Draft Environmental Impact Statement for Roca Honda Mine

Chapter 3. Affected Environment and Environmental Consequences
Ambrosia Lake Valley). The gaging stations are approximately 1 mile apart and about 5 miles
downstream of the RHR permit area (figure 37).
The San Mateo gaging station recorded daily flow of the creek from a watershed drainage area of
75.6 square miles from May 23, 1977, to October 7, 1982. Mean daily streamflow data for the 5year
period that this gage was operational are shown on figure 35. Mean monthly flow of San
Mateo Creek is shown on figure 37 for the same period.
Figure 37. Mean monthly average flow of San Mateo Creek
Prior to 1978, the San Mateo Creek daily streamflow measurements indicate that flow was
perennial. The flows ranged from 2 to 12 cfs (900 to 5,400 gpm). These flows reflect mine
dewatering discharge during prior mining activities including the Johnny M mine and discharge
during excavation of the Mt. Taylor mine shafts. After 1978, the flow pattern reflects an
ephemeral stream with sporadic high flows of the creek associated with rainfall events during the
summer/early fall and spring snowmelt runoff periods.
During peak runoff from snowmelt in the late spring or during heavy summer/fall rain storms,
San Mateo Creek may flow west as far as a few miles beyond its confluence with Arroyo del
Puerto, but according to previous investigators, flow rarely reached the Rio San José even 30
years ago (Brod, 1979; Stone et al., 1983). The flows of 30 to 40 cfs measured in 1977 and 1978
did not extend as far downstream at the Rio San José gaging station.
Since about 1995, the lower portion of San Mateo Creek has been influenced by the Homestake
Mill Superfund Site remediation. Homestake Mine is located approximately 12 miles downstream
of the RHR permit area. Homestake Mining Company diverted the channel of San Mateo Creek
to the west and southwest around the Homestake Mill Superfund Site and directed it onto one of
Homestake’s center pivot irrigation areas. Field investigations determined that the channel of San
120 DEIS for Roca Honda Mine, Cibola National Forest