Below the point of measurement the Sevier receives the following affluents: At Van Buren’s ranch is a cluster of very large springs, furnishing about 55 cubic feet per second. Between Van Buren’s and Marysvale are three streams, yielding together about 30 feet, and Bullion Creek at Marysvale carries about 40 feet. There is still another affluent at Marysvale with about 30 feet. Finally, Clear Creek, north of Marysvale Cañon, gives about 45 feet, making the total contributions between the junction of the forks and Joseph City about 200 feet.
At Monroe a stream issues from the Sevier table, and is used for the irrigation of the field cultivated by that settlement. Its flow is estimated at 40 feet in the middle of July. At Richfield, on the other side of the valley, is a stream coming from the Pavant, with a flow of about 20 feet, and at Glencove a stream of 25 feet. At Salina is a large tributary issuing from a great cañon through the north end of the Sevier Plateau, and its measurement indicated a flow of 165 feet. The total between Monroe and Salina, inclusive, would thus reach 250 feet, to which might be added some smaller tributaries, not specifically mentioned, amounting perhaps to 10 feet, giving a total of 260 feet. Adding this to the tributaries between the upper forks and Joseph City, and to the main river itself, we have, as the total above Gunnison, 1,320 feet. This estimate being for the early part of July, and obviously largely in excess of the amount which is available at the critical period, in the last week of that month and the first week in August, what allowance should be made for the diminution of supply during the month of July it is difficult to determine. The smaller tributaries, as a rule, shrink much more than the larger. Those which enter the stream lower down decline more during July than those which join it farther up. Taken altogether, I am satisfied that it would be unsafe to estimate the irrigating capacity in the first week of August at more than 60 per cent. of that found in the first week of July, and I regard 50 per cent. as a much more probable estimate. For want of a better one, I adopt it, and this gives the estimated irrigating capacity of the Sevier and its tributaries above the junction of the San Pete at 660 cubic feet per second during the critical period.
The water supply in the San Pete Valley was measured by Mr. Renshawe during the latter part of July, and found by him to be as follows:
Volume of flowing water, in cubic feet per second, of streams in San Pete Valley.
| Feet. | |
|---|---|
| Pleasant Creek | 28 |
| Ephraim Creek | 28 |
| Manti Creek | 28 |
| Springtown Creek | 14 |
| Fairview Creek | 10 |
| Wales Creek | 6 |
| Fountain Green | 10 |
| Moroni | 10 |
| Creek between Ephraim and Manti | 5 |
| Creek between Manti and Gunnison | 5 |
| Creek above Fairview | 2¹⁄₂ |
| Twelve-mile Creek | 28 |
| San Pete at Gunnison | 60 |
| Total | 234¹⁄₂ |
This estimate is also liable to reduction, being undoubtedly a little in excess of the amount available at the critical period. This reduction may be as great as 15 per cent., which would leave very closely 200 cubic feet as the water supply of the San Pete Valley, which, added to the total of the Sevier above Gunnison, gives for the whole drainage system of the Sevier River a water supply of 860 feet per second at the time when the greatest amount is required.
The next factor to be inquired into is the amount of land which a cubic foot per second of water can irrigate. This is, of course, highly variable, depending upon the nature of the soil, and the economy with which the water is applied, and the frequency of the irrigations. New lands freshly broken require much more water than the older ones which have been planted and watered for several years; and in fact the quantity diminishes with each season for a long term of years. In the San Pete Valley, which has been longest cultivated, the decrease in the amount of water applied to the oldest lands has not yet ceased, though some fields have been cultivated with regularity since 1857. The fresh soils are highly porous and absorptive, requiring a large quantity of water for their irrigation, and not retaining this moisture well under the great evaporative power of a dry and hot atmosphere. With successive irrigations, the pores of the soil are gradually closed and the earth is slowly compacted by the infiltration of impalpable silt brought by the irrigating waters. It absorbs water much more slowly, and retains it a much longer time. There is, however, a check to this increased irrigating power, arising from a wasteful mode of agriculture. It has not been the practice to employ fertilizers, nor any other conservative means of keeping up the fertility of the soil, and the yield of the crops growing smaller, the old lands are frequently abandoned, and fresh adjoining lands are broken, planted, and watered. It has been the practice to cut the straw, which is never returned as mulch; and, as there is but little rotation in crops, the result can be easily comprehended. So long as new land costs nothing but the labor to clear of the Artemisia or sage brush, there is always the tendency to invade it as rapidly as the old lands show signs of fatigue. Thus the waters are constantly irrigating every year a large proportion of new land, and the consumption of water is correspondingly great.
A serious loss of water and fertility is produced by any method of irrigation which employs more water than is just sufficient to saturate the soil. Whatever water runs off from a field carries with it great quantities of mud and fine silt, together with the most precious elements of fertility. These elements are the soluble alkaline salts and organic manner which are readily taken up by the water, and once removed are not speedily restored. A field which is so irrigated that a large surplus of water is continually running from the tail ditches during the flow will rapidly deteriorate in fertility. But a field which receives water which is allowed to stand until it has soaked into the earth, without any surplus passing into the tail ditches, will increase in fertility. These irrigating waters bring with them a sufficiency of plant food to compensate, and more too, for the drain upon the soil caused by the harvest; but they will carry off more than they bring if they are permitted to run over the field and escape from it, instead of being caught and held until they are absorbed. It is not always practicable to attain this exact distribution of water, and many cases occur where great expense and labor might be required to arrange the ditches and fields in this manner. Ordinarily, it is cheaper to throw away old land and take up new than to improve the system of irrigation, and there are many fields in the valley of the Sevier which have been abandoned because the fertility of the soil has been washed out by a reckless method of irrigation. Connected with this is another source of waste, arising from very unequal requirements of contiguous areas, in consequence of which many lands, especially old ones, are liable to be excessively watered. When a community farms a large number of small fields, using water from the same canals, it is usually impossible so to regulate the distribution of the privilege that each field will receive the exact amount it needs. Some fields can remain unwatered much longer than others, and the tendency always is to get as much water as possible—each farmer fearing a deficiency of water and wasting its surplus. Experience on the part of the watermasters and a more and more settled habit in the lands themselves gradually diminish this source of loss and create economy. Far better results, therefore, may ordinarily be anticipated in old lands than in new. Better results, also, are found where circumstances render difficult or impracticable the abandonment of old fields for new, and this is ordinarily in those portions where the water is nearly or quite sufficient for all the irrigable land, and where all the irrigable land is taken up.
Recurring, then, to the inquiry as to the amount of land which a cubic foot per second of running water will irrigate, this area is in many of the new lands as low as 40 acres, and it seldom exceeds 80 acres with the old lands. Probably there are very few regions in the world where the demand of the soil for water is so great as here where the supply is so small. In California a cubic foot of water is said to be capable of irrigating more than a hundred acres, in India 200, and in Spain and Italy a much larger area. The reason is obvious. It is the direct consequence of the extreme aridity of the climate of Utah. The irrigating capacity of the unit of water is even less in the southern counties of Utah than in those around Great Salt Lake. Mr. Gilbert’s estimate of 100 acres for this last locality being accepted as the best that can be hoped for, it will not be rating the factor too low to say that 80 acres is the best that can be hoped for in the valley of the Sevier. The present factor will not, I am convinced, have a higher average value than 50 acres.
The total acreage, therefore, which can be irrigated in the drainage system of the Sevier by the present system of watering and of agriculture may be estimated at about 43,000 acres, and the greatest improvements and economies in the system of farming and watering cannot, with the present water supply, be expected to raise the irrigable area above 70,000 acres.