Dames & Moore, 1999 - USDA Forest Service

hydrograph which includes the transducer data. The average flow at RC-I was estimated based on the
average difference between the RC-I and RC-4 measurements, observed from flow measurements during
the 1997 field season. The RC-4 flows were estimated from the rating curve because direct flow
measurements at RC-1 and RC-4 were not recorded simultaneously.
The flow at RC-'1 averaged 1 .O1 (101 percent) to 1.0 times the flow at RC-4 as estimated from the RC-4
rating curve over the course of the 1997 field season. For the purpose of this analysis, estimated flow at
RC-1 (variable "Qrcl" as noted above in equation (4-13) in this subsection) was estimated to range
between 1 .OO and 1 .O1 times the flow at RC-4 based on the field season average.
The average flow at Station RC-4 (Qrc 4) during the May/June 1997 event is estimated to be
approximately 460 cfs (Figure 4.3-4); therefore, the average flow at RC- I is estimated to range between
460 and 465 cfs based on the field season average flow relationships. The average flow at RC-4 during
September 1997 is estimated to be approximately 125 cfs; thus, the estimated flow at RC-I ranges from
125 and 126 cfs based on the field season average.
An apparent slight loss of flow between RC-1 and RC-4 was observed during the April 1997 baseflow
survey, but was not observed in September 1997. However, over the course of the field season in 1997,
an analysis of the average flow relationship between RC- I and RC-4 indicated a loss of zero to 1 percent
of the flow at RC-4. The loss of flow between these stations was observed even though the portal
drainage flows into this reach, as does runoff observed in seeps. Additionally, it is expected that overland
runoff and groundwater contribute to flow in the creek within this reach as well.
Additional baseflow measurements were collected in October 1998 between stations RC-6 and RC-4 to
further characterize the nature of the apparent flow loss between RC-I and RC-4 observed in 1997. The
results of this survey were discussed previously in Section 4.3.7.3. The data was inconclusive and
suggests that there is not a net significant change or loss in baseflow from RC-I to RC-4.
Reach 2
The flow relationships between RC-4 (Qrc4) and RC-2 (Qrc2). based on the field season averaged values,
indicates that flow at RC-2 is 1 12 percent, to 1 15 percent of the flow at RC-4 during both spring and fall
(Table 4.3-4). This percentage range is based on the values presented in Table 4.4-3 and is estimated
based on the differences between RC-2 and RC-4, and between stations' measured values and rating curve
values. A transducer was not placed at RC-2; therefore, a hydrograph presenting flow data for RC-2 is
not provided. The observed gain in flow is expected because of groundwater and seep contribution, and
tributary contribution from Copper Creek and the Copper Creek diversion. Based on the field season
average flow relationship with RC-4, the flow at RC-2 for the May/June period is estimated to range
between 5 15 and 529 cfs, and 140 to 144 cfs for the September period.
Accuracy
The accuracy of the flow measurements were found to range between an estimated 5 and 7 percent of the
actual value and are a function of the measurement technique used. Error may also result from the
development of the rating curve, although the averaging process inherent in development of the rating is
expected to compensate for some of the measurement error. The accuracy of the flow estimates are
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