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<br />During the initial seven year monitoring period at former USGS Station 09243800, average annual discharge has
<br />varied from 0.02 cfs in water year 1977 to 2.0 cfs in water year 1980. The longest no flow periods in water years
<br />1976, 1977, 1978, 1979, and 1980 were 145, 158, 184, 133, and 103 days, respectively. In contrast, the longest no
<br />flow periods during water years 1981, 1982, and 1983 were 25, 4, and 19 days, respectively. The longest no flow
<br />period on record occurred during a 291 day period from June I6, 1977 to Apri13, 1978. The changing flow pattern at
<br />former USGS Station 09243800 is believed to be primarily the result of higher precipitation, but could also involve
<br />contributions of mine dischazges from Pond A at the Eckman Pazk Mine. The maximum peak discharge on record for
<br />former USGS Station 09243800 is 55 cfs recorded on April 21, 1980. In contrast, during the drought period of water
<br />year 1977, the maximum peak discharge was only 0.16 cfs.
<br />Streamflow downstream of the mine is recorded at former USGS Station 09243900 (now TCC site 900) and at TCC
<br />Station 2005 (Figure 5-c). Data from former USGS Station 092439001ocated at the mouth of Foidel Creek describes
<br />Streamflow conditions downstream of the mine plan area. During water yeaz 1977, annual flows at this station
<br />average flow at former USGS Station 09243800 upstream of the mine plan area. Likewise, during water yeaz 1980,
<br />annual flows averaged 4.34 cfs or about two times the flow at the upstream station for the corresponding period.
<br />Maximum peak discharge for the period of record from October, 1975 through September, 1985 was 90 cfs recorded
<br />on April 22, 1980. The maximum peak discharge during the drought year of 1977 was only 2.5 cfs recorded on
<br />March 26, 1977. ALog-Pearson Type III flood frequency analysis of floods on Foidel Creek at Station 09343800
<br />indicates a 10-yeaz peak flow of 88 cfs and a 100-year peak flow of 159 cfs {see Table 10, Surface Water Data). The
<br />7-day, 10-year low flow on Foidel Creek above and below the mines was zero prior to any mine discharge.
<br />Comparison of hydrographs in Figures 5-a through 5-c show the similarity of flow patterns between the upstream and
<br />downstream stations on Foidel Creek. The baseline record from October, 1975 to October, 1979 depicts an
<br />intermittent stream with relatively long periods of now flow lasting from June or eazly July until February or,March of
<br />the following year. Starting in the fall of 1980 the Streamflow pattern has shifted to a much shorter no flow periods
<br />• and higher mean annual flows partly as a result of higher precipitation and partly due to effects of surface mining.
<br />Since the initiation of underground mine discharge from the Foidel Creek Mine im the fall of 1983, Streamflow at
<br />Station 09243900 neaz the mouth of Foidel Creek has almost become perennial, except during severe drought years.
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<br />Middle Creek is also an intermittent stream. The USGS had measured streamflows on Middle Creek at Station
<br />09243700 as shown on Map 13, Twentymile Pazk Hydrology, until TCC took over monitoring the site in November
<br />2001.. During the ten year monitoring period starting in October, 1975, annual mean flows have varied from 0.5 cfs
<br />in water year 1977 to 13.2 cfs in water year 1984. The longest no flow period on record was a 251 day interval from
<br />July 15, 1977 through March 27, 1978. Perennial flow occurred during water years 1980, 1983, 1984, and
<br />1985(Figure 5-d). Since this station is above any influence from mining it serves to illustrate the shift in flow regimes
<br />resulting from climatic conditions. The 7-day, 10-yeaz no flow for Middle Creek is zero. The maximum peak flow on
<br />record was 329 cfs recorded in water year 1981 on July 2. ALog-Pearson Type III flood frequency analysis for the
<br />period of record indicates a 10-yeaz peak flow of 216 cfs and a 100-year peak discharge of 807 cfs (see Table 10,
<br />Surface Water Data).
<br />Streamflow data for Fish Creek have been collected above and below the mine at the stations shown on Map 13,
<br />Twentymile Pazk Hydrology. Table 10, Surface Water Data, includes Streamflow records for Fish Creek downstream
<br />of the mine plan azea at Station 1003 and upstream of the mine plan area at Station 1001. Additional flow data are
<br />provided at an intermediate site, Site 26 (Station 1002). Variation in Streamflow along this reach of Fish Creek is
<br />illustrated in the monthly hydrogaph plots provided in Figures 5-e, 5-f and 5-g. These hydrographs show that Fish
<br />Creek is a perennial stream that gains considerable flow during the months of March through June as it traverses this
<br />reach through Twentymile Park. During baseflow periods Fish Creek does not appear to gain flow over this reach.
<br />Baseflow levels vary considerably from year to year on Fish Creek. Data from the upstream gauging Station 1001
<br />indicate baseflow rates from about 1 cfs in dry years to as high as 8 cfs in the relatively wet years of 1984 and 1985
<br />(see Table 10, Surface Water Data). The downstream gauging Station 1003 shows comparable ranges for baseflow
<br />conditions.
<br />PR 03-06 2.04-32 05/07/03
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