Dames & Moore, 1999 - USDA Forest Service

effect of a 0.05-foot stage increase at RC-4 as estimated from the .RC-4 rating curve), a baseflow increase of
1 to 7 cfs could be attributed to groundwater inflow along thk channel reach.
Flows measured at RC-7 and RC-2 indicate a gain in flow between these stations of approximately 3 cfs
accounting for an increase of 6 cfs between April 16 and April 17 due to a rise in stage of 0.05 feet. This
groundwater gain is approximately 3 percent of the flow measured at RC-2, which is comparable to the
maximum percentage gain estimated in Section 4.3.3.3 using the average flow increase between RC-4 and
RC-2 (subtracting Copper Creek) for all of the 1997 measurements. Flow was observed to decrease
between RC-2 and RC-5 (located downstream of RC-2; see Figure 4.3-3) by approximately 8 cfs (9 percent
relative to RC-2), which is consistent with field observations.
4.3.7.2 September 1997 Suwey
A baseflow survey was also completed during the September 1997 sampling round to evaluate the baseflow
conditions before the winter season. During this survey, flow was measured in one day at all streamflow
monitoring stations at the Site (Figures 4.3-3 and 4.3-3a). Table 4.3-8 shows the flow measurements
obtained during this survey. The results of this survey indicate a different groundwater contribution pattern
compared to April 1997. Flow measurements between RC-I and RC-4 indicate a gain of approximately 5
cfs, of which 0.3 cfs was determined to be from the portal drainage. The measured flow gain, however, is
within the measurement error @ 5 to 7 percent); consequently, the findings are not conclusive..
Flow measurements between RC-4 and RC-7 indicate a gain of 13 cfs which can be accounted for entirely
by Copper Creek flowing into Railroad Creek within this reach. This indicates that there is little
groundwater gain along this reach. During this sampling event, most of the seeps along this reach were not
flowing or were flowing at extremely low rates. Additionally, flow was apparently lost between 'RC-4 and
RC-9; however, there does not appear to be an explanation for this and may be the result of measurement
error. The flow loss as measured indicates that the reach is not gaining significantly from groundwater.
There also does not appear to be groundwater gain between RC-7 and RC-2. Flow between RC-2 and RC-5
(downstream of RC-2) indicates a similar pattern as April, with an apparent loss of flow. However,
referring to Table 4.3-8 and Figure 4.3-3% this flow is apparently regained downstream of SP-21, which
indicates that there is a groundwater contribution near the valley wall on the south side of the creek
(downstream of tailings pile 3).
4.3.7.3 October 1998 Survey
Additional baseflow measurements were made during October 1998 in Railroad Creek between stations
RC-6 (encompassing RC- I) and RC-4 in order to further characterize the nature of the flow loss that was
observed between RC-1 and RC-4 during 1997. The 1998 baseflow survey was designed to characterize
the flow loss noted between RC-I and RC-4 in 1997, and to evaluate the specific locations of this loss.
The streamflow measurement error is assumed to be on the order of 5 to 7 percent of the actual flow
because of the irregular streambed, and therefore individual measurement error could easily be greater
than the necessary detection of flow gains and losses. Therefore, the streamflow measurement strategy
included four measurements at each selected station to provide an average value of the flow.
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17693-005-019Uuly 19. 1999;4:51 PM:DRAFT FINAL RI REPORT