Absolute
maximum
temper-
ature,
Fahr.:
Yuma 81 91 100 105 112 117 118 115 113 108 92 83 118
Tucson 84 85 95 101 106 111 110 109 106 97 89 82 111
Absolute
minimum
temper-
ature,
Fahr.:
Yuma 22 25 31 40 44 52 61 60 50 41 31 25 22
Tucson 14 20 24 32 38 39 64 60 46 31 23 11 11
Mean
maximum
temper-
ature,
Fahr.:
Yuma 65.1 70.7 78.5 85.4 93.2 101.2 106.7 104.9 99.6 87.2 75.0 67.4 86.6
Tucson 62.9 67.0 74.5 81.4 91.4 100.2 99.0 94.8 92.2 82.8 71.5 63.7 81.8
Mean
minimum
temper-
ature,
Fahr.:
Yuma 42.0 46.1 50.8 55.1 61.4 68.3 77.2 77.6 70.5 58.5 48.8 44.7 58.4
Tucson 34.9 41.5 44.0 48.1 55.3 63.8 75.0 73.6 67.3 52.1 42.5 35.1 52.8
Mean
temper-
ature,
Fahr.:
Yuma 54.1 58.8 64.5 69.8 77.2 84.9 91.5 90.7 84.4 73.0 61.9 56.0 72.2
Tucson 49.4 53.2 59.5 65.6 74.0 82.3 87.2 83.5 77.7 68.5 57.0 52.0 67.4
Mean
precip-
itation
inches
and
hundredths):
Yuma 0.42 0.51 0.26 0.07 0.04 T. 0.14 0.35 0.15 0.28 0.29 0.46 3.04
Tucson 0.75 0.98 0.90 0.17 0.16 0.19 2.86 3.08 1.16 0.33 0.37 0.95 12.26
Prevailing
winds:
Yuma N. N. W. W. W. SW. S. S. NE. NE. N. N. N.
Tucson S. S. S. W. S. SW. SE. SE. S. S. S. SE. S.
Average
cloudiness
(scale 0-10):
Yuma 2.4 2.4 2.4 1.6 1.3 0.8 1.8 2.3 1.1 1.3 1.7 2.5 1.8
Tucson 3.0 3.2 3.2 1.8 1.6 1.5 4.5 4.4 1.9 1.6 1.6 2.9 2.6
The precipitation on the Sonoran province is chiefly in the form of
rain; in the winter humid season snow falls frequently on the Sierra
Madre and rarely on the outlying ranges; in both humid seasons (and in
humid spots at all seasons) dew forms in greater or less abundance. Fog
frequently gathers along the coast, especially during the winter and in
the midsummer wet season, and sometimes drifts inland for miles. The
mean annual precipitation may be estimated at 20 or 25 inches toward
the crest and half as much toward the base of the high sierra; thence
it diminishes coastward, probably to less than 2 inches; the mean for
the extensive plains forming the greater part of the province may be
estimated at 3 or 4 inches. The greater part of the precipitation is
in local storms, frequently accompanied by thunder-gusts or sudden
tempests, though cold drizzles sometimes occur, especially at the
height of the winter humid season. Except where the local configuration
is such as to affect the atmospheric movements, the distribution of
precipitation is erratic, in both time and space; some spots may
receive half a dozen rains within a year, while other spots may remain
rainless for several years; and the wet spot of one series of years may
be the dry spot of the next.
The climatal features of Seriland are somewhat affected by the
pronounced topographic features of the district. Snow sometimes falls
on Sierra Seri, and probably on Sierra Kunkaak; gales gather about
the rugged ranges at all seasons, and sometimes produce precipitation
out of season; the extreme heat of midday and midsummer is tempered
by the proximity of the tide-swept gulf; and since most of the local
derangements tend to augment precipitation and reduce temperature, it
would seem safe to estimate the mean annual rainfall of the tract at 4
or 5 inches, and the mean temperature at about 70°, with a mean annual
range of some 30° and an extreme diurnal range of fully 80°.
The configuration and climate combine to give distinctive character to
the hydrography of the Sonoran province. The melting snows and more
abundant rains of the high sierras form innumerable streams flowing
down the steeper slopes toward the piedmont plains, or soak into the
pervious rocks to reappear as springs at lower levels; sometimes the
streams unite to form considerable rivers, flowing scores of miles
beyond the mountain confines; but eventually all the running waters
are absorbed by the dry sands of the plains or evaporated into the
drier air; and from the mouth of the Colorado to that of the Taqui,
500 miles away, no fresh water ever flows into the sea. During the
winter wet season, and to a less extent during that of summer, the
mountain waterways are occupied by rushing torrents, rivaling great
rivers in volume, and these floods flow far over the plains; but during
the normal droughts the torrents shrink to streamlets purling among
the rocks, or give place to blistering sand-wastes furlongs or even
miles in width and dozens of miles in length, while beyond stretch
low, radially scored alluvial fans, built by the great freshets of
millenniums. Only a trifling part of the rainfall of the plains ever
gathers in the waterways heading in the mountains, and only another
small part gathers in local channels; the lighter rains from higher
clouds are so far evaporated in the lower strata of the air as to reach
the earth in feeble sprinkles or not at all; the product of moderate
showers is absorbed directly by earth and air; while the water of
heavy rains accumulates in mud-burdened sheets, spreading far over the
plains, flowing sluggishly down the slopes, yet suffering absorption
by earth and air too rapidly to permit concentration in channels.
These moving mud-blankets of the plains, or sheetfloods,[6] are often
supplemented by the discharge from the waterways of adjacent sierras
and buttes; they are commonly miles and frequently dozens or scores
of miles in width, and the linear flow may range from a fraction of
a mile to scores of miles according to the heaviness of the rainfall
and the consequent dilution of the mud. Such sheetfloods, especially
those produced by considerable rains, are characteristic agents of
erosion throughout most of the province; their tendency is to aggrade
depressions and corrade laterally, and thus to produce smooth plains of
gentle slope interrupted only by exceptionally precipitous and rugged
mountain remnants. A part of the sheetflood water joins the stronger
mountain-born streams, particularly toward the end of the great storm
whereby earth and air are saturated; another part forms ground-water,
which slowly finds its way down the slopes toward the principal
valleys, perhaps to reappear as springs or to supply wells. These with
certain other conditions determine the water-supply available for
habitation throughout Seriland and adjacent Papagueria.
[6] Defined and described in Sheetflood Erosion, Bull. Geol. Soc. Am.,
vol. VII, 1897, p. 87.
Another condition of prime importance arises in a secular tilting of
the entire province southwestward. This tilting is connected with the
upthrust of the Sierra Madre and the uplifting of the plateau country
and the southern Rocky mountain region north of the international
boundary. Its rate is measured by the erosion of the Grand Canyon of
the Colorado and other gorges; and its dates, in terms of the geologic
time-scale, run at least from the middle Tertiary to the present, or
throughout the Neocene and Pleistocene. Throughout this vast period the
effect of the tilting in the Sonoran province has been to invigorate
streams flowing southward, and to paralyze streams flowing toward the
northerly and easterly compass-points; accordingly the streams flowing
toward the gulf have eroded their channels effectively during the ages,
and have frequently retrogressed entirely through outlying ranges; so
that throughout the province the divides seldom correspond with the
sierra crests.
A typical stream of the province is Rio Bacuache, one of the two
practicable overland ways into Seriland (albeit never surveyed until
traversed by the 1895 expedition). Viewed in its simple geographic
aspect, this stream may be said to originate in a broad valley parallel
with the gulf and the high sierra, 200 miles northeast of Kino bay;
its half-dozen tributary arroyos (sun-baked sand-washes during three
hundred and sixty days and mud-torrents during five days of the average
year) gather in the sheetflood plain and unite at Pozo Noriega, where
the ground-water gives permanent supply to a well; then the channel
cleaves a rocky sierra 3,000 feet high in a narrow gorge, and within
this canyon the ground-water gathered in the valley above seeps to the
surface of the sand-wash and flows in a practically permanent streamlet
throughout the 4 or 5 miles forming the width of the sierra; then the
liquid sinks, and 25 miles of blistering sand-wash (interrupted by
a single lateral spring) stretch across the next valley to Pueblo
Tiejo, where another sierra is cleft by the channel, and where the
water again exudes and flows through a sand-lined rock-bed (figure
2). In the local terminology this portion alone is Rio Bacuache, the
upper stretches of the waterway bearing different names; it supplies
the settlement and fields of Bacuachito, flowing above the sands 5
to 15 miles, according to season; then it returns to the sand-wash
habit for 50 miles, throughout much of which distance wells may find
supply at increasing depths; finally it passes into the delta phase,
and enters northeastern Seriland in a zone marked by exceptionally
vigorous mesquite forests. Normally the 200 miles of stream way is
actual stream only in two stretches of say 5 miles each, some 25 miles
apart, and the farther of these stops midway between the head of the
channel and the open sea toward which it trends and slopes; but during
and after great storms it is transformed into a river approaching the
Ohio or the Rhine in volume, flowing tumultuously for 150 miles, and
finally sinking in the sands of Desierto Encinas, 30 to 50 miles from
the coast. Viewed with respect to genesis, Rio Bacuache has responded
to the stimulus of the southwestern tilting, and has retrogressed up
the slope through two sierras, besides minor ranges and 100 miles of
sheetflood-carved plains; while the debris thus gathered has filled the
original gorge to a depth of hundreds of feet, and has overflowed the
adjacent sheetflood-flattened expanses to form the great alluvial fan
of eastern Seriland. The genetic conditions explain the distribution
of the water: the product of the semiannual storms suffices to form
a meager supply of ground water, which is diffused in the sands and
softer rocks of the plains, and concentrated in the narrow channels
carved through the dense granites of the sierras; and enough of the
flow passes the barriers to supply deep wells in the terminal fan, as
at the frontier ranchos Libertad (abandoned) and Santa Ana, just as
the subterranean seepage from the Sonora more richly supplies the deep
well at San Francisco de Costa Rica. In these lower reaches the mineral
salts, normally present in minute quantities, are concentrated so that
the water from these wells is slightly saline, while deeper in the
desert the scanty water is quite salt.
[Illustration: FIG. 2—Gateway to Seriland—gorge of Rio Bacuache.]
* * * * *
In Seriland proper the distribution of potable water is conditioned by
the meager precipitation, the local configuration (shaped largely by
sheetflood erosion), and the disturbance of equilibrium of the scanty
ground-water due to the tilting of the province. The most abundant
permanent supply of fresh water is that of Arroyo Carrizal, which is
fed by drainage and seepage from the broad and lofty mass of pervious
rocks forming the southern part of Sierra Kunkaak, the abundant supply
being due to the fact that the eastern tributaries are energetically
retrogressing into the mass in deep gorges which effectually tap the
water stored during the semiannual storms. The arroyo and valley of
Agua Dulce are less favorably conditioned by reason of a trend against
the tilting of the province and by reason of the narrower and lower
mass of tributary rock in the northern part of the range, and the
flow is impermanent, as indicated by the absence of canes and other
stream plants; yet four explorers (Ugarte, 1721; Hardy, 1826; Espence,
1844; Dewey, 1875) reported fresh water, apparently in a shallow well
tapping the underflow, at the embouchure of the arroyo. On the eastern
slope of Sierra Kunkaak there are several arroyos which carry water
for weeks or even months after the winter rains, and sometimes after
those of summer; but the only permanent water—Tinaja Anita—is at the
base of a stupendous cliff of exceptionally pervious and easily eroded
rocks, so deeply cut that ground-water is effectually tapped, while
an adjacent chasm—Arroyo Millard—is so situated that the cliff-faced
spur of the sierra above the tinaja absorbs an exceptional proportion
of the surface flowage from the main crest. The tinaja (figure 3) is
permanent, as indicated by a canebrake some 20 by 50 feet in extent,
and by a native fig and a few other trees—though the dry-season
water-supply ranges from mere moisture of the rocks to a few gallons
caught in rock basins within the first 50 yards of the head of the
arroyo. No other permanent supplies of fresh water are known on the
island, though there are a few rather persistent tinajas along the
western base of Sierra Menor above Willard point.
On the mainland tract there is a cliff-bound basin, much like that of
Tinaja Anita, at the head of Arroyo Mitchell and base of Johnson peak,
christened Tinaja Trinchera; but the range is narrow and the rocks
granitic, and hence the supply is not quite permanent.[7] A practically
permanent supply of water is found in one or more pools or barrancas
at the head of Playa Noriega in Desierto Encinas. The liquid lies in
pools gouged by freshets in the bottoms of arroyos coming in from the
northward, just where the flow is checked by the spread of the waters
over the always saline playa; and, since they are modified by each
freshet, they are sometimes deep, sometimes shallow, sometimes entirely
sand-filled. When the barrancas are clogged, or when their contents
are evaporated, coyotes, deer, horses, and vaqueros obtain water by
excavating a few feet in the sand lining the larger arroyos. Commonly
the barranca water is too saline for Caucasian palates save in dire
extremity, but the salinity diminishes as the arroyos are ascended.
An apparently permanent supply of saline and nitrous water is found
in a 10-foot well, known as Pozo Escalante, or Agua Amarilla (yellow
water), near the southern extremity of Desierto Encinas, reputed to
have been excavated by Juan Bautista de Escalante in 1700, and still
remaining open; its location is such that it catches the subterranean
seepage from both Bacuache and Sonora rivers. The water is potable
but not palatable. Among the vaqueros of San Francisco de Costa Rica
there is a vague and ancient tradition of a carrizal-marked tinaja or
arroyo (Aguaje Parilla) at the eastern base of the southern portion of
Sierra Seri; and both vaqueros and Indians refer to one or more saline
barrancas about the western base of the same semirange, probably in
Arroyo Mariana.
[7] Tinaja Trinchera was entirely dry and without trace of carrizal in
December, 1894.
[Illustration: FIG. 3—Tinaja Anita.]
In brief, Arroyo Carrizal, Tinaja Anita, and Pozo Escalante are the
only permanent waters, and Pozo Hardy, Barranca Salina, and Tinaja
Trinchera the only subpermanent waters actually known to Caucasians in
all Seriland, though it seems probable that permanent water may exist
at Aguaje Parilla and in Arroyo Mariana, and impermanent supplies
near Bahia Espence. There may be one or two additional places of
practically permanent water in smaller quantity, and a few other places
in which saline water might be found either at the surface or by slight
excavation, and which may be approximately located by inspection of
the map under guidance of the principles set forth in the preceding
paragraphs; but this would seem to be the limit of trustworthy water
supply. During the humid seasons the waters are naturally multiplied,
yet it is improbable that any of the arroyos except Carrizal and Agua
Dulce and a few minor gulches along the more precipitous shores shed
water into the gulf save at times of extraordinary local flood.[8]
[8] The physiographic features of the Sonoran province in general are
treated in greater detail in a paper on Sheetflood Erosion, Bull. Geol.
Soc. Am., vol. VIII, 1897, pp. 87-112, and in a paper on Papagueria,
Nat. Geog. Mag., vol. IX, 1898, pp. 345-371; while certain local
features are described in a paper on Seriland, prepared jointly with
Willard D. Johnson, Nat. Geog. Mag., vol. VII, 1896, pp. 125-133. The
aggregate available fresh water of Seriland is estimated on p. 181*.
* * * * *
The geologic structure of the Sonoran province is complex and not
well understood. So far as the meager observations indicate, the
basal rocks are granites, frequently massive and sometimes schistose,
sometimes intersected by veins of quartz, etc. The granitic mass is
upthrust to form the nuclei of Sierra Madre and other considerable
ranges; it also approaches the surface over large areas of plains.
Resting unconformably on the granites lie heavy deposits of shales
and limestones, commonly more or less metamorphosed; these rocks
outcrop on the slopes of most of the main ranges and form the entire
visible mass of some of the lower sierras and buttes, while they,
too, sometimes approach the surface of the sheetflood-carved plain.
The rocks, both calcareous and argillaceous, combine the characters
of the vast Mesozoic limestone deposits of eastern Mexico and the
immense shale accumulations of corresponding age in California, and
hence probably represent the later half of the Mesozoic. This is the
only sedimentary series recognized in the province. Both the granites
and the sedimentary beds are occasionally overlain by volcanic
deposits, chiefly in the form of much-eroded lava-sheets and associated
tuff-beds, which sometimes form considerable ranges and buttes (notably
Sierra Kunkaak, of Isla Tiburon); these remnantal volcanic deposits
are probably late Mesozoic or early Tertiary. Newer volcanics occur
locally, forming mesas, as about Agua Nueva (40 miles northwest of
Hermosillo), or even coulees apparently filling barrancas of modern
aspect, as in the vicinity of Bacuachito,[9] or rising into cinder
cones surrounded by ejectamenta, as at Pico Pinacate, in northwestern
Sonora. The various rocks are usually bare or meagerly mantled with
talus in the mountains; over the greater part of the plains they are
commonly veneered with sheetflood deposits, ranging from a few inches
to a few yards in thickness; while the central portions of the larger
valleys are lined with alluvial accumulations reaching many hundreds of
feet in thickness.
[9] Noted by Willard D. Johnson.
The clearly interpretable geologic history began with extensive
degradation and eventual baseleveling of a granitic terrane in
Paleozoic or early Mesozoic time; then followed the deposition of
the shales and associated limestones during the later Mesozoic; next
came elevation, accompanied or followed by corrugation, chiefly in
folds parallel with the present coast, whereby the granite-based
sierras were produced, and accompanied also by the earlier vulcanism
to which the volcanic sierras owe their existence. A vast period of
degradation ensued, during which the land stood so high as to induce
greater precipitation than that of today and to permit the streams
to carve channels far below the present level of tide, and during
which the present general configuration was developed; then came the
southwestward tilting and consequent climatal desiccation, the filling
of the deeper valleys, the inauguration of sheetflood erosion, some
local vulcanism, and the progressive shifting of the divides.
The geologic structure affects the hydrography, especially that factor
determined by subterranean circulation, or ground-water; for the
superficial sheetflood and alluvial deposits are highly pervious and
many of the volcanics hardly less so, while the shales and limestones
are but slightly pervious and the granites nearly impervious. The
geologic structure also determines the character of the soil with
exceptional directness, since the dryness of the air and the dearth
of vegetation reduce rock decay to a negligible quantity. The
characteristically precipitous sierras and cerros are of naked ledges,
save where locally mantled with a mechanical débris of the same rocks
(much finer than the frost product of colder and humider regions); the
soil of the normal plains is but the little-oxidized upper surface of
sheetflood deposits made up of the mechanical debris of local rocks and
varying in coarseness with the slope; while the soil of the valleys is
detrital sand and silt, derived from tributary slopes, passing into
adobe where conditions are fit, and essentially mechanical in texture
and structure save where cemented by ground-water solutions at the
lower levels.