At a stand of five feet the observations show variations in fall from 19.5 feet to 21.7 feet.

At a stand of four feet the observations show variations in fall from 20 to 22 feet.

At a stand of three feet the observations show variations in fall from 21.6 feet to 22.4 feet.

At a stand of two feet the observations show variations in fall from 21.3 feet to 22.5 feet.

At a stand of one foot the observations show variations in fall from 22.3 feet to 23.2 feet.

At a stand of six inches the observations show variations in fall from 22.5 feet to 23.5 feet.

In one instance in June, 1865, when the Montrose gauge stood at 1.9, the fall was only 19.6 feet on the rapids. If there was no error in the observation, this would indicate a back-water rise from the Des Moines at the low stage of about three and a half feet on the Lower chain.

The effect of wind up or down the stream probably makes the majority of the fluctuations in the total fall, and appears to amount to about a foot in medium stages. The observations are not complete enough to decide exactly what amount is due to the wind.

The above includes all the practical information to be derived from the beforementioned plot, and it is not thought necessary to introduce it here.

The best observations we have on the relative rise at different parts of the rapids are those made by General Wilson during the rise in March and April, 1867.

These were taken at the respective readings on the Montrose gauge 2.3, 5.5, 7, and 7.5, and they show that the influence of back water is sensibly felt as far up as Waggoner's, the foot of Lamillies chain, and above that the water does not back, for it so happened that these observations were made during a heavy rise in the Des Moines river.

The plane of high-water in 1851 shows back water up to Waggoner's, but above there soon becomes parallel with the observed plane of April 23, the highest water of the spring of 1867.

We are therefore somewhat authorized to conclude that a rise at Montrose will always produce an equal rise as low down as the foot of Lamillies chain, and that below that point nearly all the fluctuations of back-water and wind are confined. The reading of the Montrose gauge can then be taken for the stage of the river for at least the upper half of the rapids.

VOLUME OF THE MISSISSIPPI.

Observations made at Keokuk for discharge on the 27th of April, 1867, when the river stood on the gauge 13.35, gave the volume at 195,000 cubic feet per second.

The highest water was April 24, gauge reading 15.3. Assuming the velocity not to be greatly different, as it was much affected by back-water from the Des Moines river, we have the high-water discharge of April 24, 1867, 215,000 cubic feet per second; applying the same reasoning to the high water of 1851, (which, however, is very uncertain as the railroad embankment on the Illinois side opposite Keokuk was not built at that time, though the back-water of the Des Moines acted much in the same way,) we would have the maximum high-water discharge of the Mississippi at 265,000 cubic feet per second.

The lowest water discharge is approximated to in the following manner : Measurements made by General Warren, October 23, 1866, at Burlington, gave a discharge of 36,100 cubic feet per second. This was an average low-water year. The gauge at Montrose, October 23, stood at one (1) foot, and making

this deduction at Burlington, and supposing the mean velocity to not change, we should get the extreme low-water discharge of 1864 at 31,913 cubic feet per second. This is probably in excess, but it would be safe to take it at 30,000 cubic feet per second.

General Wilson in his report says the average area of cross section is 17,550 feet at ordinary low water; mean surface velocity is 2.88 feet per second; and its mean velocity deduced therefrom is 2.304 feet. From these data the discharge has been calculated, and is found to be 40,435 cubic feet per second.

Measurements made of the discharge of the Des Moines river, April 29, 1867, ascertained it to be about 35,000 cubic feet per second. The Des Moines was then 24 feet below high water of April 24, 1867, on which day the discharge. was not less than 42,000 cubic feet per second. The high-water discharge of 1851 in the Des Moines could not have been less than 55,000 cubic feet per second, its height then being 74 feet above the level on April 24, 1867.

At the time of our observations on the Mississippi, April 27, 1867, a rise of one foot corresponded to an increased discharge 10,000 cubic feet per second; consequently the volume of the Des Moines at its floods would be able to secure the Mississippi perhaps five feet above what it would be without it.

An examination of the maps will show that no great rise can occur in the Des Moines valley without a corresponding one in the Mississippi. The Des Moines runs through the whole length of Iowa, and its valley embraces about 12,600 square miles. The area of the valleys of the other tributaries of the Mississippi in the same latitude amounts to about 40,000 square miles, and the country above would probably contribute as much more water contemporaneous with it, so that the Des Moines can never form more than one-sixth part of any considerable flood in the Mississippi.

SEDIMENT.

There is a decided rise in the river produced by the spring floods in the streams in the latitude of the Des Moines, and the water is much more filled with sediment at the rapids at such times than it is at any other. Such fact was very observable to us while at Keokuk, about the 20th of April, but before we were prepared to measure the amount the rise came from the rivers further north and restored it to its usual purity. Out of two quarts of water taken from the Mississippi on the 25th of April careful filtering and weighing obtained but 33 grains of sediment, or about of the weight. This is the character of the river during very much of the season, so that there would be but little deposition of mud from it in a canal.

The greatest amount of filling up which a stream produces is well known to arise from the material swept along near the bottom, and the amount of it is very difficult to measure. But the observed fact that the upper Mississippi throws out no material amount of sand on its present bottom lands, which are submerged at high water six to twelve feet, and which rise about ten feet above the low water, gives assurance that walls or banks of canals would not be much affected by the material drifted along the bottom, if carried up as high as the average of the bottom-land banks when submerged at high stages.

The following table exhibits the number of days at which the water stood above the different-numbered feet on the gauges at Montrose and Keokuk, by actual observation. The zeroes of these gauges are at the low water of 1864, at which time there was about one foot of water on the rapids. The rise given on the Montrose gauge extends quite uniformly down the rapids as far as the foot of the Lamillies chain; below this it often and generally rises faster, owing 21 w-vol. ii

to the back-water and such influences, the greatest amount of which is meas

ured at the Keokuk gauge:

The following table, from observations kept at the Rock Island bridge, gives the duration of different stages there. The zero is the low water of

1864:

The maximum range of the Mississippi at Davenport, according to Lieuten ant Warren's report of his survey in 1853, is 23 feet, and at the head of these rapids 13 feet. During the time the Rock Island bridge gauge has been kept, the highest water observed was May, 1862, viz., fifteen feet two inches, which would thus appear to have been at this point seven feet ten inches below the range of highest floods, such as that of 1851.

It is difficult to deduce from these observations at the head and foot of the rapids the corresponding rise of the river at the places between the rapids, or above and below them. That it is somewhat more rapid at the foot is apparent from the levelling made by General Warren's parties in October, 1866, at Clinton and at Burlington. These show the extreme ranges at Clinton from lowest to highest water is about 18 86 feet, at Burlington 18.75 feet, and at Quincy

20.31 feet.

The following statement is taken from Lieutenant Warren's report as to the relative effect of rises on the rapids and on the bars above and below, and is the best that can now be given. Supposing that the natural channel was deepened so as to give a low-water depth of four feet, we would then have

With 3 feet on the bars, 4 feet over the rapids.

With 4 feet on the bars, 4 feet 3 inches over the rapids.

With 5 feet on the bars, 4 feet 7 inches over the rapids.
With 6 feet on the bars, 5 feet over the rapids.

With 7 feet on the bars, 5 feet 8 inches over the rapids.

With 8 feet on the bars, 6 feet 6 inches over the rapids.

From this it is evident it would be required to increase the improved navigation to a low-water depth over the rapids of at least five feet, in order to accommodate the passage of boats, when six feet navigation would be afforded.

II.

Before proceeding to the discussion of the different plans of improvement, the board will state their opinions in reference to the dimensions that any improvement should have, in order to fully accommodate the requirements of navigation.

First. If it be an improvement without locks.

The least width at any place should not be less than 300 feet. The depth should not be less than five feet in extreme low water, in order that boats may pass over the rapids with six feet, when they can carry six feet over the bars. The ordinary low water is about five feet on the bars.

It will be seen by referring to the table taken from Lieutenant Warren's report, which was prepared with great care, that when there is five feet on the bars, (supposing the rapids improve to four feet in extreme low water,) there would be only four feet seven inches on the rapids; and hence a boat drawing the maximum draught up as far as the foot of the rapids would still be unable to pass over them. The ordinary low water, that is, the average depth from about the 1st of July till the 1st of December, will give about five feet on the bars, and boats will invariably load to that depth, if they can get freight and pass over the bars in safety, even though they rub along the bottom; but they cannot be allowed to rub over the rocky bottom of the rapids-they ought to have at least six inches to spare. This would require five feet six inches on the rapids when there are five feet on the bars, and the improvement will then give five feet depth on the rapids in extreme low water.

This the board regard as the least depth that can be given, and make an improvement that will adequately accommodate the requirements of commerce. Second. If it be an improvement involving the use of locks.

The same depth, viz., five feet in extreme low water, must be had on the mitresills of the locks, and the width of the water-way should not be less than 250 feet in excavation, and 300 feet in all other cases, when it could be had without a great increase in the cost of the work.

If it be a canal improvement, taking the pilots' gauge at Montrose as a guide for extreme low water at that place, the canal bottom at the upper end, and the level of the top of the mitre-sills of the guard-lock, should be fixed at five feet below extreme low water. The lower mitre-sill of the lower or outlet lock should be fixed at the same distance-five feet below extreme low water (viz., that of 1864) at that place. The lockage to be effected by means of two lift locks, using the guard-lock as a lift lock, after the water at the head of the、 rapids rises four feet above the extreme low water.

Third. Width of the lock chambers should be eighty feet, in order that the largest boats navigating the Mississippi or its tributaries may pass through it. According to the information in the possession of the board, this width of lock chamber will pass any steamer on the Mississippi, with probably only one or two exceptions, and will allow an increase in beam of the majority of steamers that now navigate the upper Mississippi.

It is believed that the time is not far distant when eighty feet width of lock chamber will be as little as will lock through ordinary steamers on the upper Mississippi, though it may now seem large. Eighty feet is the width of lock

chamber adopted for the canal around the falls of the Ohio; less than that should not be adopted here.

Fourth. Each lock should have two pairs of gates.

Fifth. Length of lock should be 350 feet between the quoins.

Sixth. The locks should be filled by means of valve or slide gates in the main gates, and culverts and openings passing at right angles through one of its side walls the number and size of the culverts to be sufficient to fill the dock in as short a time as possible consistent with the safety of the masonry and boats. The locks to be emptied in like manner through culverts in the opposite side wall, and valve gates in the lower gates. The details of construction and mode of operation should be left to be determined by the engineer in charge of the work. Seventh. That the middle and lower locks should be built high enough to maintain eight feet depth of water in the levels, so that boats of a maximum draught of seven feet may pass over the mitre-sills with one foot to spare. guard-lock should be built high enough to lock boats through when the gauge at Montrose shall indicate twelve feet above the low water of 1864.

The

Eighth. The bottom of the canal should slope about one and one-half inch to the mile, when rock excavation on the bottom occurs, in order that a more rapid current may be given to the water at those places, should it be necessary to wash out sediment.

Ninth. The width of the canal embankment should be not less than ten feet on top, including the riprap covering, the slopes of the embankment should be made with one and a half base to one vertical on both sides, and the average thickness of the riprap covering, on the river side, need not exceed two and a half feet, and on the canal side two feet, and on top one and a half feet. The general range of the top of the embankment should be made not less than two feet above the range of extreme high water. In excavating, however, a large amount of rock will be at the disposal of the engineer, and can be advantageously used in increasing the thickness of the riprap covering of the embankment if deemed necessary. The guard-lock walls to be two feet above the extreme high water, the other lock walls to be high enough to maintain eight feet depth in the levels.

In case any other than a canal plan should be adopted requiring locks to overcome the fall, they should have the same general dimensions and the same general arrangement of their several parts, so far as may be applicable to the case. III.—Plans of improvement discussed-Plan of improving the natural channel of the river by excavating the rock.

This plan was first recommended by Lieutenant Lee of the engineers in 1837, and some work was accomplished during the next two succeeding years under his direction.

The plan was further partially carried out under the direction of J. G. Floyd, United States agent, some twenty years later. About $350,000 have been spent, and as nearly as can be ascertained about 25,000 cubic yards of rock excavated.

It is conceded by pilots, notwithstanding the many assertions to the contrary that the navigation on Lamillies and Lower chains has been somewhat benefited, but in no degree commensurate with the amount of money expended, when the price of labor and materials at that time is taken into consideration.

General Wilson, in his report, speaking of this plan of improvement, says: "The plan of excavating the channel is for a variety of reasons exceedingly difficult to execute at these rapids, either by blasting under water or by the use of coffer dams. In order to enlarge the channel to 200 feet wide and four feet depth in extreme low water, (low water of 1864,) * according to data recently obtained, it will require the excavation of 176,519 cubic yards of rock, which, at an average of $15 per yard, will cost $2,662,797.

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*

"Should this channel be completed it will not accomplish all that is required,