the sandstone, and there is no doubt but what this wearing out was caused by a heavy jam of logs forming on this pier and extending up stream, over the site of these cavities, the logs obstructing the current of water, turning it underneath the logs and impinging a strong current of water directly against the sandstone in the bed of the river, and thus excavating these deep cavaties, No doubt but what the other cavities in the bed of the river were produced by the same cause.

The pier, marked in the accompanying map in the up-stream side of my embankment, no doubt caused the excavation of the cavity from which the original breach into the tunnel found its way. The large cavity in the bed of the stream nearest the easterly shore of the river was formerly the site of a large boom-pier, which held a jam of logs until it formed a cavity which undermined the pier and let it tilt into the cavity, discharging the ballast and allowing the hulk of the pier to float away.

I think the mill companies should be required to remove all of these piers from the bed of the river above the head of the ledge as far up as the suspension-bridge, and the bed of the river should be riprapped around those piers above the suspension-bridge, so as to prevent any further forming of cavaties around them.

In my report to General Warren, dated December 7, 1869, I gave a description of the limestone formation at the Falls of Saint Anthony, and the causes that were operating to destroy it, and description and estimates of works necessary for its preservation. Also report and estimates for lockage to continue navigation past the falls, with description of the ownership of property adjacent to the falls, to which I would respectfully refer, as the foregoing is supplementary to it. It may be found in House Executive Document 118, Forty-first Congress, second session.

I am, colonel, very respectfully, your obedient servant,

FRANKLIN COOK,

of Falls of Saint Anthony.

Colonel JOHN N. MACOMB,
Colonel of Engineers, U. S. A., Engineer in charge of surveys
and improvements of the Upper Mississippi River.

H 13.

PRESERVATION OF THE FALLS OF SAINT ANTHONY.

Proceedings of the board of engineers convened in pursuance of the following order, viz:

[Special orders No. 91.]*

HEADQUARTERS CORPS OF ENGINEERS,
Washington, D. C., July 31, 1872.

*

2. A board of officers of the Corps of Engineers, to consist of Colonel J. N. Macomb, Lieutenant-Colonel J. D. Kurtz, Major G. Weitzel, Major O. M. Poe, Major D. C. Houston, will assemble at Minneapolis, Minnesota, on the 10th August next, or as soon thereafter as practicable, for the purpose of considering and reporting upon the whole subject of the preservation of the Falls of Saint Anthony, and of the work now in progress therefor, as will be more particularly stated in instructions of this date, from the Chief of Engineers.

By command of Brigadier-General Humphreys.

JOHN G. PARKE,
Major of Engineers.

The board met at 9 a. m. on the 10th of August, all the members being present. The day was passed in making a personal examination of the falls and the works executed for their preservation. In the evening a large number of citizens called upon the board at the Nicollet House, and presented their views as to the proper method of securing the falls from further injury.

Upon the 12th the board met at 9 a. m., and, after consultation, agreed upon the following

REPORT.

The Falls of Saint Anthony are caused by the damming up of the waters of the Mississippi by a natural barrier, consisting of a layer of limestone-rock, which is 11 feet thick at the crest of the falls and 3 feet

thick at its upper end, about 1,000 feet above the crest, overlying a mass of sand-rock about 100 feet in thickness. The total fall is 50 feet. The river passes over the ledge in two channels, separated by Hennepin Island. It is apparent that the crest of the falls has been continually receding, and the record shows that the crest near the middle of the river is from 300 to 600 feet above its location in 1857.

The cause of this is found in the action of the water in undermining the sand-rock which sustains the limestone-rock, so that the latter, becoming unsupported, breaks off by its own weight. Unless this action is arrested by artificial protection, the falls will continue to recede until the limestone-ledge entirely disappears, after which, the bed of the river being soft rock, the falls will be destroyed and the bed of the river worn down until the velocity is so reduced as not to wear away the sandrock.

The water-power caused by the falls has been utilized by dams, canals, and works, the position of which are shown upon the accompanying map. These works have been so located as to increase the destructive action above referred to, by unnecessarily contracting the water-way by which the waste water passing over the falls west of the island is made to converge upon the bed of the river across a space but 200 feet in width.

In addition to the causes of destruction just mentioned, the falls are threatened with danger by the passage of water under the limestone ledge from above, by which the soft sand-rock is worn away so as to allow the limestone-rock to cave in. This danger was developed by the opening of a tunnel from the foot of the falls at Hennepin Island, up to and under Nicollet Island. The water of the river found its way through the soft strata of sand-rock, under the limestone, to such an extent as to wash out large quantities of the soft rock and allow the lime-rock overhead to cave in. Unless this flow had been checked it would doubtless have soon resulted in the destruction of the falls.

From this statement it appears that two things are necessary for the preservation of the falls: first, to prevent the wearing away of the sandstone at the foot of the falls; second, the permanent stoppage of the current from above the falls through the soft sand-rock under the limestone ledge.

The works already executed for this purpose are a timber-apron at the foot of the falls, partially completed, and the construction of a coffer-dam and masonry wall around the gorge caused by the water finding its way into the tunnel. These works are fully described in the report of Colonel Macomb, to be found in Senate Ex. Doc. No. 59, Fortysecond Congress, second session.

In addition to the above-mentioned works an effort is being made by the citizens to line the tunnel and fill up the gorge at its head. This plan is described in the accompanying communication, marked X.

In the opinion of this board the plan proposed for the preservation of the falls by Colonel Macomb, in his report to be found in the executive document just referred to, is the correct one, but it involves a cost far in excess of the funds now available. The large interests involved in the preservation of the water-power demand that the works for the purpose of stopping the flow of water in the tunnel should be of such a character as to be rapidly executed, and be of sufficient stability to resist the action of probable freshets.

Having accomplished this, the works for the permanent preservation of the falls can be executed without interfering with the water-power or being constantly exposed to disaster threatening the integrity of the

falls. This permanent improvement should not be commenced without an available sum of $200,000.

The board is also of the opinion that the apron at the foot of the falls, when completed, will prevent the undermining of the rock and injury to the falls on that side, but deems it important that such modification of the dams and other works be made as will restore the water-way to its natural width.

The board would therefore recommend that the present appropriation be expended in strengthening, as far as necessary, the coffer-dams already built around the gorge, including the work done since July 1, 1872, so as to secure them against freshets, and in continuing the lining of the tunnel on the plan already adopted. It is believed that the immediate danger will be thus avoided. While this work is in progress a thorough study should be made of the details of the plan for the permanent preservation of the falls.

The board would recommend that the sum of $37,500 of the present appropriation be applied in accordance with the petition of the citizens of Minneapolis addressed to Colonel J. N. Macomb, dated July 11, 1872, and as that petition asks that the money be expended under the direc tion of Mr. J. T. Stevens, engineer of the union committee, the board recommends that, if consistent with the law, the said Stevens be called upon to give bonds for the faithful disbursement of the funds.

The board would further recommend that the balance of the appropriation be expended under the direction of the engineer officer in charge of the works constructed by the United States, in maintaining and strengthening them, as far as is found necessary. The board are led to these recommendations because the sum appropriated is too small to apply with advantage on the plan recommended by Colonel Macomb. They deem it advisable to test the method of lining and plugging the tunnel on the plan adopted by citizens of Minneapolis, upon which a large sum has been expended, especially as the board is informed that, in case the Government funds are thus applied, the additional funds required to complete their plan will be furnished by the city of Minneapolis.

In conclusion the board cannot too strongly condemn the practice, which prevails so commonly throughout the country, and of which the locality now under consideration affords a manifest instance, of erecting structures in and across the natural water-way of navigable streams by the authority of State or States, or municipalities, and sometimes even without any authority whatever, and in all cases without any authority from Congress.

The board therefore recommends that the attention of Congress be invited to this matter, with a view to such legislation as will cause these objectionable structures to be removed or altered, and to prevent their erection in future.

Respectfully submitted.

H 14.

PRESERVATION OF THE FALLS OF SAINT ANTHONY.

To Colonel J. N. Macomb, Lieutenant-Colonel J. D. Kurtz, Major G. Weitzel, Major O. M. Poe, and Major D. C. Houston, United States Army, board of Government engineers, on the preservation of the Falls of Saint Anthony:

The undersigned, of the "Board of construction for the preservation of the Falls of Saint Anthony," and of the "Union Committee " of citizens for the same purpose, and in behalf of the citizens of the city of Minneapolis, respectfully submit that each of the above-named boards have had in charge portions of the work undertaken by the cities of Minneapolis and Saint Anthony, and the Saint Anthony Water-power Company, and the Minneapolis Mill Company, for the preservation of the falls.

That the "board of construction" was authorized to construct an apron of timber and rock at the falls, to conduct the water from the brink of the same, and protect the soft sand-rock which underlies the lime-rock, from being worn away by the under-tow of water, and have expended in the construction of this apron about $140,000, appropriated by said cities and water-power companies, with a small amount from a late Government appropriation, and have so far completed the work as to have arrested the recession of the falls. They have in hand funds sufficient to place the structure beyond immediate danger. It is estimated that a further expenditure of $65,000 will complete this struc

ture.

The "Union Committee" have had in charge the arresting of injury to the falls, occasioned by the breaking of the river into a tunnel excavated from below the falls upward underneath Hennepin Island, and extending to the lower end of Nicollet Island. This committee have expended about $95,000 in the prosecution of their work, which was also provided by the city of Minneapolis and private citizens.

The expenditure has been made under the direction and pursuant to the plan of J. T. Stevens, an engineer of said committee, and in obedi-, ence to the pressing demand for the protection of the tunnel against further damage from freshets, regarding that the point of greatest immediate danger, and in view of the fact that the Government appropri ation was insufficient to go on with the work in progress at the head of the ledge, and at the same time effectually close up the tunnel. After a thorough examination of the work to be done it was concluded that the only way to do any work that would be durable, and render immedi ate protection, was to plug the tunnel, as all work at the head of the break demonstrated that it was impossible to hold the head of water in the sandstone without a base of several hundred feet.

The mode of doing this work was as follows: First, all débris and loose rock were excavated and removed from the tunnel, and the sides then cut perpendicular in the solid sand-rock and the bottom level, commencing at the lower end. The bottom of the tunnel was then covered with grout, composed of broken rock and the best water-cement, as the work advanced, on and in which sills of 10 by 12 inch timber were laid, and on the sides posts of the same size timber were erected on the ends of the sills reaching and wedged to the lime-rock above, with supports across the tunnel, at the middle and top, to withstand pressure from without, and distant from the sand-rock from 12 to 30 inches. Outside of these posts 4-inch plank were carried up from the

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