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In the several falls special examinations were made, the details of which have been reported to you from time to time.
Obstructions to navigation in the shape of bowlders, points of ledges, &c., were examined, and, where practicable, measured. As the river was low during the work and special examinations as far as possible made at times when little or no mill-water was running, it is believed that most of the sunken rocks requiring removal have been noticed.
Levelings in Hunt's Falls* connect with the zero of Essex Company, and of proprietors' locks and canals' scales of heights through the upper and lower monuments respectively at the head and foot of Hunt's Falls], established by Commissioners to determine the rights of flowage by the Lawrence Dam at certain stages of the river. Above Pawtucket Dam, levels commence at the foot of Cromwell's Falls (where the Lowell mill-pond ends), 174 miles above Lowell, and they were carried continuously up to the top of Amoskeag Dam in Manchester, a distance of 14 miles; they were checked by releveling from Amoskeag Dam to the starting point; they also connect with the Amoskeag base. It was not deemed advisable to ascertain by spirit level the fall of the river on the Lowell mill-pond, involving the running of 354 miles of level lines. For the purposes of navigation this part of the river is practically level. For the sake of referring the heights above Cromwell's Falls to the United States Engineers datum plane, the surface of the mill-pond has been considered horizontal on Sunday, September 18, 1881, at 2 p. m., the most favorable opportunity which presented itself. Should these 35 miles of levels be run, it is possible that a correction (the adding of a constant) will have to be applied to the heights above Cromwell's Falls, as given on the accompanying drawing No. II.
The slope of the river, represented in profile on this map, has been constructed from simultaneous observations on the gauges established, and corresponds to the average height prevailing during the survey (mill-water running), and I have called it the stage of ordinary low-water with mill-water running.
Map No. II gives, besides the slope of the river, the heights of gauges, bench-marks, &c.; also the location of the various obstructions referred to in the estimate. The heights are drawn to scale 1: 100; the horizontal distances have for the sake of clearness not been drawn to scale, but are stated in number at the foot of the sheet.
The working up of the first-mentioned eight sets of maps to a uniform scale, and the plotting of the hydrographic survey thereupon, has not been attempted on account of the limited time; only in places where improvements are needed have the soundings been plotted, especially so below Lawrence, at Gage's Ferry Shoal, and at Andover Bar, and between Lawrence and Lowell in Hunt's Falls, and adjoining parts of Merrimac and Concord rivers.
The map of Hunt's Falls (No. 1), with proposed improvements, has been already forwarded to you with a special report dated November 9, 1881.
In other parts of the river the notes only have been referred to, so as to report intelligently upon the channel.
The navigation of Merrimac River above Haverhill, Mass., has, since the opening of the channels through Mitchell's Falls, been practicable to the head of those falls for craft drawing 44 feet of water in ordinary low-water with mill-water running.
To further extend this channel to the locks in Lawrence, it is only necessary to deepen it at two places, namely, Gage's Ferry Shoal and Andover Bar, and to remove a small quantity of rocks at a comparatively slight expense (see special report here with of October 15, marked B).
Since the opening of the channel through Mitchell's Upper Falls the effect of the tide is perceptibly felt above, in the difference of velocities during ebb and flood, so that tidewater now virtually extends to Lawrence.
To carry navigation above Lawrence a fall of 314 feet has to be overcome from the lower lock to the mill-pond above Lawrence Dam. The locks in Lawrence Canal are 20 feet wide, 100 feet long, and they admit 2.6 feet draught over the miter sill of the lower lock; should this condition be altered either by lowering the miter sill to the requisite depth or by building a new lock in front of the present one with its miter sill at the proper grade, there are yet many impediments to the navigation of such craft as could be navigated above the Lawrence Dam. The streets in Lawrence, the gas and water mains cross the canal on bridges which leave only 3 feet space between them and the canal level. To provide these bridges with suitable draws, and to carry the gas and water in syphons below the canal bottom, will be so expensive that the question arises whether it would not be more profitable to construct a new canal and locks on the south side of the river.
Above Lawrence Dam there is no obstruction in the form of shoals or bars to a 4-foot channel as far as Hunt's Falls; only scattered rocks need be removed, beginning 7 miles above Lawrence Dam.
*This set of levels check with Commissioners' levels within 0.03 foot. Checking within 0.09 foot.
At Hunt's Falls, 9 miles above Lawrence Dam, there is a fall of 11 feet to overcome in a length of 5,700 feet. Different plans of making these falls navigable have been discussed in special report dated November 9, 1881. Considering the immense value of the water power in Lowell, and that any works built in the river for the purpose of procuring slackwater navigation would tend to reduce the area of cross-section, and thus damage the water power by raising the river, it appears to me the most feasible way to excavate straight channels through the chutes in Hunt's Falls, and by stationary engines on the shore to haul the vessels or barges over the falls.
Above Hunt's Falls there is a fine basin with deep water in the city of Lowell; it extends for about a mile to a distance of 11 miles from Lawrence Dam; three-fourths mile beyond is the Pawtucket Dam and the Lowell mill-pond, the surface of which is 36 feet above the basin below.
To further extend navigation above the Pawtucket Dam in Lowell, the Pawtucket Canal is available for craft drawing 3 feet and of 12 feet width, the lock chambers being 100 feet long. This canal is entered by the Concord River, which joins the Merrimac directly above Hunt's Falls. Between the entrance to Pawtucket Canal and the mouth of Concord River there are still some slight rapids where additional excavation would have to be done to make a channel 3 feet deep.
As the same difficulties exist in the Pawtucket Canal as in the Lawrence Canal, in regard to streets, water and gas mains crossing the canal at a low grade, it is perhaps worth considering whether a new set of locks and a canal from the head of the basin three-fourths mile below Pawtucket Dam to the mill-pond above could not be so located as to avoid the cost of changing existing bridges, streets, &c., in Lowell; and at the same time provide for greater width of lock chamber than 12 feet, which is the minimum width the present laws of Massachusetts oblige the canal company to maintain. The depth of 3 feet which the Pawtucket Canal Company maintains in the locks might in a new canal be increased to continue 4-foot navigation above Lowell to Nashua, N. H., where the channel is deep enough now.
Between Lowell and Nashua the river is deep and there is no less depth in the channel than 8 feet on this distance of 13 miles. The opportunities for traffic by water between these two cities compare favorably with those between Newburyport and Haverhill, and very little improvement is needed to make it safe. Opposite Tyng's Island, about 44 miles above Lowell, we meet with the Old Wiscassee Dam, which is now submerged and has from 1 to 2 feet of water over it; but a passage has been cleared through it, 10 feet deep, which only needs to be properly buoyed out. Some rocks and a point of ledge below this old dam require removal.
A short distance above Tyngsborough Bridge, about 6 miles from Lowell, bowlders need removal.
Two ledges and a rock below Nashua should also be removed to make this part of the river safe to navigate.
Above Nashua the channel becomes shoaler and the rocks more frequent; 4 miles above Nashua (or 17 miles above Pawtucket Dam) the smooth water of Lowell millpond ends and a series of falls and rips commence, which continue up to Manchester, N. H., with intervening pools.
The rise on this part of the river is as follows:
From foot of Cromwell's Falls (elev. 93.01) to head of Merrill's Falls (elev. 126.17).
i. e., from Manchester, the river falls 334 feet in 13 miles.
The top of Amoskeag Dam (elevation 177.99) is again 51.82 feet above the head of Merrill's Falls.
It is not practicable to make these several falls navigable by excavation of channels in the rapids. Cromwell's Falls, and perhaps Short Falls, might, by a large outlay of money, be treated so that stationary engines could help barges through the rapids. The only practicable way is to restore slackwater navigation on this part of the river by means of locks and wing-walls parallel to the shore, transferring the upper levels to the locks at the lower levels.
The river, before the building of the railways, was navigated from Boston via the old Middlesex Canal to Lowell, thence to Manchester and above to Concord, N. H. Remains of the locks and wing-walls, by which the falls were then overcome, still exist in the river, and the present project contemplates their restoration, with little or no change in location. The canal-boats at that time were 8 feet wide, 80 feet long, and the dimensions of the locks were suited to admit these boats. If restored, the locks should be 20 feet wide and 100 feet long, allowing a draught of 3 feet, for which the following estimate has been made. The locks were formerly built of granite, in a very solid manner, and some of the old lock-walls are yet in a good state of preservation, especially so in Griffin's Falls. The wing-walls and dams were made of cribwork, and are now nearly all razed to the low-water line. The locks should be rebuilt in solid granite masonry, the stone doweled together. To build the wing-walls and dams of solid granite masonry would secure permanent works; but the cost of it would be so greatly in excess of crib-work (excess estimated at $690,000) that it is thought crib-work for the wing-walls and dams (where the latter are needed) would answer the purpose, if damage done by ice freshets was promptly repaired.
To secure a navigable channel 3 feet deep between the falls it will be necessary to remove a great many bowlders in places specified in the following estimate.
In Cromwell's Falls the wing-wall should be carried a distance of 900 feet from the entrance to the lock (to B 13), and there cross the river as a dam, instead of being carried through the upper part of the falls for an additional distance of 21,000 feet.
The bar above Ram Rock, between Little Cohass Falls and Goff's Falls, is one of the worst places in the river; if a dam could be built at Little Cohass Falls, the water could be raised sufficiently on the bar to give 3 feet navigation; but this would, at the same time, back the water up at least 1 foot (more at high stages of the river) in front of the mills on Great Cohass Brook; and the damage to this water power would probably exceed the cost of clearing an artificial channel through the bar, which consists of heavy bowlders, and no doubt would maintain its depth if excavated. The estimate has been made for such a channel.
At Goff's Falls ledges run across the river; at the most favorable place to pass the ledge, and directly above the proposed lock site, its elevation is 115.56, while the water level of the pool above is only 14 feet higher (117.06). A channel through this ledge would be very expensive, and as the river channel between Goff's Falls and Short Falls is shallow, it will be preferable to raise the level above Goff's Falls by closing up the gaps between the ledges by dams having a least elevation of 118.56.
At the head of Baker's Island, above Griffin's Falls, the navigation is again difficult; here a shifting gravel bar obstructs the passage; there is, however, depth enough in the gulley, but this is narrow and may change its location through the bar. It should be watched, and its location marked by buoys or shore ranges.
Between Merrill's Falls, in the city of Manchester, and the head of navigation, near the new iron bridge (McGregor Bridge), is still a small fall of about one-half a foot, caused by the rocky bar below Factory Bridge. A channel can be cleared here by the removal of bowlders.
The estimated cost of the various improvements is given below in tabular form.
Estimated cost of making the channel of Merrimac River navigable between Mitchell's Falls, below Lawrence, Mass., and Manchester, N. H.
Bowlders in channel below Hunt's Falls.
Channel in Hunt's Falls, Lower Chute
Channel in Hunt's Falls, Upper Chute..
Channel at mouth of Concord River
BETWEEN LOWELL, MASS., AND NASHUA, N. H.
Rocks below Wiscassee Dam
Ledge at Wiscassee Dam.
Rocks at Tyngsborough Bridge
Ledge above Little's Station.
Rock below Nashua
BETWEEN NASHUA AND MANCHESTER, N. H.
Rocks near Hill's Ferry.
Rocks off Whiting's Point.