Ledges (f this description occur in many of the Atlantic rivers e the United States, as the Potomac, the James River, and others; an they mark with precision the paving° of the rivers from the tmdulatin r hilly region to the low plains along the coast. There are of course epids where these ledges occur.
The tower course of rivers usually lies through a plain. In general Isere are no hills which constitute the outer margin of its course, and onsetinently there is no bottom or valley through which it runs.
'he banks arc very little raised above the surface of the waters, and he level ground extends to a greater distance. The current is slow, he fall being very small. Thus it was observed by La Condamine, hat the Amazonas from the narrow at Obydos to its mouth, a listanee of 700 miles, does not fall quite 12 feet, or little more than of an inch per mile. It can hardly be conceived that a river with o small a fall could propel its waters, and as the current of the \mamma is considerable, it can only be accounted for by suppo Mg that the enormous volume of water which the river brings town, drives on by its pressure that which is before it until it *aches the sea. [Amazosas, in GEOGA Div.] The surface of the Elbe at Hamburg, about 70 miles from the North Sea, is not nore than 6 feet above the sea, and the fall per mile very little uxceeds an inch. The surface of the plain through which a river runs ;enerally consists of an alluvium, which the river has deposited during the inundations. The matter of which this alluvium consists is soft and loosely bound together, and consequently the current, slow as it is, has lower enough to remove a portion of the banks from one side, and to deposit the detached matter on the other. Thus great changes are produced in the courses of rivers in the lapse of time. Major Rennetl surveyed a large portion of the lower course of the Ganges, and his maps were very exact at the time. He also observed the changes which the river had produced in its bed. About fifty years afterwards the course of the Ganges was again surveyed, for the purpose of establishing a steam navigation, and it was found that the course of the river hardly in any place agreed with the maps of Rennell. The most remarkable circumstance however is, that a river frequently divides into a number of arms, each of which runs to the sea, though some branches re-unite and again detach themselves from one another. Thus the Danube reaches the sea by seven arms, as the
Nile formerly did, according to the ancient accounts. though there are now only five arms in the Nile. Our best maps represent the number of the mouths of the Ganges as amounting to ten at least. This division of a river into several arms is easily understood when the soft nature of the alluvium is considered : and if we suppose that the river in its operation of changing its bed, finds in its way a piece of rock or other matter harder than the alluvial soil, by setting against such an obstacle the current is divided, and flows on both sides of it : the following inundation removes still more of the alluvium, and thus, in course of time, a new arm is formed.
The country which is enclosed by the arms of a river is called its delta, from the form of the Greek letter a, which the delta of the Nile, that which was best known to the ancients, greatly resembles : but the term is generally appropriate, as most river deltas hare that form. To the base of the triangle, however, the deposit of matter brought down by the river adds a curved projection seaward, rendering the entire form of what is called the delta that of the sector of a circle. It is a common conjecture that the space which is now occupied by the delta of a river was once a part of the sea, which was filled up by the ddbris and earthy matter brought down the river from the mountainous and hilly country through which its upper and middle course lie. This supposition is strongly supported by the nature of the soil, which evidently consists of matter brought down by rivers, and not of such as the sea leaves behind when, from any cause, it retires. (On this subject see Captain Sprat's Investigation of the Delta of the Nile,' as referred to in the article QUICKSANDS.) It may be added, that this operation of rivers goes on during the inundations, for after the waters have subsided the surface of a delta is found to be covered with a very thin layer of mud, which soon becomes dry earth. The deltas of rivers which are annually swollen by rains, which is the case between the tropics, are generally much more extensive than those which are formed by rivers whose inundations are only produced by the melting of snow.