Plate XLI. Ancient Work on Clear Creek.
One such bare area occurs close to the edge of the terrace about a mile and a half above the mouth of the creek. In its center is a structure called for convenience a reservoir, although it is by no means certain that it was used as such. It occurs about 100 yards from the creek, opposite the Wingfield place, and consists of a depression surrounded by an elevated rim. It is oval, measuring 108 feet north and south and 72 feet east and west from rim to rim. The crown of the rim is 5 feet 8 inches above the bottom of the depression and about 3 feet above the ground outside. The rim is fairly continuous, except at points on the northern and southern sides, where there are slight depressions, and these depressions are further marked by extra large bowlders. At its lowest points, however, the rim is over 2 feet above the ground, which slopes away from it for some distance in every direction. Plate XLI shows the eastern side of the depression; the large tree in the middle distance is on the bank of Clear creek and below the terrace. Plate XLII shows the northern gateway or dip in the rim, looking southward across the depression. The large bowlders previously referred to can be clearly seen. A depression similar to this occurs on the opposite side of the valley, about half a mile from the river. In this case it is not marked by bowlders or stones of any description, but is smooth and rounded, corresponding to the surface of the ground in its vicinity. In the latter as in the former case, the depression occurs on a low knoll or swell in the bottom land, and the surface of the ground slopes gently away from it for some distance in every direction.
Plate XLII. Gateway to Ancient Work, 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]
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.
On the other hand, there is the hypothesis that these depressions represent house structures; but if so these structures are anomalous in this region. The contour of the ground does not support the idea of a cluster of rooms about a central court, nor does the débris bear it out. Mr. F. H. Cushing has found depressions in the valleys of Salt and Gila rivers somewhat resembling these in form and measurement, and situated always on the outskirts of the sites of villages. Excavations were made, and as the result of these he came to the conclusion that