Imagens das páginas
[ocr errors]






Report of the Chief of Engineers, covering report by General Michler on the

examination and survey of the Potomac river.

MAY 18, 1868.-Referred to the Committee on Commerce and ordered to be printed.


Washington City, May 18, 1868. SIR: I have the honor to send herewith a communication of May 16 from the Chief of Engineers, with General Michler's report on the examination and survey of the Potomac river, made in accordance with the act of Congress of March 2, 1867. Your obedient servant,


Secretary of War. Hon. SCHUYLER COLFAX,

Speaker of the House of Representatives.


Washington, May 16, 1868. Sir: I transmit herewith the report, with accompanying maps, of Major and Brevet Brigadier General N. Michler, corps of engineers, on the examination and survey of the Potomac river, made in accordance with the act of Congress of March 2, 1867.

The suggestions of General Michler are generally concurred in.

The removal of the causeway of the Long bridge is deemed of the first importance, from its injurious effect upon the Washington channel of the river.

It should, for the present at least, be replaced by a roadway upon piles, that being the least expensive and most suitable until the expected advantage of such removal is fully proved.

This causeway begins near the Washington shore, and extends more than half-way across the river. The effect of this dam in certain stages of the floods of the river has formed the chief cause of the filling up of the upper portions of the former Washington and Middle channels. T'he effect of the same structure upon the tidal currents and tidal action has been also an active agent in the filling up of those channels.

It is also important that the bar in the Virginia channel should be dredged to a depth of 12 feet, but to a width greater than that estimated for by General Michler, say to 200 feet; and also that the Washington channel be deepened to the same depth, wherever needed, between the bridge and the mouth of the Eastern Branch.

General Michler's recommendation of a new cut between Easby's wharf and the Maryland draw of the Long bridge is deemed essential, with some modifications in its size and curvature, to restoring, as near as may be, the former navigable condition of the river.

It should be wider, and, at first, need not be dredged to the full depth be recommends. A depth of eight feet, at low water, would be sufficient for the first work.

The obstructions in the vicinity of Easby's Point should be removed; but it is not deemed advisable to construct a breakwater or deflector across the Virginia channel. The effects of the dredging should be tested before any dikes are resorted to.

It is, therefore, respectfully recommended that an appropriation be asked for, to be expended for the improvement of the Potomac river, as follows, viz: 1. For the removal of the causeway of the Long bridge, and to replace the same by a bridge upon piles.

$25,000 00 2. For dredging the Virginia and Washington channels..... 8,000 00 3. For dredging a channel eight feet deep and from 250 to 300

feet wide, between Easby's Point and the eastern draw of the Long bridge, about 300,000 cubic yards...

105,000 00


138,000 00

Very respectfully, your obedient servant,


Brigadier General of Engineers, Commanding. Hon. E. M. STANTON,

Secretary of War.




United States Capitol, Washington, April 30, 1868. GENERAL: By section 4 of an act of Congress approved March 2, 1867, (Public, No. 59,) “ making appropriations for the repair, preservation and completion of certain public works, heretofore commenced under the authority of law,and for other purposes,” the Secretary of War was directed to cause exam. inations or surveys, or both, to be made at certain points ; among others, of the Potomac river, in the District of Columbia. Subsequently, by Engineer Orders No. 67, dated July 27, 1867, the examination and surveys of the Potomac river were assigned to me. I now have the honor to submit a report of the same, accompanied by detailed maps of the different surveys made of that portion of the river lying between the Aqueduct bridge, at Georgetown, and Giesboro' Point, including a part of the Eastern Branch. The last survey, in 1867, only extended from Easby's wharf, in Washington, nearly opposite the foot of Analostan or Mason's island, to the Long bridge, the improvements of the channels of this particular reach being of most immediate and practical importance for the two cities. At your request, general, the Superintendent of the Coast Survey, Professor B. Peirce, kiudly consented to have the coast survey made; Captain C. P. Patterson, hydrographic inspector, directed the operations, and the party in charge of Mr. Clarence Fendall, assistant, executed the details of the work, and subsequently prepared the maps of the same. To each of these gentlemen I am greatly indebted for the interest displayed in prosecuting their labor, and especially to the latter for the completeness and accuracy of its execution.

The map is drawn on a scale of zooo, showing not only the hydrography of the river, but also the topographical conformation of the shore; the depth of the water, at mean low tide, is represented in feet. Observations of various kinds were made at many points as to the nature of the soil at the bottom, the direetion and strength of the currents in the channels, and the action produced by the rise and fall of the tides. It is found in water less than six feet that the bottom is hard sand on the surface, through which an iron rod, one inch in diameter, can be thrust six feet by the force of two men. In greater depth than six feet the rod can be pushed down from seven to ten feet with the same physical effort, and the bed on the surface is of soft mud. The points on the map which are colored in purple locate the positions where current observations were made; the arrows indicate the direction and velocity, one inch in length corresponding to one knot. For the purpose of carefully examining what, if any, marked changes have taken place in the nature and depth of the channels, as well as of the general hydrographic condition of the river since previous surveys were made, a plan has been prepared showing the alterations that have taken place during a period of more than sixty years. The earlier dates are furnished principally from a map published in 1792, which approximately locates the channel and general features of the river at that time, and previous to the construction either of the Long bridge or of the causeway connecting the head of the Analostan island with the Virginia shore; this is not probably very reliable in its details, although confirming the recollections of some of the older inhabitants with whom the subject has long been familiar, but it is nevertheless very interesting and suggestive. The later information is obtained from more recent and accurate surveys, made respectively in the years 1857, 1862, and 1867. That in 1857 was made by Mr. R. W. Burgess, civil engineer, under the orders of Captain J. C. Woodruff, corps of topographical engineers United States army, and those in 1862 and 1867 by coast eurvey parties, under the direction of Captain Patterson. From the maps prepared by them, tracings of which are herewith submitted, we are enabled to make a comparison of the changes that have taken place at different intervals.

By an examination of the chart published in 1792, it will be observed that in that year there existed three distinot channels. The first may be called the swash or shore channel, washing the banks of the city; the second is the one generally known as the Washington or city channel; and the third, at present the only one not entirely closed to navigation above the Long bridge, is denominated the Virginia or Georgetown channel. In the first a depth of seven feet could be then had, and in the second over nine; through the third, or main one, the soundings show that at least 17 feet could have been carried through its entire length. The least width of these channels is about 200 yards, large marshy flats separating them from each other. A large rock, called Braddock’s Rock, at the base of the hill upon which the United States Observatory now stands, is said to mark the spot where the British general, in 1755, landed a portion of his war material when contemplating his move upon Canada; a channel of seven feet then existed where now extends a low grassy expanse. In the year 1806, the appropriation for which was made by Congress and approved January 19, 1805, the causeway, connecting Mason's island with the Virginia shore, was thrown up for the purpose of deflecting the large body of water that then passed between the two into the main bed of the river between the island and the Maryland bank. In consequence of this the channel south of the island, through which a draught of 18 feet could be had, has been entirely closed , immediately below the causeway the depth of water is still over 30 feet ; a grassy marsh has since been formed, and now extends for some distance below the extremity of the island. The Long bridge was constructed in 1809; that portion of it known as the causeway, 1,967 feet in length, having been built on the “flats” separating the channels, and the remainder on wooden piles and crib-work; two draws for the passage of vessels are respectively 134 and 148 feet in the clear. Although the flats had previously existed to a wide extent, and have no doubt since been enlarged by the greater quantity of debris borne along by the river and deposited where the opposing forces of the current of the one and of the tidal influence from the ocean first encounter each other, these deposits yearly increasing in consequence of the more advanced culture of the rich regions through which the waters flow, still examinations recently made indicate that the flats have become more extensive owing to the obstructions offered by the causeway, not quite so apparent above the bridge as below it. The recent surveys, those made in 1857, 1862, and 1867, at intervals of five years, indicate very decided changes in the regions of the river as compared with that exhibited by the oldest accessible chart. Where formerly three channels existed, now only one is practically useful for the present tonnage of the sea-going vessels sailing between Georgetown and the Atlantic ports ; it has been slowly but gradually filling up for a short distance, so that upon the bar which has formed some eight feet of water only can be found at mean low tide; vessels bound up or down have to wait for the flood tide, creating delay and uncertainty. The old swash or shore channel has entirely disappeared, scarcely a foot of water over the flats upon the ebb tide; so also with the one known as the Washington or city channel, the upper portion of which is closed to navigation, the present head being about midway between the north draw of the bridge and the Washington monument, and even at that point scarce seven feet of water can be depended upon ; only very light-draught boats, chiefly used for transportation of wood and lumber, pass through the draw of this channel to the

entrance of the Washington canal, and then only on a high stage of water. These great changes in the last fifty years have been the result of the constantly accumulating deposits in the bed of the river from the surface drainage and sewerage of the two cities brought down by the Tiber and Rock creeks, from the washings from above and the wear of the banks by the varying currents, and from the effect of the ever shifting bars formed by sunken vessels or the obstructions arising from the building of wbarves and other necessary river encroachments. A careful examination of the more recent surveys indicates marked changes even during the brief intervals between the respective years in which they were executed. The plan previously referred to, exhibiting these alterations, accompanies this report. The curves laid down and indicated by different colors show the positions of the curves of 6, 12, and 18 feet of water in the river, as ascertained at the different times the surveys were made, the several depths being represented by full, broken or dotted lines. For example, before either the causeway at the head of the island or that at the Long bridge were built, the curves of 18 feet approached each other from above and below within 950 feet, with a channel of over 12 feet connecting them. Whilst up to the present period we find that there has been very little, if any, change in the depth of water between the island and the Washington shore since the first chart was published, the position of the upper curve of 18 feet remaining almost identically the same, still in descending the river by the Virginia channel such does not continue to be the case; the depth had diminished since that time, and by survey of 1857 the curve of 18 feet had receded over 7,000 feet, and its position was then only a short distance above the south draw of the bridge. In an interval of five years, as shown by survey of 1862, the same curve had commenced to advance up stream, although not very materially, having gained in that time about three hundred and sixty (360) feet; by the last survey, that of 1867, it is demonstrated that the same curve has continued to advance during a like interval, having moved up stream over two thousand three hundred (2,300) feet in the last five years ; at the latter rate it will not take long to regain its position of 1792. These alterations have, no doubt, been brought about by a combination of the causes above enumerated, aided by those powerful agents which brook no control, the immense freshets arising from the melting of the snows of the mountains and the heavy falls of rain, together with the great accumulation and pressure of the ice of winter when broken up and let loose by the aduance of spring. Similar changes can be observed in the 6 and 12-feet curves of the Virginia channel during the same intervals, at one time receding and then again advancing, although not to the same extent as in those of the 18 feet. Along the course of the same channel, between the upper and lower termini of the 12-feet curves, a distance of 2,000 feet, a gradual deposit bas been forming for years back, creating at one point a bar which has already seriously interfered with its navigation; during the last survey not more than soundings of eight feet were found on it at mean low tide. While these alterations have been taking place in the Virginia channel, others of considerable importance have been gradually made in the old Washington channel, exhibiting a tendency on the part of the current to re-open the bed of the latter. Where formerly a depth of 12 feet could be had in its most shallow parts, the examinations of the last year show that the 12-feet curves, up and down stream, are now about five thousand seven hundred (5,700) feet distant from each other, and the six feet curves are separated by some three thousand six hundred (3,600) feet; at intermediate points from one to five feet only can be found upon the intervening flats. · A comparison of the last three surveys shows, however, that during the interval of ten years, from the first to the last, the distance between the 12-feet curves has considerably diminished, and that the current now sets in the same direction as it originally had in the old channel. This is an interesting fact, and indicates that the river has a tendency to resume its old regime. Very little alteration has taken place in the shore-lines above the Long bridge, and consequently from this cause the river has not been much deflected from its natural course. Immediately below the Little Falls the depth of water is over 80 feet, and continues very deep until the Aqueduct bridge is reached, there the soundings indicate 24 feet in the channel. Thence to a short distance below the southern extremity of Mason's island it decreases considerably, varying from 22 feet to 18, although at some intermediate points there is as much as 35 feet of water, particularly opposite Easby's wharf. Below that point, as has been already described, the river spreads out over a wide surface, the current becoming more sluggish and the channel gradually contracting, with, for some considerable distance, scarce eight feet at mean low tide. Then, as the Long bridge is approached, the Virginia channel again widens out and gradually deepens; at the draw it is over 24 feet, and continues to increase in depth as the descent is made. The Washington channel, now heading but a short distance above the Long bridge, with only seven feet at its highest point, obtains a depth of 12 feet where it passes through the east draw, afterwards continuing to deepen, although but slightly; it in a short time joins, south of Greenleaf's Point, on the arsenal grounds, the waters of the Eastern Branch. Vessels drawing 18 feet can ascend the latter as high as the wharves at the navy yard ; formerly they could reach Bladensburg, but from natural causes the bed has been filling up. About half a mile below their junction the combined currents unite with the

« AnteriorContinuar »