ENGINEERING.

SCOPE OF WORK.

The following brief statement of results accomplished by the engineers follows the plan of the first annual report on irrigation, there being no essential change in methods or locality of work. These results, however, are of necessity given in the briefest form, as the valuable material is so great in amount that it must be left for subsequent publication. The surveys vary in character from rapid reconnaissance work, covering great areas, to the most careful and detailed instrumental work in chosen spots, followed by elaborate computations and plans. Between these extremes are surveys of every intermediate degree of care. In each engineering division broad reconnaissance work for the purpose of discovering reservoir sites was done; at the same time preliminary surveys were carried on in a few cases, and a smaller number of very careful examinations were made in localities whose adaptability for storage reservoirs was at once recognized. An exhibition of this work in full requires more space than can be given in an annual report.

It should be noted in this connection that each engineer has been compelled to make statements regarding the feasibility of the larger projects, in a most general and qualified manner, pointing out the impossibility of arriving at definite conclusions. until more is known of the actual water supply and the behavior of the rivers and smaller streams. The engineer must know not only the average flow of certain water courses, but also their greatest and least discharge for a number of years, before he can give decisive opinion as to the desirability of constructing important storage systems. It is a comparatively simple matter to plan a dam at a given place and to estimate the cost of such a structure of given dimensions; and, moreover, the calculation of the contents of the reservoir thus made is a mere matter of detail. Thus, the engineers can say that at this place or that a dam can be built for a certain sum, and that this

dam will hold back so much water; but the question of far greater importance and whose answer governs all other matters is, will it pay? In other words, do we know that there is sufficient water to fill the reservoir each and every year (for the loss of a single year may mean ruin to a community), and do we know that no great flood will occur of such violence as to overtop the structure and pull it down? It is only after these questions have been satisfactorily answered that the central fact can be determined, namely, whether the cost of storing this water and bringing it to the arable land will be such as to make it feasible.

These questions of water supply, location, and proportioning of dam are so interdependent that the estimates, cost, and future returns are hopelessly involved in doubt, while there is uncertainty as to the volume of water flowing in the rivers. It is true that there are many undertakings of local importance and value which can be carried out without any but the most general knowledge of water supply and its vagaries, but the more important, comprehensive systems, upon whose proper execution depends the prosperity of great irrigation districts and in whose stability the public as a whole have a vital interest-these, as in all similar constructions made throughout the world, must await more definite knowledge of the hydrography and of the topographic and climatic features which modify them.

From the best information to be obtained, it appears that 1889 was an extraordinarily dry year, and in 1890, in spite of the abundant rainfall in certain parts of the arid regions, the rivers as a whole did not fully recover from the effects of the drought. Thus, the published results represent but one side of the case, the minimum condition, and the water supply in the localities where measured appears less than is actually the case. In the reservoir projects, which from necessity are based upon data of a dry year only, there are vital qualifications to be borne in mind in discussions of their value, and while the attempt has been made to give the best and most satisfactory treatment to each scheme, this uncertainty as to fundamental facts hangs over all.

MONTANA DIVISIÓN.

Mr. H. M. Wilson, engineer, took charge of this division on June 14, 1889, under general instructions to devote his attention primarily to a thorough reconnaissance, especially with regard to storage facilities, and to undertake only such field work and segregation as should not divert him from the task of gaining a preliminary knowledge of the country. In this he was aided by Mr. R. S. Tarr, one of the hydrographers of the Survey, who had been, during the previous month, preparing information relative to irrigation for the use of the select committee of the Senate.

After a brief reconnaissance it was found that in the Sun River and Teton Basins, in Cascade and Chouteau Counties, there was an abundance of irrigable land and excellent opportunities for water storage. A survey of the reservoir sites in the Upper Sun River Basin was therefore begun at once and continued under the charge of Mr. John B. Rogers until the end of October, 1889.

In this

The field operations being well under way, Mr. Wilson devoted his time to a thorough personal reconnaissance of as much of the State as could be examined in a season. work, lasting from July 24 until September 25, he traveled 2,000 miles on horseback and 1,650 miles by rail, examining irrigable land and water supply, and looking for possible storage sites.

This preliminary examination was for the purpose of discovering storage sites and determining the relative importance and necessity in the various localities of detailed engineering surveys, in order that these might be begun and be carried on in a systematic manner, in places where the largest and best results would ensue. Estimates sufficient for this purpose were made based upon observations by the eye, aided by simple hand instruments, but were not to be considered as final, as in matters of this character it is impossible to make accurate estimates without careful surveys. The statements of the acreage of irrigable land were based upon assumptions as to the water duty, which may not be verified in the future and may require revision as new facts are developed. The duty 11 GEOL., PT. 2-8

of water should be great, but on account of its abundance no efforts have been made to use it frugally. From statements of farmers and canal-owners, it is estimated that the duty varies from 60 to 80 acres in ordinary soils, and with clay subsoil is as high as 100 acres. In the estimates made a water duty as high as the surrounding circumstances seemed to justify was assumed in each case.

In order to determine upon the practicability of the projects reported, it is necessary to make preliminary surveys of the various reservoir sites indicated and a thorough study of the catchment areas of the streams, after which detailed mapping of the localities selected is essential in order to make reliable estimates of the cost of construction. A brief notice of the localities visited and reference to the storage facilities there discovered will serve to indicate the scope and importance of this reconnaissance.

Beginning with the Missouri River in western Montana, there is on the east side of the river, the Missouri valley 25 miles long north and south and varying in width from 1 to 8 miles. It is found that the water supply for the tract of good land, which comprises about 60 per cent of the valley, is abundant and convenient and may be derived from either of two sources, namely, from a great canal taken from the Missouri River, heading in the canyon near Toston and carried around the eastern edge of the valley; or by augmenting by storage the discharge from small streams coming out of the Belt Mountains, by constructing reservoirs in the hills and tanks on the bench lands. Whether the one or the other of these methods, or a combination of both, will prove the best, is a matter which can be decided only after a survey of the whole has been made.

The Missouri River was examined at a point 23 miles above Toston, and it is believed that a diversion weir of masonry, rising 10 or 15 feet above mean level, will turn as much of the stream as is necessary into a canal heading at that point, but from the hasty reconnaissance it is believed that a system of storage reservoirs will be cheaper and quite as satisfactory. A number of what appeared to be good reservoir sites.