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During the initial seven-yeaz monitoring period at former USGS Station 09243800, average annual dischazge has
<br />varied from 0.02 cfs in water yeaz 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 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 Park Mine. The maximum peak discharge on record for
<br />former USGS Station 09243800 is 55 cfs recorded on April 21, 1980. )n contrast, during the drought period of water
<br />yeaz 1977, the maximum peak dischazge was only 0.16 cfs.
<br />Streatnflow downstream of the mine is recorded at former USGS Station 09243900 (now TCC site 900) and at TCC
<br />Station 2005 (Figure Sc). Data from former USGS Station 09243900, located at the mouth of Foidel Creek, describes
<br />sVeamflow conditions downstream of the mine plan area. During water yeaz 1977, annual flows at this station
<br />significantly exceeded the annual flow at former USGS Station 09243800 upstream of the mine plan azea. Likewise,
<br />during water year 1980, annual flows averaged 4.34 cfs or about two times the flow at the upsVeam station for the
<br />corresponding period. Maximum peak dischazge for the period of record from October, 1975 through September,
<br />1985 was 90 cfs, recorded on Apri122, 1980. The maximum peak dischazge during the drought yeaz of 1977 was only
<br />2.5 cfs, recorded on March 26, 1977. ALog-Pearson Type III flood frequency analysis of floods on Foidel Creek at
<br />Station 09343800 indicates a ] 0-year peak flow of 88 cfs and a ] 00-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 />dischazge.
<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 eazly July unfit February or March of
<br />the following year. Starting in the fall of ] 980, 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 sw#'ace mining. Since
<br />the initiation of undergound 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 tbe site in November
<br />2007. During the ten yeaz 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 yeaz 1984. The longest no-flow period on record was a 251 day interval from
<br />July I5, 1977 through March 27, 1978. Perennial flow occured 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-year low flow for Middle Creek is zero. The maximum peak flow
<br />on record was 329 cfs, recorded on July 2, 1981. ALog-Pearson Type ID flood frequency analysis for the period of
<br />record, indicates a 10-yeaz peak flow of ZI6 cfs and a 100-year peak dischazge of 807 cfs (see Table 1Q, Surface
<br />Water Data).
<br />SVeamflow 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 azea at Station 1003, and upstream of the mine plan area at Station 1001. Additional
<br />flow data aze provided at an intermediate site, Site 26 (Station 1002). Variation in sVeamflow 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 doting the months of March through June as it
<br />traverses this reach through Twentymile Pazk. During baseflow periods, Fish Creek does not appeaz to gain flow over
<br />this reach. $aseflow levels vary considerably from yeaz 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 ] 0, 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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