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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 16, 1977 to April 3, 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 discharges from Pond A at the Eckman Park 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 5c). Data from former USGS Station 09243900, located at the mouth of Foidel Creek, describes
<br />streamflow conditions downstream of the mine plan area. During water year 1977, annual flows at this station
<br />significantly exceeded the annual flow at former USGS Station 09243800 upstream of the mine plan area. Likewise,
<br />during water year 1980, annual flows averaged 4.34 cfs or about two times the flow at the upstream station for the
<br />corresponding period. Maximum peak discharge for the period of record from October, 1975 through September,
<br />1985 was 90 cfs, recorded on April 22, 1980. The maximum peak discharge during the drought year of 1977 was only
<br />2.5 cfs, recorded on March 26, 1977. A Log -Pearson Type III flood frequency analysis of floods on Foidel Creek at
<br />Station 09343800 indicates a I0 -year peak flow of 88 cfs and a 100 -year peak flow of 159 cfs (see Table 10, Surface
<br />Water Data). The 7 -day, 10 -year low flow on Foidel Creek above and below the mines was zero prior to any mine
<br />discharge.
<br />Comparison of hydrographs in Figures 5-a through 5-c, shows the similarity of flow patterns between the upstream
<br />and downstream stations on Foidel Creek. The baseline record from October, 1975 to October, 1979 depicts an
<br />intermittent stream with relatively long periods of no -flow lasting from June or early July until February or March of
<br />the following year. Starting in the fall of 1980, the streamflow pattern has shifted to much shorter no -flow periods and
<br />.higher mean annual flows, partly as a result of higher precipitation and partly due to effects of surface mining. Since
<br />the initiation of underground mine water discharge from the Foidel Creek Mine in the fall of 1983, the streamflow
<br />regime at Station 09243900, near the mouth of Foidel Creek, approaches perennial, except during severe droughts.
<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 Park Hydrology, until TCC took over monitoring the site in November
<br />2001. During the ten year monitoring period starting in October, 1975, mean annual 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 19801 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 -year low flow for Middle Creek is zero. The maximum peak flow
<br />on record was 329 cfs, recorded on July 2, 1981. A Log -Pearson Type III flood frequency analysis for the period of
<br />record, indicates a 10 -year peak flow of 216 cfs and a 100 -year peak discharge of 807 cfs (see Table 10, Surface
<br />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 Park Hydrology. Table 10, Surface Water Data, summarizes streamflow records for Fish Creek
<br />downstream of the mine plan area at Station 1003, and upstream of the mine plan area at Station 1001. Additional
<br />flow data are provided at an intermediate site, Site 26 (Station 1002). Variation in streamflow along this reach of Fish
<br />Creek is illustrated in the monthly hydrograph plots provided in Figures 5-e, 5-f and 5-g. These hydrographs show
<br />that Fish Creek is a perennial stream that gains considerable flow during the months of March through June as it
<br />traverses this reach through Twentymile Park. During baseflow periods, Fish Creek does not appear to gain flow over
<br />this reach. 13aseflow levels vary considerably from year to year on Fish Creek. Data from the upstream gauging
<br />Station 1001 indicate baseflow rates from about 1 cfs in dry years to as high as 8 cfs in the relatively wet years of
<br />.1984 and 1985 (see Table 10, Surface Water Data). The downstream gauging Station 1003 shows comparable ranges
<br />for baseflow conditions.
<br />TR06-52 2.04-32 02/17/06
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