VIM. PIMAN INDIANS OF THE SONORAN DESERT
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into the Colorado River. The water which remains after transpiration, irrigation, absorption, and evaporation have taken their toll eventually arrives
to lose its potability in the Gulf of California.
a. Climate. Rainfall at various points on or near these rivers is generally
greater than elsewhere in Pimeria Alta, especially in the east. Tucson,
which is on the Santa Cruz River at an elevation of 721 meters, gets a
mean summer rainfall of 16.2 cm and a winter rain of 12.5 cm, for an
average annual total of 28.7 cm. Magadalena, Sonora, which is on the Rio
Magdalena, gets 42.1 cm of rain per year, while Nogales, midway between
Magdalena and Tucson, has an annual mean of 41.1 cm. Caborca, Sonora,
in the Lower Colorado Valley vegetation zone and downstream from Magdalena below the junction of the Rio Altar and Rio Magdalena, gets a
fairly scant 18.5 cm/year, a situation comparable to that of Phoenix, Arizona, a short distance north of the Gila and which gets 19.1 cm/year. Farther down the Gila, at Gila Bend, the annual rainfall figure drops to
15.3 cm. At Casa Grande and Maricopa, both somewhat south of the Middle Gila, the annual totals are 16.7 and 17.2 cm, respectively (Bryan,
1925, pp. 34-40; Hackenberg, 1964a, p. 135; Hastings and Humphrey,
1969b, pp. 8 and 19). Temperature trends are the same, with some of
the riverine sections of Pimeria Alta, even discounting adjacent mountains,
being the coolest in the entire region.
b. Physiography. Riverine Pimeria Alta, like the rest of it, lies within
the Basin and Range physiographic province, but the San Pedro and parts
of the Santa Cruz and San Miguel Rivers lie in the Mountain Region rather
than in the Desert Region. The highest point of the Catalina Mountains,
which rise between the Santa Cruz and San Pedro, is the 2790-meter
Mount Lemmon. And Mount Wrightson (Old Baldy) in the Santa Rita
Mountains, also between these two rivers, looms to 2880 meters above
sea level. The transition life-zone (pine forest) and the Canadian lifezone (Douglas fir forest) are represented in the upper reaches of several
of these ranges (Lowe, 1964, pp. 63-73). The river valleys themselves
are higher above sea level in their upper reaches than intermontane plains
elsewhere in Pimeria Alta. The Santa Cruz and San Pedro are inclined
from south to north; the San Miguel from northeast to southwest; and
the Gila and Magdalena-Concepcion generally from east to west.
c. Plant Life. The vegetation along the banks and margins of streamways, as well as in their surrounding flood plains, is the most dense and
supports the largest specimens of species common to the Sonoran Desert
and its immediate surrounds. Mesquite, ironwood, palo verde, and acacias
are larger in these river valleys than elsewhere in Pimeria Alta, and it is
in the canyons of the neighboring mountains, except those of the Middle
509
into the Colorado River. The water which remains after transpiration, irrigation, absorption, and evaporation have taken their toll eventually arrives
to lose its potability in the Gulf of California.
a. Climate. Rainfall at various points on or near these rivers is generally
greater than elsewhere in Pimeria Alta, especially in the east. Tucson,
which is on the Santa Cruz River at an elevation of 721 meters, gets a
mean summer rainfall of 16.2 cm and a winter rain of 12.5 cm, for an
average annual total of 28.7 cm. Magadalena, Sonora, which is on the Rio
Magdalena, gets 42.1 cm of rain per year, while Nogales, midway between
Magdalena and Tucson, has an annual mean of 41.1 cm. Caborca, Sonora,
in the Lower Colorado Valley vegetation zone and downstream from Magdalena below the junction of the Rio Altar and Rio Magdalena, gets a
fairly scant 18.5 cm/year, a situation comparable to that of Phoenix, Arizona, a short distance north of the Gila and which gets 19.1 cm/year. Farther down the Gila, at Gila Bend, the annual rainfall figure drops to
15.3 cm. At Casa Grande and Maricopa, both somewhat south of the Middle Gila, the annual totals are 16.7 and 17.2 cm, respectively (Bryan,
1925, pp. 34-40; Hackenberg, 1964a, p. 135; Hastings and Humphrey,
1969b, pp. 8 and 19). Temperature trends are the same, with some of
the riverine sections of Pimeria Alta, even discounting adjacent mountains,
being the coolest in the entire region.
b. Physiography. Riverine Pimeria Alta, like the rest of it, lies within
the Basin and Range physiographic province, but the San Pedro and parts
of the Santa Cruz and San Miguel Rivers lie in the Mountain Region rather
than in the Desert Region. The highest point of the Catalina Mountains,
which rise between the Santa Cruz and San Pedro, is the 2790-meter
Mount Lemmon. And Mount Wrightson (Old Baldy) in the Santa Rita
Mountains, also between these two rivers, looms to 2880 meters above
sea level. The transition life-zone (pine forest) and the Canadian lifezone (Douglas fir forest) are represented in the upper reaches of several
of these ranges (Lowe, 1964, pp. 63-73). The river valleys themselves
are higher above sea level in their upper reaches than intermontane plains
elsewhere in Pimeria Alta. The Santa Cruz and San Pedro are inclined
from south to north; the San Miguel from northeast to southwest; and
the Gila and Magdalena-Concepcion generally from east to west.
c. Plant Life. The vegetation along the banks and margins of streamways, as well as in their surrounding flood plains, is the most dense and
supports the largest specimens of species common to the Sonoran Desert
and its immediate surrounds. Mesquite, ironwood, palo verde, and acacias
are larger in these river valleys than elsewhere in Pimeria Alta, and it is
in the canyons of the neighboring mountains, except those of the Middle