Dames & Moore, 1999 - USDA Forest Service

Railroad Creek and Copper Creek. The probability of an earthquake occurring sufficient enough to exceed
the theoretical threshold for horizontal acceleration to initiate the shallowest movement is relatively
moderate to high. However, the deeper hypothetical failure was found to be less likely.
Erosion Potential
Tailings Pile Slo~es
Surface erosion of the tailings due to precipitation and wind results in a relatively minor amount of tailings
material being transported down slope and down wind. The potential for erosion is highest for the steeper
slopes of the tailings piles. The materials transported down slope have the potential for being delivered into
Railroad Creek during storm events.
The results of preliminary mapping conducted as part of the RI indicated that the distribution of wind-blown
tailings deposited on the ground surface is generally limited to the area between the tailings piles and the
confluence of Tenmile Creek and Railroad Creek. The maximum thickness of the deposits was noted to be
generally less than several inches immediately north of the Railroad Creek, down wind of the piles. The
thickness of the deposits decreased with the distance from the tailings piles, with the thickness near the
mapped limits generally less than an inch.
Existing Riurau
The existing riprap was placed along Railroad Creek between 1989 and 1991 with the intent of preventing
erosion of the tailings pile slopes by Railroad Creek. The majority of the existing riprap originated from a
rock quarry in the eastern portion of the Railroad Creek watershed, near Dan's Camp. An assessment of the
existing riprap was conducted as part of the RI.
The results of the assessment indicated that a number of rocks exposed at the surface are in relatively poor
condition and eroding relatively rapidly. The riprap of highest quality was observed at the northwest comer
of tailings pile 2, immediately downstream of the Copper Creek confluence. at the location where it is most
needed to prevent erosion by Railroad Creek. The majority of the remaining riprap was variable in quality
and resistance to erosion.
Further erosion of the riprap could reduce its effectiveness for preventing removal of the tailings materials
by Railroad Creek during storm events. The implications of the eroding riprap are further discussed below
in the Surface Water erosion section.
8.2.2.4 Aquatic Reference Reaches
The geology for the aquatic reference reaches is similar to the Railroad Creek geology, consisting primarily
of metamorphic bedrock with igneous intrusives. The bedrock in the main stem of the Stehekin River
drainage was scoured by a glacier that carved out the Lake Chelan Basin. The drainages in which the three
reference reaches are located were carved by smaller glaciers and overlain by a combination of glacial and
alluvial soils, similar to Railroad Creek. Similar to Railroad Creek, the bedrock is generally exposed along
the valley walls and ridges, and less often within the stream drainages. The area also contains a number of
economic mineral deposits similar to the Railroad Creek drainage, but none as extensive as the Holden mine
deposit.
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17693-005-0 19Vuly 28.1999;10:24 AM;DRAFT FmAL RI REPORT