Dames & Moore, 1999 - USDA Forest Service

May and June, which coincide with the snowmelt period. Low flows also occur from late summer through
fall; however, there is a possibility of large flow events occurring in the late summer and early fall due to
rainstorms. The hydrologic pattern reflects a steady baseflow provided by glacial melt and groundwater in
the summer and winter punctuated by seasonal storm events of short duration and snow melt of longer
duration.
4.3.3 Site Climate and Hydrology
4.3.3.1 Site Climatic Conditions
Average precipitation at Holden Village from 1962 to 1997 is approximately 38 inches annually, with the
highest monthly amounts occurring between November and January, and the lowest between May and
August. Table 4.3-2 lists average monthly precipitation totals for the period of record at Holden Village
along with estimated monthly totals for the Railroad Creek basin as a whole (above Lucerne). During the
winter of 199611 997 the second highest recorded snowfall occurred, with approximately 500 inches falling
through the winter (NOAA weather records for Holden Village, Appendix H).
The snowpack accumulation during mid-April 1997 was approximately 72 inches at the weather station, and
was 43 inches by May 1, indicating that approximately one-half of the accumulated snow had melted by the
beginning of May. The snowpack had disappeared from the measuring location and most open areas by
May 17 (NOAA weather records for Holden Village, Appendix H); however, field observations during the
RI indicated that one to two feet of snow remained on the ground at this time in shaded areas. Due to the
large snowpack, avalanches upslope of the Site were frequent during the winter of 199611997 (personal
communications with Holden Village residents, 1997).
Monthly average daily temperatures for Holden Village are shown on Table 4.3-3, along with estimated
average daily temperatures and potential evapotranspiration for the basin as whole. 'Potential
evapotranspiration from Holden was estimated based on the average temperature values and estimated
percent cloud cover. The cloud cover data was an average of percentages observed at Seattle and Yakirna
(data is unavailable for Holden). Based on these data, average annual potential evapotranspiration from the
basin h6 been estimated to be on the order of 16 inches (see Section 4.3.5) and actual evapotranspiration
approximately 10 inches.
4.3.3.2 Streamflow Monitoring
Streamflow Monitoring Stations
Streamflow monitoring stations used during the RI are shown on Figures 4.3-3 and 4.3-3a. Stations RC-1
(established to be upstream of the mine-affected area), RC-2 (immediately downstream of the tailings piles),
and RC-3 (at Lucerne) were continued from previous work conducted by PNL and the USFS. New stations
include RC-4 (immediately upstream of the tailings piles), RC-5 (approximately one-half mile downstream
of tailings pile 3), RC-6 (established upstream of RC-I to confirm the upstream limit of the mine-affected
area), RC-7 (adjacent to tailings pile 2), RC-8 (established upstream of RC-3 in an attempt to collect data
where Railroad Creek flows directly atop bedrock), RC-9 (an aquatics sampling station established upstream
of the Copper Creek confluence adjacent to tailings pile I), RC- 10 (approximately three miles downstream
of tailings pile 3), and RC-I 1 (established immediately upstream ofthe Holden Creek confluence after it
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