INVESTIGATIONS IN IDAHO.

By D. W. Ross.

MINIDOKA PROJECT.

This project provides for the reclamation of what is known as the "Minidoka" tract, a body of land situated in the Snake River Valley almost midway between the eastern and western boundaries of Idaho.

SOURCE OF WATER SUPPLY AND POINT OF DIVERSION.

Water is to be taken from Snake River at the head of the Minidoka Rapids, about 6 miles south of Minidoka station on the Oregon Short Line Railroad. This is one of the two points of diversion on Snake River from which the high bench lands between Blackfoot River and the west line of the State can be reached; the other point is at the "Cedars", the head of the proposed Twin Falls canal system. The river at this point has cut through the lava ridge, which is a southerly extension of the great lava fields that approach the river from the north between American Falls and Minidoka. The top of the ridge at this place is about 37 feet above the river at low water.

The formation in the bed of the river is solid lava rock, as shown by borings which have been made to depths of from 50 to 80 feet. The conditions are very favorable for the construction of a dam in the river at this point. The top of the ridge on the south side could be used as a spillway.

On

The river from this point up to American Falls is in a canyon, or a very narrow valley from 40 to 150 feet below the bench lands. the south side the country has a steep lateral slope between these points, rising rapidly toward the foothills, while on the north side the lava fields referred to extend down to the river throughout the greater part of this distance.

Between the foot of American Falls and the head of the Minidoka Rapids the river has a fall of 103 feet. The construction of a dam at Minidoka Rapids would back the water up a distance of about 35 miles.

The fall from the head to the foot of the Minidoka Rapids, a distance of about one-half mile, is 11 feet at low water, but below this point the river, for a distance of nearly 30 miles, or to the head of the lower canyon, has only the slight fall of 0.45 foot per mile.

The banks of the river throughout this course are from 5 to 15 feet above high water, except at a few places where narrow pieces of first bottom, a few acres only, are flooded during extreme high water.

Through the upper end of the canyon the fall is very rapid as far as the site of the proposed Twin Falls dam, from which point, for more than 50 miles, the river flows at the bottom of a great canyon, from 150 to 800 feet deep, over a succession of rapids and cataracts, all on the grandest scale imaginable. (See Pl. XXXII, B.)

AREA TO BE IRRIGATED.

A large area of irrigable land is situated on the north side of the river, on a bench at a height of from 15 to 30 feet above the stream. It is a flat, unbroken tract of nearly 60,000 acres, about 25 miles long and about 12 miles wide through its widest part.

The country farther north rises abruptly in a second bench, the lower edge of which is from 60 to 80 feet above the river, and which slopes upward gradually toward the north, reaching an elevation about 50 feet higher along the line of the Oregon Short Line Railroad.

The lands on the south side of the river appear to be very flat, when viewed from a distance, but the slope is decided near the river, and the surface rises gradually to the foothills to the south, 8 to 25 miles distant.

The low bench on the north and the lands on the south have a most excellent soil--a deep sandy loam, and free from alkali-now producing a thrifty growth of sagebrush.

PREVIOUS INVESTIGATIONS.

An investigation was made of this project in 1890 by the United States Geological Survey, preliminary lines being run on both sides. of the river, beginning at the dam site at the head of the rapids referred to. The line on the north side of the river was run for a distance of about 15 miles and the one on the south for a distance of 35 miles."

During the season of 1895 a survey was made by the State of Idaho under the direction of the State engineer. The line on the north was run for a distance of about 35 miles, and one on the south was run for about the same distance. As it would be necessary to construct a dam in the river at the point of diversion, these canals were run on a grade plane originating at an elevation of 42 feet above low water at the dam site.

Private parties have been interested in this undertaking from time to time during the last fifteen years, but no careful surveys were ever made by them.

a See Eleventh Ann. Rept. U. S. Geol. Survey, pt. 2.

INVESTIGATIONS UNDER RECLAMATION ACT.

A large body of land, including the area susceptible of reclamation under the project, was withdrawn from public entry by the order of the Secretary of the Interior November 17, 1902, under the provisions of the reclamation act.

In March, 1903, a party was organized by the United States Geological Survey for the purpose of investigating the possibility of reclaiming. these lands. This party was under the direct charge of Mr. J. T. Burke, assistant engineer, who began the survey by running a series of trial lines on both sides of Snake River. These trial lines were run as a basis for making a study of the land and ascertaining the feasibility and the probable dimensions of the undertaking.

On the north side a line was first run, beginning at an elevation 30 feet above low-water level of the river. This line followed the outer margin of the land near the foot of the steep slope leading to the second bench. It was soon discovered that practically none of the land lying between it and the river could be watered from a canal constructed on this line on account of a wide depression which followed the edge of the tract, and which could not be crossed by distributing laterals.

Lines of levels were then run over the lands along the section lines, and the course of a controlling ridge was located, when another trial line was run connecting the ridge with the point of diversion. It was found that this ridge began about 6 miles from the head of the canal system, in the northeast corner of sec. 13, T. 9 S., R. 24 E., and that from that point water could be conducted on a line practically through the middle of the tract, it being possible to distribute the same toward both edges of the land from this location.

The controlling point in the grade elevation of the canal system which will irrigate this body of land is near the upper edge of the ridge mentioned. From this point to the dam site the country, while comparatively flat, is slightly broken by mounds and ridges of lava, rendering the location of a large canal difficult.

A careful topographic survey was made of the ground along this line, for the first 6,000 feet below the dam site, on a scale of 100 feet to the inch, with contour intervals of 2 feet. The remainder of the 6 miles was done on a scale of 200 feet to the inch, with contour intervals of 1 foot. Careful borings with a 6-foot coal augur were made along this line and at many points on either side, these test holes being located on the plane-table sheets.

From the upper end of the ridge referred to, a line was run west along the section line for a distance of about 2 miles, then about S. 45° W. for a distance of about 8 miles. It would be quite feasible to construct the main canal on this line and distribute the water into main laterals which would have a course due west and south.