Dames & Moore, 1999 - USDA Forest Service

this, it is possible that the snow which accumulated on the tailings piles could have infiltrated in 1997,
except in areas of steeper slopes and where the ground became saturated. However, during days of high
snowmelt, runoff may be generated in areas where the infiltration capacity is lowest, as illustrated by the
fact that the drainage ditches were observed flowing for periods of time in May and June. Few rainfall
events are expected to exceed the infiltration capacity, except during the most extreme events, including
hypothetical 50- and 100-year storms.
4.3.3.8 Other Tributaries to Railroad Creek
Several tributaries to Railroad Creek (Figure 4.3-2), other than Copper Creek (and the Copper Creek
diversion), were sampled during 1997 and 1998 field programs and included Tenmile Creek, Holden Creek,
RC-I 1, and Big Creek. The flow data collected for the tributaries was limited to one measurement. RC-I 1
is included as a tributary but is actually on Railroad Creek upstream of the Holden Creek confluence and is
considered an upper basin tributary to the main channel of Railroad Creek.
Flow in Tenmile Creek in September 1997 was approximately 7 cfs, which was comparable to Copper
Creek, excluding the Copper Creek diversion. The watershed area of Tenmile Creek is approximately 4
square miles (larger than the drainage of Copper Creek); however, the flow was significantly lower, by
about one-half assuming the diversion was flowing at about 7 cfs. This indicates that flow contribution
from the north side of the Railroad Creek basin may be less than the south side during spring due to the
south-facing aspect of the slopes (solar gain during the winter months likely melts some of the snow before
spring arrives).
Flow in the upper tributaries generally was not measured directly, but was visually estimated. During the
October 1997 sampling event Holden Creek was estimated to be flowing at approximately 40 cfs, and
Railroad Creek at RC-I I was estimated to be flowing at between 150 and 200 cfs. At the time of these
estimates, flow at RC-4 was approximately 280 cfs. The watershed area of Railroad Creek above RC-1 I is
approximately 14 square miles, and the watershed area of Holden Creek is approximately 3 square miles.
Based on watershed area ratios, the expected flow at RC- I 1 would be about 5 1 percent of the flow at RC-4
(watershed area 27 square miles) and Holden Creek would be about 1 1 percent of the flow at RC-4.
4.3.4 Railroad Creek Streambank Erosion
Channel observations in Railroad Creek indicate that sediment sources are limited within the basin, and that
the channel carries a relatively low sediment load. For mountain streams in the Cascade Mountains, this
condition is characteristic of channels exhibiting frequent cascading reaches alternating with relatively
straight channel reaches with uniform bed material, and large clast riffle pool channels with bar
development (Montgomery and Bufington, 1997); all of these conditions exist in Railroad Creek. This is
also evidenced by relatively low amounts of sedimentation and limited delta development at the mouth of
Railroad Creek. Because of the low sediment load and narrow valley for the majority of the basin, channel
braiding is not a dominant feature of Railroad Creek. However, in the wider areas, braiding does occur, but
likely does not shift frequently.
Channel erosion, therefore, is limited throughout most of the Railroad Creek channel reach, except for those
cases where the presence of obstructions causes flow energy to be directed toward a bank. As noted above,
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17693-005-019Uuly 19. 1999;4:51 PM.DRAFT FINAL RI REPORT