6043

• UCREFRP Catalog Number: 6043
• Author: Desert Fishes Council ‎(Edwin Pister, e.
• Title: A Summary of the Proceeding of the Tenth Annual Symposium.
• USFW Year: 1978.
• USFW - Doc Type: November 16-18, 1978.
• Copyright Material: NO

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sedimentary rocks, consist of well-bedded semi-consolidated to non-
consolidated valley-fill material overlain by floodplain alluvium.
The valley-fill upstream from Benson ranges from 90 m - 644 m thick,
but has.generally been eroded away further downstream (Reichhardt
et al., 1978).
Floodplain alluvium,'consisting of gravel, sand, and silt, along
the channels and floodplains of the San Pedro mainstream and its.
tributaries ranges in thickness from 12 to 43 m. This alluvium is
very porous and is capable of producing water flows in excess of
2,000 gallons per minute (7.57 cubic meters per minute). Ground water
in the alluvi-um is under strong artesian pressure in the vicinity of
Palominas - Hereford, St. David-Benson, and Mammoth. Further down-
stream, ground water occurs at increasing depths beneath the surface
(Roeske & Werrell, 1973)•
Because of high sediment porosity, ground water withdrawn from
the alluvium is rapidly replaced by infiltration from periods of
high surface runoff. During the past 25 years, net change in ground
water level- ha _s been negligible except in the Sierra Vista-Fort
Huachuca area due to domestic-industrial uses. The relatively
constant ground water level may be due in part to replacement of
native riparian vegetation by irrigated agricultural land (Roeske
and Werrell, 1973)•
Lack of substantial lowering of ground water since the early
1950's, however, does not mean that the flow regime of the San Pedro
has not been radically altered. Diversions,, headwater impoundments,
depletion of underground aquifiers, and a possible regional tendency
toward increasing aridity have reduced surface flow in the mainstream
to that of an intermittent stream, with permanent water present'in
three main sections which comprise only 50 km of the total river
length of ca 250 km. That the San Pedro historically contained
consistently high surface flows is documented by occurrence of
Colorado River squawfish, Ptychocheitm tucim (Miller, 1955), and
razorback sucker, Xynauchen texanuz, species confined to swift,
big-river habitats in excess of one meter deep (Minckley, 1973)•
The San Pedro drainage historically supported at.least 13
native fish species (Table l). 'Aravaipa Creek, the largest
tributary, presently contains 7 of 13 fishes reported, and is the
only portion of the entire drainage that supports a relatively
secure native fish fauna. Of these 7 species, 4 have not been
reported elsewhere in the basin since at least 1963; of the remaining
6 species historically occurring in the San Pedro drainage as of
'November 1978, Tiahoga and Meda deserve special mention because both
genera are endemic to. the Gila River basin and have, except for
populations in Aravaipa Creek, been virtually eliminated from Arizona.
Meda persisted in small numbers in portions of the Verde River in
1972 and still occurs there as far as is known. A few Tdanoga were
collected in the Blue River, Greenlee County, in 1977.' Also, the species
may still be-present in the Black River drainage; specimens were collected'
there in the late 1960's.