University of Virginia Library

Weather, Climate, and Water Supply:

A moot point among archaeologists working in the Chaco area is
the possibility of climatic change during the past thousand years, and
even during the last hundred years. At one time Gregg and Simpson
were misquoted to prove that only three or four generations ago there
was a perpetual stream flowing in the Chaco Canyon.[5] The droughts
indicated by tree rings (for the period 750 to 1150 A. D.) did not cause
the abandonment of the Chaco, nor were they more intense or more frequent
than during the last four hundred years.[6] Undoubtedly there are
cyclic fluctuations which affect agricultural populations, especially on
marginal steppe areas as in the "dust bowl" of the United States, but
these fluctuations do not constitute secular change. There does remain
evidence, however, for a fuller and more diversified vegetation in the
Chaco a thousand years ago. Without entering into a full discussion
of possible factors, it will be sufficient at this time to mention the close
relationship between ground water supply and vegetation, and the oscillations
produced in physiographic processes and in areal distribution
of vegetation before a balance is attained after any disturbance of the
water supply-vegetation equilibrium. Any stream channeling will
initiate a lowering of water level and a reduction of vegetation. Concommitantly,
any reduction of vegetation (whether by disease, fire,
man, or other agent) will induce an accelerated runoff, and this will
initiate a cycle of erosion. On the basis of the evidence in hand, one
might plausibly conclude that weather and climate are the same today
as they were in 937 A. D., but the processes of denudation and erosion
have become so greatly augmented that marked differences exist in
landforms and vegetation. The change is physiographic and not necessarily
climatic.

Weather records have been kept in the Chaco Canyon continuously
only since June of 1932. A broken record extends back to May of 1922.
These records comprise only precipitation, and maximal and minimal
temperatures. Summarized for the Chaco Canyon National Monument
station, these records are:[7]

Station: Pueblo Bonito

Elevation: 6,000 feet.

Length of record: 6 to 9 years

                           

	Max. Temp.	Mean Max. Temp.	Min. Temp.	Mean Min. Temp.	Mean Temp.	Min. Precip.	Max. Precip.	Mean Precip.
Jan.	60°F.	40.7	—13	14.8	27.7	T	.67	.30
Feb.	70°	40.5	—15	18.7	29.6	T	2.34	.93
Mar.	80°	54.0	5	24.7	39.3	T	.94	.31
Apr.	84°	62.3	9	32.3	47.3	T	.47	.27
May	88°	73.6	24	41.3	57.4	.08	1.56	.60
June	99°	85.3	30	49.0	67.1	T	1.23	.27
July	100°	89.7	44	57.2	73.4	.66	2.11	1.19
Aug.	99°	86.7	42	55.6	71.1	.22	2.72	1.49
Sept.	102°	78.5	24	46.0	62.2	.32	2.42	1.02
Oct.	81°	66.6	10	37.0	51.8	T	2.62	.61
Nov.	71°	51.2	1	22.3	36.7	0	1.38	.70
Dec.	60°	40.9	—24	16.9	28.9	0	.89	.49
Annual	102°	64.2	—24	34.6	49.4	7.86	11.72	8.18

Greatest precipitations in 24 hours: 1.14 inches October 27, 1935; and
.90 inches August 4, 1936.

Modal date last killing frost: second week in May.

Modal date first killing frost: first week in October.

Average growing season: 150 days.

A general statement of the climate of the Chaco Canyon area,
based on records in the Chaco, interpolations from neighboring stations
of longer record at Aztec, Bloomfield, Crownpoint, Farmington, Fruitland,
Haynes, Shiprock and Tohatchi, and statements from local inhabitants,
follows:

The rainy season (with 46 per cent of the annual total) falls in
the summer months of July, August, and September. The summer rains
are normally convectional, and spotted in distribution, often accompanied
by high winds and hail. Annual precipitation means vary from
less than six inches in the western area to more than fifteen inches on
the higher eastern and southern mesas. Normally about fifty days of
the year have .01 inch or more of precipitation. More than twenty
inches of snow fall annually on the higher mesas. Occasionally roads
are blocked with snow, as in January, 1937. Annual precipitation will
vary more than fifty per cent either way from the mean, from year to
year. The wind blows prevailingly from the southwest, but in the summer
months there are diurnal shifts in the Chaco Canyon. A west wind
will blow up the canyon from 10:00 a. m. until 7:00 p. m.; this will
be replaced by an east wind down the canyon between 9:00 p. m. and

8:00 a. m.[8] March and April normally have the most violent windstorms,
although high winds may blow in July and August. The winter
months may get bitterly cold, but the summers are never intolerably
hot. Even after the hottest summer days, night and early morning
temperatures are cool, due to the rapid radiation under clear skies at
an elevation of 6,000 feet and higher.

Most visitors to the Chaco Canyon would class it immediately as a
desert. The comparative lack of water and the sparse vegetation would
seemingly justify such a classification. The writer has taken the
record from complete years in the Chaco Canyon and applied the
Koeppen system for the determination of climate.[9] According to this
analysis the Chaco Canyon is normally a cold desert, bordering on the
steppe (BWkfw, near BSkfw). Should the Chaco Canyon average one
more inch of rain a year, it would have a steppe climate. Certain years
are steppe years, although seven to eight out of every ten are seemingly
desert years. While the Chaco Canyon is desert in climate, the adjoining
mesas are probably steppe.

As has been mentioned, the Chaco River is an ephemeral stream
which drains most of the area. However, in between the Chaco tributaries
are a number of semi-permanent lakes or ponds, occupying small
areas of interior drainage. These are located principally on the Chaco
plateau to the south of the Chaco Canyon. Besides these ponds, and
artificial tanks constructed for the watering of sheep, there are a number
of springs and seeps, and countless tinajas and charcos. Undoubtedly
the prehistoric inhabitants of the Chaco relied in part on
these waterholes on the mesa tops, as trails lead up to them from the
various pueblos below in the canyon. The mesa-top waterholes are
filled only by seasonal rains, but a number of the springs are perpetually
fed by seepage, down through the sandstones and along bedding
planes, from large areas. The largest springs of the area are to be
found at the base of outcroppings of the Ojo Alamo formation, to the
north of the Chaco Canyon. Attempts at well drilling by white settlers
in the Chaco area have demonstrated that (1) there is no permanent
water table in the Chaco Canyon away from the underflow in the bed
of the river; (2) there are artesian basins to the south of the Chaco
Canyon; (3) the most likely source of water is in the porous sandstone
immediately above the Mancos shale; (4) most of the water from wells
is hard, quite often salty. The water at present provided by surface
and sub-surface run-off after summer rains is normally sufficient to
mature crops of maize and beans in the Chaco Canyon, when directed
to the fields by diversion dikes.