Dames & Moore, 1999 - USDA Forest Service

Precipitation in the region can be highly variable due to strong orographic effects resulting from the
mountainous terrain. Weather patterns generally follow a west to east movement, with precipitation falling
heavily on the western.side of the Cascades and diminishing as the weather system moves east across the
mountains. In the Lake Chelan basin, precipitation is generally higher at greater elevation and decreases to
the east and south (USGS, 1975). Average annual precipitation in the basin ranges from a high along the
Pacific Crest of more than 100 inches, to less than 25 inches east of Chelan.
Streamflow patterns within the Lake Cheian basin are fairly consistent between individual watersheds,
although total runoff can vary significantly due to basin aspect, elevation and extent of glacierization. Flow
regimes are'dominated by snowmelt with peak flows generally occurring in May and June. Low flows
typically occur in late fall and persist until spring thaw. Comparison of average monthly flows for the
Stehekin River, Railroad Creek and the Entiat River reflect elevational differences within each watershed
and the presence of a relatively large percentage of glaciers in the Stehekin upper basin (Table 4.3-1 and
Figure 4.3- I).
4.3.2 Overview of Railroad Creek Watershed Hydrology
The Railroad Creek watershed is elongated and steep, and oriented west to east. Figure 4.3-2 shows the
Railroad Creek watershed and surrounding areas. The valley is characterized by steep-sided slopes carved
by the ,most recent glaciation. The creeks on the side slopes have steep gradients. Elevations of Railroad
Creek range from 6,500 feet msl at its headwaters in the Glacier Peak Wilderness area to approximately
1,100 feet msl at Lucerne, located on Lake Chelan. The average elevation of the basin upstream of Lucerne
is 4,930 feet (USGS, 1984). Most of the Holden Mine facilities and the tops of the tailings piles are between
3,200 and 3,400 feet above msl, which is as much as 200 feet above Railroad Creek and Holden Village.
The Railroad Creek watershed is approximately 65 square miles in size upstream of Lucerne. Lakes '
comprise approximately 0.5 percent of the total watershed area (located at high elevations), and glaciers
comprise approximately 1 percent (Figure 4.3-3). Approximately 68 percent of the basin is forested (USGS,
1984). The average slope of the main stream channel is 4.5 percent; however, the slope varies depending on
location in the watershed. In the vicinity of Holden Mine Site, the channel slope varies from 1 to 2 percent
and becomes steeper downstream. Approximately one half of the watershed is upstream of the Site (3 1
square miles).
The watershed experiences rapid response times from precipitation influx. During the September 1997 field
program, streamflow was observed to rise quickly (less than one day) in response to rainfall, and recede as
quickly when rainfall ceased. Because of these conditions, there is potential for flash flooding on Railroad
Creek, particularly from a combination of high-density rainfall and rapid snowmelt. Historically, a flood of
this type probably occurred on May 28, 1948, when soils were saturated from spring runoff, resulting in an
estimated discharge of 3,900 cfs at Lucerne. During this same storm, a discharge of 3,000 cfs was estimated
at Holden (PNL, 1992).
The Railroad Creek system is partially glacier fed, which contributes to the baseflow during the summer
along with groundwater contribution. Referring to Figure 4.3-3, glaciers comprise approximately 2 percent
of the watershed area of Railroad Creek upstream of the mine. Railroad Creek stream discharge in the Site
area is characterized by generally low flows during winter and peak flows occurring during the months of
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17693-005-01 9Uuly 19.1999:4:5 1 PM:DRAFT FINAL RI REPORT