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this will be discussed below). Other species collected were Notropis formosus,
<br />Pimephales promelas, Catostomus plebeius, and Gambus~a affinis (exotic).
<br />(B) Rio Santa Maria at Santa Ana de Bavicora (Fig. 1, ~~9), 107° 28'
<br />N long., 29° 02' W lat., 1850 m, 29 May 1979. -Juvenile to adult Gila n~rescens
<br />were abundant at this locality. The only previous record of this species here
<br />is UANL 1850 (13 specimens). Although this is essentially a large-river
<br />habitat (10-15 m wide), the water present was due to a rheocrene; immediately
<br />upstream (above the hwy. bridge), for a distance of 8 km, the river bed was
<br />dry. The habitat did not comprise isolated pools; rather, the water was
<br />continuous, without visible current,. deeper against the steeply-cut western
<br />bank, with a bottom of sand, fine gravel, and mud substrates. Aquatic vegetation
<br />consisted of narrow-leaf Potamogeton and green algae with willows and cottonwoods
<br />lining the shores. Small chubs were captured in the open water and shallow,
<br />vegetated habitats. The larger specimens were taken in deeper water (usually
<br />greater than 1 m), in shady areas, and against the steeply-cut bank. One adult
<br />Gila ni~rescens was taken in a shady, shallow area among some fa11eH logs.
<br />Other species captured were: Notropis formosus, Pimephales promelas, Catostomus
<br />plebeius, Cyprinodon sp., and Cyprinus carpio,
<br />(C) Rio Santa Maria at Bachiniva (Fig. 1, ~f10), 107° 15` N long., 28° 45' "
<br />W lat., ca. 2025 m, 29 May 1979. --This locality is the farthest upstream station
<br />that we collected in the Rio Santa Maria basin. Only 2 small juveniles of the
<br />Chihuahua chub were taken; no previous records of this fish are known from
<br />Bachiniva. This is an arroyo with permanent water subject to much fluctuation;
<br />both shallow areas and deep pools were present. The substrate consisted of
<br />sand, gravel, rocks, and cobbles. A large area was choked with dense mats of v
<br />filamentous green algae and fine-leaf Potamogeton; also present were Nasturtium
<br />and Hydrocotle. The shoxe adjacent to the arroyo was a garbage dump among the
<br />rooted Salix. Other species collected were: Notropis formosus:; Pimephales
<br />promelas, Catostomus plebeius, and Cyprinodon sp, ,
<br />IV. Rio. del Carmen Drainage
<br />(A) Rio. del Carmen, near Ahumada (Fig. 1, X111) 106° 31' N long., 30° 37`"
<br />W lat., ca. 1210 m, 31 May 1979. --Our search for water in the region surrounding
<br />Ahumada revealed that it had been diverted out of natural river channels for
<br />irrigation. Grassing the desert westward from Ahumada toward Ojo Carbonera
<br />across the Carmen basin, only dry stream beds and arroyos were encountered.
<br />Gila nigrescens was last collected in this region in 1975 (UANL 2223, 1
<br />specimen). "
<br />(B) Rio del Carmen, from Hwy 10 crossing W of Ricardo Flores Magon to
<br />7 km S (Fig. 1, ~~12), 106° 59' N. long., 29° 57' W lat., ca. 1480 m, 30 May
<br />1979. -At R. F Magon the entire river had been diverted into two concrete
<br />raceways and the former river bed (ca. 50 m wide) was completely dry. We
<br />drove upstream for 1-0 km to a large earthen dam (ca. 50 ~n high.) with a hydro-
<br />electric power plant at the base. Behind the dam was a large reservoir (we
<br />estimate that we saw at least 32 km before the reservoir turned west between
<br />some mountains), with steep banks, containing brown, turbid water; the banks
<br />were too steep and soft and the water too deep for collecting in the reservoir..
<br />The Chih>ahua chub was last taken near R. F. Magon in 1975 (UANL uncatalogued),
<br />
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