Lawrence to the east of Nova Scotia. In fact the Atlantic coast north of the Hudson furnishes innumerable examples of submerged river valleys.
The tributary streams which enter low down on a river"s course, after submergence enter the sea directly in the bays formed by the submerged valley. Such rivers may be called dismembered rivers. Thus a coast region which was formerly well dissected by streams will on submergence become penetrated by a great number of irregular channels and bays.
FAN FILLED VALLEY.
Notice how the river is forced to wander around the edges of the fans.
Delaware and Chesapeake bays, where the early settlers each had a nice little sea inlet instead of a rough wagon road as his means of communication with his neighbors, are fine examples of submerged river systems. These drowned river valleys enabled the early settlers to penetrate easily into the country, and determined many of the early settlements, like Philadelphia, New York and Providence.
161.Alluvial Cones and Fans. -When a stream having a steepgrade and bearing a heavy load of sediment emerges upon a flat coun- try where the grade is suddenly reduced, it so quickly deposits its sediment as to be continually obstructing its own course and forcing itself to find new channels. It thus builds a fan or cone-shaped deposit pointed toward the place where the stream reached the plain. If the material is coarse, the deposit will have a steep slope; if fine, a gentleLAKE DELTA.
Notice the triangular formation.
slope resembling a spreading fan. Sometimes these fans so overlap each other as to form an irregularly sloping plain.
Plains of this kind are found along the base of many mountain regions. If such formations occur in regions of little rainfall, they yield themselves with peculiar facility toirrigation, as ditches can be easily led out from the apex of the cone in all directions. Southern California offers many examples of easy irrigation due to such cones.
162.Deltas. -When a river enters a body of quiet water, its cur-rent is gradually checked and it deposits its material in somewhat the same way as on emerging upon a flat country. But here the deposition is more gradual and the slope of the deposited material less steep. The sediment, too, is sorted by the water, and the finer material is carriedCONE-SHAPED DELTA IN LAKE GENEVA.
far out from the river mouth. Formations of this kind are called deltas, from the Greek capital letter Delta (?) which has the shape of a trian- gle. Few deltas have this ideal shape, but there is a general correspon- dence to it.
If the delta-forming stream descends steeply, it may build a delta with a steep upper surface rising cone-shaped above the water. Many of the deltas in Lake Geneva are of this kind. If the grade is slight, the delta will be simply a continuation of the flood plain of the river. Such is the Mississippi delta.
The layers of sediment composing the
delta, slope away from the point where the river enters the still water. Here,LAKE BRIENZ, FROM ABOVE INTERLAKEN.
A rapidly eroding stream at the extreme right has built a great delta dividing the ancient lake into two parts.
as in the alluvial cone, the river is continually silting up its outlet and being forced to seek new channels. In large deltas the river generally enters the sea through several channels or distributaries, as they are called. This is seen in the map of the Mississippi delta.
Deltas have rich, fine-textured soils and are very fertile. The Nile delta during all history has been noted for its fertility. But they are treacherous places, as they are liable to inundations by the overflowing of the river at time of flood. Because they are pushed out into the sea, they are peculiarly exposed to the sweep of the waves in great storms.
The rate at which deltas grow depends upon the amount of material carried by the river and upon the tides and currents at its point of outlet. In seas where the currents and tides are strong, no deltas are formed,except by very large streams such as the Yukon, the Huang-He and the Ganges. In quiet seas deltas are readily built. The delta of the Mississippi is more than 200 miles long and has an area of more than 12,000 square miles. The Po in historic time has built a delta more than 14 miles beyond Adria, a former port which gave its name to the Adriatic Sea.
163.History and Rivers. -From earliest times rivers have playeda most important part in the world"s history. At first almost all human movement was along river valleys, as they offered the easiest route of travel. Here, too, men found the fertile and easily worked land so nec- essary in their primitive agriculture. Thus their settlements were usu- ally placed upon the banks of rivers. In war the river offered a means of defense, as the Tiber so often did to Rome.
Before the time of railways, rivers and lakes supplied almost the only means of inland commerce. In our own country the hundred and fifty miles of unobstructed riverway stretching from New York to the north was the great road from Canada and the Lakes to the sea, fought for persistently in French and Indian Wars as well as in the Revolution. If in the Revolution the British could have obtained control of the Hudson, they would have effectually separated the colonists in the north from those in the south and would probably have been able to crush each separately.
The Mississippi River served for years as the only artery of transportation from the interior of the country to the sea. When Spain held the mouth of this river and Congress was unable or unwilling to exert itself to obtain the privilege for American boats to pass to the sea, it seemed for a time that the sturdy colonists along the Ohio and Mississippi would either form an independent country and fight for the privilege or else in some way ally themselves with Spain, so vital to them was the need of this waterway. In the Civil War vast amounts of blood and treasure were spent in fighting for the control of this river.