Geologic Publications for Mount Rainier

Floodplains statement of findings: Carbon River area access management plan

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Author(s): Scott R. Beason

Category: PUBLICATION
Document Type: Environmental Assessment
Publisher: Mount Rainier National Park
Published Year: 2011
Volume:
Number:
Pages: 25
DOI Identifier:
ISBN Identifier:
Keywords:

Abstract:
This Statement of Findings (SOF) was proposed as part of the Carbon River Area Access Management Environmental Assessment (EA). The Carbon River Road corridor (FIGURE 1) was originally constructed in the early 1920s and has historically been an important cultural resource to the region, providing access to a uniquely wet habitat on Mount Rainier National Park's northwest side. The road corridor has also been classified in the National Register of Historic Places as part of the Mount Rainier National Historic Landmark District (NHLD). Additionally, vast tracts of designated wilderness are accessible from the northwest side of the park along the roadway. The goal of the Carbon River Road Area Access Management plan is to preserve year-round sustainable public access to the northwest corner of the Carbon River Valley. Executive Order 11988 (Floodplain Management) requires the National Park Service (NPS) to evaluate likely impacts of actions in floodplains. NPS Directors Order #77-2 (Floodplain Management) provide policy and procedural guidance for complying with these orders. This SOF documents compliance with these orders.

The Carbon River's headwaters are at the Carbon Glacier, the lowest elevation alpine glacier in the continental United States at approximately 3,500 feet (1,067 meters) above sea level (ASL). The Carbon River then flows north and west to the park boundary at 1,750 feet (533 meters) ASL. The Carbon Glacier begins its downward movement from near the summit of Mount Rainier at Liberty Cap, approximately 14,112 feet (4,301 meters). Along the way, the glacier scrapes and scours the volcanically-formed andesite rock below and adjacent to the glacier. The glacier acts as a giant conveyor belt and carries this rock and debris downstream to the headwaters of the Carbon River, for the river to carry out of the park. The river flows as a braided stream through a wide glacially-formed valley, constantly changing its braids and bars as sediment and water discharge fluctuate. Over time and owing to the river's exceedingly large sediment source, the riverbed is rising, or aggrading, as more sediment is provided to the river than can be conveyed out of the system. The Carbon River has historically aggraded up to 0.559 feet/year (0.170 meters/year) in a period between 1915 and 1971; or raising a total of 31.329 feet (9.549 meters) in 56 years (Beason, 2006). The Carbon River's 52.023 square mile (134.739 square kilometer) drainage basin at the park entrance receives 99.4 inches of rain and is covered with approximately 57.9% forest (TABLE 1).

In November 2006, almost 18 inches of rain fell park-wide and lead to the single longest closure in the park's history (6 months between November 6, 2006-May 5, 2007; The Carbon River Road currently remains closed to public vehicle traffic at the Carbon River Entrance). The Carbon River valley was one of many areas in the park that received significant infrastructure damage. Between November 5, 2006 at 2:00 P.M. and November 7, 2006 at 2:15 P.M., 8.76 inches (22.25 cm) of precipitation was recorded at the USGS stream gauge on the Carbon River near Fairfax, WA (USGS Gauge #12094000). Flood stage of 13.5 feet (4.1 meters) was recorded at the gauge around noon on November 6th and the stream gage reached its highest recorded gauge height of 16.93 feet (5.16 meters) about six hours later. The flood significantly damaged the Carbon River Road, especially near Falls Creek (2,600 linear feet; 792 meters) and just before Ipsut Creek Campground (1,350 linear feet; 411 meters). In these locations, the road was washed away and replaced with a gully approximately 6-10 feet (2-3 meters) deep. Also, one lane of the Carbon River Road was washed away in two locations and both lanes were removed in one location between the Green Lake Trailhead and just before the Ipsut Creek Campground. Low recurrence interval (approximately 15-year) floods since 2006 have caused more damage to both the roadway and park infrastructure, mainly the loss of a structure by bank erosion at the Carbon River maintenance area.

ENTRIX (2008) have shown that there may be an increase in the frequency and intensity of flood events as recorded by United States Geological Survey (USGS) stream gauges near the park. For instance, on the Carbon River at Fairfax, WA, the 100-year flood during the period of record from 1930-1977 now has a recurrence interval closer to 70 years when compared with the entire period of record (1930-2006) (FIGURE 2). Therefore, ENTRIX (2008) states that design conditions are changing and larger, more intense floods should be anticipated. On the Nisqually River, on the park's southwest side, there were no 10-year recurrence interval floods that occurred before 1970. Since then, there have been 6, including two events with recurrence intervals greater than 50 years. The general trend for the Nisqually River and Carbon River is an increase in the size of annual peak flows since the period of record began in 1940 and 1930, respectively. According to research by the University of Washington Climate Impacts Group (UW CIG), it is anticipated that by 2080, average yearly temperatures in the Washington Cascades region will be approximately 5.9°F warmer with an overall increase in precipitation of about 1-5%. Most of the anticipated increases in temperature will be between October and January (Mote, personal communication, 2008). The trend is for dryer summers and wetter winters, which is significant in that the largest and most destructive floods occur in the late fall during the period of record at both the Nisqually and Carbon Rivers.

The Carbon River valley has had a long history of flooding since the establishment of the Carbon River Road. Large floods in 1990, 1996 and 2006 caused major damage to the roadway (the second, third and largest floods on record since 1930, respectively) (FIGURE 3). Following the 1996 flood, Mount Rainier National Park spent approximately $787,000 on a repair to the road. Two medium-size floods five weeks later destroyed the recently-repaired sections of roadway, washing out a 1,200 foot (366 meter) section of roadway to a depth of about 2-3 feet (0.6-1.0 meters). Even low recurrence interval floods have historically caused damage to the roadway and associated park infrastructure near the river (FIGURE 4). The Mount Rainier General Management Plan (GMP) signed in 2002 stated that the park would no longer maintain the Carbon River Road after the next major washout. The GMP did not define what a "major washout" of the road would be but under the guidance of the GMP, Mount Rainier National Park is not considering repairing and reopening the entire road corridor in its previous condition as part of the current EA.

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Suggested Citations:
In Text Citation:
Beason (2011) or (Beason, 2011)

References Citation:
Beason, S.R., 2011, Floodplains statement of findings: Carbon River area access management plan: Environmental Assessment, Mount Rainier National Park, 25 p..