The SRS and the sediment plain (a) in approximately 1990 prior to the outlet works reaching their upstream capacity (b)
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From page 76... ...
FIGURE 2.13 Aerial view of the SRS and sediment plain taken both in (a)
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From page 77... ...
Helens itself is located in Skamania County, Washington, but the concerns that are most central to this report (especially those asso ciated with the Toutle and Cowlitz Rivers) focus primarily on impacts likely to be felt in adjacent Cowlitz County.
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Major employers in the county include Weyerhaeuser Timber Company, International Paper (formerly Long-Bell Lumber Company) , K apstone Paper (formerly Longview Fiber Company)
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From page 79... ...
The Cowlitz Indian Tribe The Cowlitz Indian Tribe is another influential force within the region. It and other tribes in the region are sovereign nations and therefore important stakeholders in decisions regarding Spirit Lake and the Toutle River system.
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From page 80... ...
HOW THE SETTING AFFECTS SYSTEM RISK ASSSESSMENT AND LONG-TERM MANAGEMENT Information on the regional setting contained in this chapter has a number of implications both for assessing risks and for making decisions about long-term management of the Spirit Lake and Toutle River system. Key among these are the fact that the physical setting is complex and dynamic, that socioeconomic considerations, including tribal concerns, are part of the decision landscape, and that decision making needs to be based on a holistic conceptualization of the region and issues.
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From page 81... ...
For example, the SRS has resulted in the trapping of sediments upstream of the structure, changing the landscape of that region and having wide-ranging effects on fish species that are valued by many local stakeholders. The tunnel designed to regulate water levels in Spirit Lake may be vulnerable to natural hazards such as earthquake-related movement.
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From page 82... ...
Spirit Lake, the Toutle River, and associated engineered works are elements within a broader regional system that includes many other features such as the Monument, the Cowlitz and Columbia Rivers, and downstream flood protection levees. Elements within that system are managed by different entities -- a point that will be emphasized in Chapter 3 -- and are often considered in isolation from one another.
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