[Illustration: Plate XLI. ANCIENT WORK ON CLEAR CREEK.]
The purpose of these depressions is not at all clear, and although popularly known as reservoirs it is hardly possible that they were used as such. The capacity of the Clear creek depression is about 160,000 gallons, or when two-thirds full, which would be the limit of its working capacity, about 100,000 gallons. The minimum rate of evaporation in this region in the winter months is over 3 inches per month, rising in summer to 10 inches or more, so that in winter the loss of water stored in this depression would be about 10,000 gallons a month, while in summer it might be as high as 35,000 or even 40,000 gallons a month. It follows, therefore, that even if the reservoir were filled to its full working capacity in winter and early spring it would be impossible to hold the water for more than two months and retain enough at the end of that time to make storing worth while. It has been already stated, however, that these depressions are situated on slight knolls and that the land falls away from them in every direction. As no surface drainage could be led into them, and as there is no trace on the ground of a raised ditch discharging into them, they must have been filled, if used as reservoirs, from the rain which fell within the line that circumscribes them. The mean annual rainfall (for over seventeen years) at Verde, a few miles farther northward in the same valley, is 11.44 inches, with a maximum annual fall of 27.27 inches and a minimum of 4.80 inches. The mean annual fall (for over twenty-one years) at Fort McDowell, near the mouth of the Rio Verde, is 10-54 inches, with a maximum of 20.0 inches and a minimum of 4.94 inches.[8]
[Footnote 8: Report on Rainfall (Pacific coast and western states and territories), Signal Office U.S. War Dept., Senate Ex. Doc. 91, 50th Cong., 1st Sess., Washington, 1889; pp. 70-73 (Errata, p. 4).]
If these depressions were used as reservoirs it is a fair presumption that the bottoms were plastered with clay, so that there would be no seepage and the only loss would be by evaporation. Yet this loss, in a dry and windy climate such as that of the region here treated, would be sufficient to render impracticable a storage reservoir of a cross section and a site like the one under discussion. Most of the rainfall is in the winter months, from December to March, and it would require a fall of over 12 inches during those months to render the reservoir of any use in June; it would certainly be of no use in July and August, at the time when water is most needed, save in exceptional years with rainfall much in excess of the mean.