Dames & Moore, 1999 - USDA Forest Service

with little or no braiding. Upstream of the vehicle bridge, cutbanks supply sediment to the stream and are
also associated with accumulations of large woody debris (logs and root wads), probably due to
undercutting of trees on the banks as the cutbank erodes. Also at this location, sediment accumulation and i
channel braiding occurs because of proximity to the cutbank source. i
Based on the review of a historic map of Railroad Creek, an old alluvial channel or braid of Railroad Creek
appears to exist beneath the western portion of the Site and tailings pile 1 (Figure 4.3-3b); the map does not
display information east of the northwest comer of tailings pile 2. Referring to Figures 4.2-6b and 4.2-1 1,
geophysical investigation line E-E' (immediately east of tailings pile 3) appears to confirm the eastern extent
of the historical alluvial channel. Reportedly, the main channel was-relocated to its present location when
the tailings piles were constructed. The tailings piles are also underlain by apparent alluviallfloodplain
deposits, suggesting that, prior to mining, the channel reach adjacent to Holden Village occupied a relatively
wide floodplain within which the channel could form braids and develop bars similar to the reaches
upstream and downstream of the tailings.
Presently, a short segment of the creek bed at the east end of tailings pile 3 (immediately downstream of
RC-2) is slightly higher in elevation than the adjacent ground surface. This condition would not be expected
within a natural channel system, and probably exists because sediment which would normally accumulate
within the floodplain has been transported within the straightened reach adjacent to the piles and
accumulated just downstream of the tailings.
Adjacent to Holden Village, from RC-4 to RC-7 (downstream of the Copper Creek confluence), the north
(left) bank exhibits floodplain and occasional woody debris. As stated earlier, the stream gradient within
this reach varies from 1 to 2 percent and streambed materials consist of cobble with occasional small
boulder and some gravel. Below the confluence with the Copper Creek diversion (CC-DICC-Dl) the
streambed exhibits iron staining and accumulations of iron (orange) oxide flocculent/precipitate within the
interstices of the cobbles (see Figure 4.3-3c).
Also in this reach, near the confluence of Railroad Creek and Copper Creek, isolated portions of the south
bank of the creek (SP- 1 area, RC-9 to Copper Creek confluence, and SP-3 area) are cemented with apparent
iron oxyhydroxides (ferricrete) (refer to Figure 4.3-3d). The cementation was found in three portions of the
streambank along tailings piles 1 and 2 and appeared to range in thickness from about 0 to 4 feet. Along this
reach, the streambed itself does not appear to be entirely composed of ferricrete.
Below Copper Creek, 'ferricrete was generally not observed exposed in the streambed. However,
approximately 50 meters of the south bank near the northwestern comer of tailings pile 2, which is
coincident with seep SP-3 (Figure 4.3-3d), was observed to be cemented. It is possible that other isolated
occurrences of ferricrete exist within this reach of the creek, but are masked by the presence of the riprap.
The cobble substrate was generally loose, although a fine iron-oxide precipitate coating was present on the
cobbles and boulders and within the interstices of the substrate. The depth of iron-oxide precipitate
accumulation was not extensive, less than the embedded depth of the small boulders within the streambed
(evidenced by the lack of orange staining on the underside of the larger bed particles).
From the eastern end of tailings pile 3 (RC-2) to RC-5, near the Tenmile Creek confluence, the ferricrete
was not observed and the iron-oxide precipitate was noted as suspended material, but was not measurable on
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17693-005-019Uuly 19. 1999:4:5 I PM;DRAFT FINAL RI REPORT