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Imperial irrigation district, this development and general prosperity is to be ascribed to the successful operation of the Imperial Canal. The extension of irrigation in our own as well as in foreign territory by a unified single canal system has been accomplished without international treaty, by the unique method of operation in Mexico through the agency of a Mexican corporation whose stock is owned and whose affairs were directed by the predecessors of the Imperial irrigation district and are now being directed by this district.

THE YUMA PROJECT

The Yuma project is a United States reclamation projeet located in Arizona and California on both sides of Colorado River. The construction of project works was recommended by a board of engineers on April 8, 1904, and construction was authorized by the Secretary of the Interior on May 10, 1904.

The Laguna Dam at the head of the project canal on Colorado River, about 10 miles northeast of Yuma, was completed in March, 1909. Its cost, including some river bank revetment and the diverting works, has been about $2,100,000.. Some water was used on the project lands prior to the completion of the Laguna Dam through ditches which had been acquired from private owners. The main canal of the project is located on the California side of Colorado River from the Laguna Dam down to a point opposite Yuma, and its water is taken thence to the Yuma Valley, on the Arizona side, through a large inverted siphon under the Colorado River at Yuma. The project plan provides for large pumping plants below Yuma on the east main canal for raising water to irrigate about 50,000 acres of mesa land.

Its proposed extent is about 120,000 acres, but there may be future additions. In the Yuma Indian Reservation, on the California side of the river, there are about 15,000 acres of irrigable land; in the Yuma Valley about 55,000 acres; and on the Yuma mesa about 50,000 acres.

The lands of the project adjacent to Colorado River are protected aganist overflow by heavy rock-faced levees and ample heavy equipment; also two quarries and a railroad on the entire length of the levees are maintained in order to afford means for protection during flood stages of the river. A drainage system is required and is now under construction.

The population in the project area, including Yuma, is about 12,000. The value of the crop output in 1917 from about 35,000 acres under cultivation was about $3,750,000. In 1918 the cultivated area was about 45,000 acres, and the value of the crops from this area exceeded $5,000,000.

COLORADO RIVER AS INTERNATIONAL BOUNDARY Had the character of the lower Colorado River been better known, it would never have been made a part of the boundary line between the United States and Mexico. The river is not, as is now well known, in a stable location. Throughout some 20 miles of the river's course where if forms the boundary, and now for some 40 miles or more farther downstream, where the river is entirely on Mexican territory, the river, as already stated, has been and now is a menace to large areas of lands and large property interests in the United States and Mexico. If allowed to do what human operations in Mexico have predisposed it to do, it would discharge inland into the Salton Sea, and it would be only a question of time before this sea would expand to the full limit of the Salton Basin, with a surface area of some 1,250,000 acres, extending from above Indio, in California, to about 20 miles south of the international boundary.

Due to the unfortunate location of the boundary line, the United States has no jurisdiction over the territory in which the flood menace to Imperial Valley lies. Mexico seems impotent to cope with the situation, or at any rate appears to take no note of the urgency and seriousness of the situation as we are endeavor. ing to sketch it. In such circumstances the Imperial Valley, or more particularly the Imperial irrigation district, representing the largest organized interests in the valley, has been constrained to construct and maintain at large cost extensive protective works on foreign territory.

These facts are recited because the usefulness of an all-American or any other canal for the irrigation of lands in the Imperial Valley would soon be in large measure destroyed if adequate protection is not had against the danger from from the south, which threatens the area already under irrigation. This danger, moreover, will continue to grow so long as the Colorado River is allowed to run wild in the Volcano Lake region. It is evident that the problem of irrigation in the Imperial Valley is interwoven with the other problem of protection against the river at its high stages.

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If the United States and Mexico were cooperating on lower Colorado River problems, this board would not now find itself embarrassed by being denied the opportunity to survey and propose other possible high-line canal routes than such as are wholly on United States territory. To the westward of Pilot Knob the mesa slopes to the southward and southwestward. It breaks off in a comparatively steep slope a few miles to the southward of the international boundary. It is known that a canal without material sacrifice in water-surface elevation could be placed on lower ground than north of boundary by swinging the canal line across the boundary. It is not known definitely what the material advantage of such a location would be, though old surveys and a partial reconnaissance indicate that a reduction in excavation to the extent of about 10,000,000 cubic yards might be expected.

WATER CONSERVATION BY STORAGE

Whenever the river drops to allow stage early in the season, as was the case, for example, in 1915 and in 1918, the demand of the irrigator upon the lower river will be in excess of the water supply: This situation will become more. pronounced when all the land in Imperial irrigation district and in the Yuma project susceptible of cultivation shall have been brought under irrigation.

The irrigated lands and the lands susceptible of irrigation with water from the lower Colorado River are therefore directly concerned with the conservation of the river's flow, and particularly with the regulation thereof by storage. The construction of storage reservoirs should go hand in hand with the building of any irrigation system that will add so large a body of arid land to the irrigable area as would the construction of a full-capacity all-American canal.

When, therefore, project features are outlined for the full utilization of such a canal, or its equivalent, there should be assurance that some water will be obtainable for it from storage. It is assumed that within the time that such a canal could be built the United States Government will have made suitable provision for storage works that will be of general benefit. It may also be assumed that water from storage will be supplied at a charge which will fairly compensate the United States and will return to it the proper allotment of original investment within a period of 40 to 50 years and that there will be no immediate demand upon the irrigation interests benefited for any part of the first cost of reservoir construction. On these assumptions the new area to be served with water from the all-American canal will not be burdened with the necessity of providing funds to meet the first cost of storage works, and all complications that might otherwise result from negotiations with up-river interests will be avoided.

THE SILT PROBLEM

On the subject of silt carried by Colorado River much information has already been collected. The silt studies which have been made by the United States Reclamation Service at Yuma since 1909 and earlier studies by Prof. R. H. Forbes, of the Agricultural Experiment Station of Arizona, have led to the following general conclusions on this subject:

The water of Colorado River ordinarily carries the least percentage (by weight) of silt in suspension during the months June to November.

The water of Colorado River ordinarily carries the highest percentage of silt in suspension from January to March, and this percentage is about three to four times as great as when the river is at its highest summer stage and as in the fall when the river is at a low stage.

The percentage of material in suspension in the water of Colorado River is ordinarily at a maximum when the river has a flow of about 30,000 to 40,000 cubic feet per second and is at a minimum at the low and at the high stages of the river.

The percentage of material in suspension increases with depth below the water surface; in other words, there is more material in suspension near the bottom of the river than near the water surface.

The annual average load carried in suspension is about 0.85 per cent by weight and the range throughout the year is from about 0.20 per cent at low water and 0.30 per cent at high water to an ordinary maximum of about 1.45 per cent and to an occasional maximum at certain intermediate stages of about 4 per cent.

There are numerous wide departures from these general conditions.

It is estimated from these figures and the mean annual flow of Colorado River at Yuma that the river's annual load of silt carried in suspension is about 196,700,000 tons, which at 100 pounds to the cubic foot, represents about 90,000 acre-feet of compactly deposited dry material.

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This does not include the river's bottom or bed load of sand, concerning which little is known other than that the river at its high stages occasionally makes rapid changes in depth and that the bottom, being loose sand, yields readily to the erosive force of the water.

The studies which have been made in the Imperial Canal indicate that at an annual discharge of about 2,000,000 acre-feet the average bed load of the canal has been at least 190,000 cubic yards per month, or about 2,280,000 cubic yards per year. It will be safe and conservative to assume that the river has å bed Ioad at least as many times greater than this as its annual flow is greater. Accepting the records of the United States Geological Survey, the river's annual discharge may be placed at about 17,000,000 acre-feet, or 8.5 times the amount diverted into Imperial Canal. The bed load of the river may therefore be estimated at not less than 8.5 times 2,280,000 cubic yards, or about 19,500,000 cubic yards, or 12,000. acre-feet.

The combined load, in suspension and pushed along the bottom, is therefore equivalent to an average annual deposit of at least 102,000 acre-feet, or enough material to cover 158 square miles i foot deep.

In making this estimate it has been assumed that the average weight of 1 cubic foot of dry deposit is 100 pounds. This is based on recent careful determinations of the amount of material in a cubic foot of deposit taken from the bed of Imperial Canal. It is quite possible that this assumption is somewhat in excess of the average weight of the top layer of ground in the delta, but is likely to represent fairly the average weight when the great bulk of the river's deposits, including: what lies below the surface layer of soil is considered.

The all-American canal project is for the irrigation of an area of such large extent that when the lands commanded by it, together with those of the Yuma project, are all under cultivation the entire natural flow of the river would be diverted at Laguna Dam for use on both sides of the river during an average of at least one month per year and in some years, as in 1902, 1903, 1904, 1910, 1915, and 1918, from two to six months. At such times, until conditions are modified by storage, water would not be available for desilting operations without a reduction of the diversions to the detriment of the irrigators; but desilting is essential in order that as little as possible of the river's bed load of sand may get into the canal systems and that the suspended load in the waters of the canals be kept at a minimum.

During the months when the irrigation demand is equal to or exceeds the quantity of water in the river at the Laguna Dam there is then urgent need for water from a stored supply to make up the natural deficiency with an ample allowance for sluicing purposes. It is believed that the desilting operation can be carried on successfully at the Laguna Dam without sacrificing more than 5 per cent of what would otherwise be the continuous flow of the canals.

The following table presents in condensed form the results of the studies made at Yuma from 1909 to 1916 of the quantity of silt carried in suspension by the water of Colorado River.

TABLE 3.--Silt contents, Colorado River at Yuma, 1909 to 1918 (Information furnished by the United States Reclamation Service. Based on semiweekly determinations

by weight)

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The flow at Yuma and silt contents are affected by the sluicing operations at the Laguna Dam. The influence of sluicing is probably negligible at the river's high stages, March to August. If taken into account for the remainder of the year the silt contents would probably appear slightly less than shown in this table. The mean diversion of water at the Laguna Dam throughout the year has not exceeded 1,000 second-feet. There was practically no diversion preceding 1910.

THE FLOOD MENACE

The contract between the Secretary of the Interior and the Imperial irrigation district, under which this board has been appointed, is specific and definite in its requirement that an all-American canal route be surveyed and examined. No alternative has been left open. The surveys and investigations which have been made relate, therefore, to a canal, located throughout, upon American territory. Our investigations could not be broadened out to a full consideration of the wisest and best treatment of the irrigation problem of the lower Colorado River in its broadest aspect. This is to be regretted, because the lower river presents problems of unique perplexity. The river is not alone the sole dependency for irrigation water for large areas in both the United States and Mexico, but its floods have become a menance to property interests in both countries by reason of the fact that these floods threaten extensive areas with permanent destruction. In the case of most regions which are subject to overflow from rivers the occasional flood is held in check by levees, which if they break cause more or less inundation with inconvenience and loss of property and perhaps of lives, but the water recedes in the course of a week or a month as the case may be.

On the lower Colorado, however, the situation is entirely different. The river from the head of its delta to the Volcano Lake region is from 300 to 400 feet higher than the lowest portions of the Imperial Valley. The river in fact flows on the southeasterly rim of this valley—if a very broad flat delta ridge may be regarded as delimiting the valley in this direction—at an elevation above sea level from low to high water in round numbers of 100 to 120 feet near Pilot Knob at the head of the delta and at an elevation of 30 to over 40 feet in the Volcano Lake region. These elevations of the river are to be compared with sea level as the general elevation of the ground near Calexico on the boundary between the United States and Mexico, 50 feet below sea level at El Centro, 110 feet below sea level at Brawley, and over 280 feet below sea level at the deepest points of the bed of Salton Sea.

On the present route of the lower Colorado, practically along the top of the delta ridge to Volcano Lake, any overtopping of the river's west and north or right bank, if not restrained by artificial barriers, would send water inland down a slope into the Salton Basin. This basin to the southernmost portion of which the name Imperial Valley has been given, has a surface extent of about 2,000 square miles. Being a depressed basin, there can be no return flow therefrom to the river until the basin is full. A large part of the north central portion of the river delta—that is to say, land near and to the northward of the present Abejas section of the river-has for some years past escaped submersion. The result has been that vegetation has died out to a large extent, thus leaving this region more exposed to erosion at flood if barriers are broken than was the case under natural conditions. Any flood waters of the river which go over bank toward the north must, therefore, at all costs be returned to the river and not allowed to flow down the northern slope of the delta cone. This is being done by the maintenance of levees judiciously placed on high ground, frequently some miles distant from the present river channel.

From the head of the river delta at Pilot Knob to its outfall into the Gulf of California the right or west bank of Colorado River is in Mexico. Consequently all of the levee work for the protection of the Imperial Valley has been done in Mexico. This levee work, although upon foreign territory, has been done without contribution by Mexican interests. Not only were several millions of dollars expended in 1906 and 1907 in turning the river from its suddenly established northerly course back into the gulf; but since then a million dollars, contributed by the United States, were expended on levee work in Mexico and in addition thereto, the Imperial irrigation district and its predecessors have spent more than another million in levee maintenance, levee extension, and betterments, all giving protection to Imperial Valley and other lands, and all necessarily in foreign territory.

Without attempting to give a full history of the efforts which were made to keep the head of the Imperial Canal from being choked with river silt and of the many other difficulties relaing to maintenance of flow in the canal and to protection against flood which were successfully overcome, the following facts bearing upon the flood menace may be briefly recalled:

The original canal heading or intake from the Colorado River was in California, a few hundred yards north of the international boundary line. Owing to trouble with silt it could not be kept open at sufficient capacity to meet the growing demands of the irrigators, and when in 1904 the concession was obtained from Mexico to make a diversion on Mexican territory another intake channel was constructed just south of and paralleling the boundary line, delivering into the canal, which was there only a few hundred feet from the river bank. As this did not sufficiently improve the situation, another cut was made about 4 miles farther downstream, at the so-called lower Mexican heading. In 1905 the river scoured out this cut to river dimensions and in November of that year completely changed its course, sending a flood of water over the broad flat areas of Imperial Valley. In December, 1906, the river was turned back into its old channel by the construction of the so-called Hind Dam, but broke out again a few weeks later. It was turned back a second time in February, 1907, by the construction of the Clarke Dam, which is practically a southern extension of the Hind Dam. In 1909, about 20 miles by river below the California boundary line, the river again left its old channel and cut a new channel westerly, dropping its waters into a delta channel known as the Abejas or Bee River. It abandoned its old bed below this point. On the new route it flows westerly into the Volcano Lake region. Its waters there inundate a broad extent of country which is being raised by silt deposit and in which the copious annual watering has stimulated the growth of willows, brush, and plant growth generally. To the northward the spread of the waters is checked by the Volcano Lake levee, now an embankment about 13 feet high, against which in 1917 for 10 miles or more water stood 7 to 8 feet deep. To the southward the water finds its escape from the Volcano Lake region through various channels and over low areas, dropping finally into the Hardy Colorado and its feeders and thus reaching the Gulf.

The Imperial irrigation district is under the necessity of adding constantly and at an ever-increasing rate to the extent and magnitude of the defenses which must be maintained to keep the river from turning to the north. The river has in the last few years deposited so much silt toward the west and south in the Volcano Lake region, over which its water flows, that the obstruction of the higher ground thus formed, together with that offered by the rank vegetation of the southerly delta lands which have been peridically under water, has caused the flood menace in this region to increase from year to year. Based on the information at hand since 1914 it appears that the rise of the flood plane now averages about 1 foot per year. Before the river changed its course in 1909 the annual high-water stage in Volcano Lake was about 37.6 feet. In 1917 it rose to about elevation 44.5 above mean sea level (United States Geological Survey datum).

The situation as described is serious, and the menace is growing. The control of the lower river is an international problem. The area to be protected by putting the river upon a direct course to the gulf lies in both Mexico and the United States. No time is to be lost in dealing with the problem, which can not be adequately handled by the Imperial irrigation district.

The construction of an all-American canal, while it may relieve the irrigated areas in California from compliance with the conditions named in the Mexican concession, will not solve this other problem, which is also of vital importance to Imperial Valley.

Should an all-American canal be constructed and the carrying of irrigation water through Mexico cease, there will undoubtedly be some provision thereupon made by Mexico to maintain the productiveness of the Lower California lands by a system of irrigation canals heading at the river upon Mexican territory. New works then established may and most likely would introduce new complications and would create a new menace. All such works should be under more or less supervision and control from our side of the line because their failure would menace property interests in the United States which are, for the time being, far in excess of those in Mexico and will always be of great magnitude. This board, therefore, suggests an arrangement with Mexico which is fully explained at the end of this report.

At the river's high stages there will always be an abundance of water. And, when the flood season has passed, those who have not established their claims to an adequate part of the season's diminishing natural flow should be supplied with

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