It is lowest in November, and rocks, shallows, and the remains of old dams then render it almost unnavigable.
Two great dams on the river (one built by the Federal government) provide good water power.
The Monongahela has been improved by locks and dams to Fairmont.
South of Mosul, at which point navigation is blocked by two ancient dams, erected, apparently, to control the river for the Assyrian city of Calah, the ruins of which are called Nimrud by the natives after these dams, which they conceive to be the work of that mythical hero.
Were it not for these dams steamers might reach Mosul itself, at an elevation of 353 ft.
The wheels, called naoura, are of the most primitive construction, made of rough branches of trees, with palm leaf paddles, rude clay vessels being slung on the outer edge to catch the water, of which they raise a prodigious amount, only a comparatively small part of which, however, is poured into the aqueducts on top of the dams. These latter are exceedingly picturesque, often consisting of a series of well-built Gothic arches, and give a peculiar character to the scenery; but they are also great impediments to navigation.
The experiment was so far successful that, with incredible difficulty, the two vessels did actually reach Meskene, but the result of the expedition was to show that practically the river could not be used as a high-road of commerce, the continuous rapids and falls during the low season, caused mainly by the artificial obstructions of the irrigating dams, being insurmountable by ordinary steam power, and the aid of hundreds of hands being thus required to drag the vessels up the stream at those points by main force.
Midhat caused many of the dams to be destroyed and for some years occasional steamers were run between Meskene and Hillah in flood time, from April to August.
From Tewkesbury; the locks from Evesham upward to Stratford (17 m.) are decayed, but the weirs, and mill-dams still higher, afford many navigable reaches to pleasure boats.
It is irrigated by an elaborate system of dams, cuts and canals in connexion with the rivers Cauvery and Coleroon, and the soil is exceedingly productive.
The improvement of the river, by the removal of snags and the construction of dams and locks in order to give it a navigable depth of to ft.
Seven years later the Bokharians razed the city to the ground, broke down the dams, and converted the district into a waste.
The beds of these rivers, as well as that of the Danube, are continually changing, forming morasses and pools, and rendering the country near their banks marshy, Notwithstanding the work already done, such as canalizing and regulating the rivers, the erection of dams, &c., the problems of preventing inundations, and of reclaiming the marshes, have not yet been satisfactorily solved.
The regulation is effected by locks and movable dams, the latter so designed that in times of flood or frost they can be dropped flat on the bottom of the river.
Floods must be provided with channels of sufficient size to carry them safely past the mine openings, and intercepting ditches should be excavated for this purpose, and dams and embankments constructed to divert the flood waters.
The flood-water is controlled by a system of dams and channels constructed so as to utilize every drop, and the extent of cultivation is limited more by the supply of water available than by the amount of suitable soil.
Beavers are sociable animals, living in streams, where, so as to render the water of sufficient depth, they build dams of mud and of the stems and boughs of trees felled by their powerful incisor teeth.
"In places," writes Hearne, "which have been long frequented by beavers undisturbed, their dams, by frequent repairing, become a solid bank, capable of resisting a great force both of ice arid water; and as.
Their houses are formed of the same materials as the dams, with little order or regularity of structure, and seldom contain more than four old, and six or eight young beavers.
In numerous depressions, some of which may have been the beds of lakes formed by beaver dams, the soil is deep and largely of vegetable formation.
Dams on the Fox River furnish a good water-power.
A great series of dams was therefore constructed (mapped and described in " Topography of Nineveh," J.R.A.S.
By a series of locks and dams the Muskingum river has been made navigable for small vessels to the Ohio and above Zanesville to Dresden, where connexion is made with the Ohio Canal extending north to Cleveland.
In 1890-1893 one of the largest dams in the world, an immense structure of granite masonry, 1200 ft.
A new feature in diking was the construction of dams or sluices across the mouths of rivers, sometimes with important consequences for the villages situated on the spot.
Owing to the shrinkage of the soil in reclaimed lands, however, that is, lands which have been drained after fen or other reclamation, the sides of the polder are often higher than the middle, and it is necessary by means of small dams or sluices to make separate water-tight compartments (afpolderingen), each having its own unit of measurement.
In the Mediterranean provinces of Spain there are some very remarkable irrigation dams. The great masonry dam of Alicante on the river Monegre, which dates from 1579, is situated in a narrow gorge, so that while 140 ft.
In the jungles of Ceylon are to be found remains of gigantic irrigation dams, and on the neighbouring mainland of Southern India, throughout the provinces of Madras and Mysore, the country is covered with irrigation reservoirs, or, as they are locally termed, tanks.
The casual traveller in southern India constantly remarks the ruins of old dams, and the impression is conveyed that at one time, before British rule prevailed, the irrigation of the country was much more perfect than it is now.
When a river partakes of the nature of a torrent, dwindling to a paltry stream at one season and swelling into an enormous flood at another, it is impossible to construct a system of irrigation canals without very costly engineering works, sluices, dams, waste-weirs, &c., so as to give the engineer entire control of the water.
- Map showing the Damietta and Rosetta dams on the Nile.
Hence, as the survival of the fittest, there are many artificial waters, with low dams consisting exclusively of earth - and sometimes very sandy earth - satisfactorily performing their functions with no visible leakage.
The most important dams are those which close the outlets of existing valleys, but a dam may be wholly below ground, and according to the commoner method of construction in Great Britain, sufficient) im ermeable construc- ?
Dams Any well-made earthen embankment of moderate height, and of such thickness and uniformity of construction as to ensure freedom from excessive percolation at any point, will in the course of time become almost impermeable to surface water standing against it; and when permeable rocks are covered with many feet of soil, the leakage through such soil from standing water newly placed above it generally diminishes rapidly, and in process of time often ceases entirely.
Hence the extreme importance in high dams with clay cores of loading the clay well for some time before water pressure is brought against it.
The highest dams of this class in the British islands impound water to a level of about IIo ft.
On the other hand, the partial or total failure of smaller dams of this description, to retain the reservoir water, has been much more common in the past than is generally supposed.
Throughout Great Britain there are still many reservoirs, with earthen dams, which cannot safely be filled; and others which, after remaining for years in this condition, have been repaired.
Asphalt, whether a natural product or artificially obtained, as, for example, in some chemical manufactures, is a most useful material if properly employed in connexion with reservoir dams. Under sudden impact it is brittle, and has a conchoidal fracture like glass; but under continued pressure it has the properties of a viscous fluid.
High - almost wholly above the original ground level, which is said to be impermeable; but other dams of the same system, with core walls of less than ioo ft.
Masonry dams are, for the most part, merely retaining walls of exceptional size, in which the overturning pressure is water.
Or with the reservoir full, near the lower part of the outer face ac. The principles hitherto adopted in designing masonry dams, in which the moment of resistance depends upon the figure and weight of the masonry, involve certain assumptions, which, although not quite true, have proved useful and harmless, and are so convenient that they may be continued with due regard to the modifications which recent investigations have suggested.