DRAINAGE SYSTEMS. 67

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     

 

 

 

 

 

 

    

 

 

 

Fig. 66. Fluvio-Marine Formations,

and the inflowing tide neutralize each other. The
impediment so formed permits of the rapid de-
position of silt, which fills up the portions of the
ocean so shut off; and converts them into shallow
bodies of water called sounds. These sounds, by
gradual rising of the land, are afterward con-
verted into river-swamps, According to Dana,
the eastern and southern coasts of the United
States, from Virginia to Texas, are an almost con-
tinuous fluvio-marine formation. Albemarle and
Pamlico Sounds and the Great Dismal, Alligator,
and Okefinoke Swamps are but different stages in
the formation of these deposits.

—— 070300

CELALPITIGRE Vs.

Drainage Systems.

181. Continental Drainage is dependent on the
position of the mountain-systems and the direc-
tion of their slopes. The mountain-ridges or
peaks, or the high plateaus, form the water-sheds.
In some cases, from a single peak or plateau, the
water drains into distinct river-systems, emptying
into different oceans.

182. North America——The central plain of
North America is drained by four large river-
systems: the Mackenzie into the Arctic Ocean;
the Saskatchewan and the Nelson into Hudson
Bay ; the St. Lawrence into the Gulf of St. Law-

 

 

 

 

rence; and the Mississippi into the Gulf of Mex-
ico. The basin of the Mississippi occupies the
long slopes of the Rocky Mountains and the
Appalachians. The Missouri and the Ohio are
the principal tributaries of the Mississippi.

Numerous streams descend the eastern slopes
of the Appalachian system into the Atlantic.

Owing to the position of the predominant sys-
tem, the streams which empty into the Pacific are
comparatively small. The principal are the Yu-
kon, the Columbia, and the Colorado.

There are several remarkable isolated water-sheds or
drainage-centres in North America. These are—

(1.) In the central part of the Rocky Mountain system,
where the land drains in different directions into the sys-
tems of the Mississippi, the Columbia, and the Colorado
Rivers.

(2.) In the northern part of the Rocky Mountains,
where the drainage is received by the systems of the
Yukon, the Mackenzie, and the Saskatchewan Rivers.

“. 183. South America resembles North America

in its drainage systems. The long, gentle slopes
of the Andes, and those of the systerns of Brazil
and of Guiana, are occupied at their intersections
by the three great river-systems of the continent:
that. of the Orinoco, in the north; that of the

- Amazon, near the centre; and that of the La

Plata, in the south. Nearly the entire continent
is drained by these rivers and their tributaries
into the basin of the Atlantic.

The Pacific receives no considerable streams.
Only impetuous mountain-torrents are found.

The Magdalena, which drains north, corresponds to the
Mackenzie; the Orinoco and the Amazon, which drain
east, to, the Nelson and the St. Lawrence; and the La
Platte, which drains south, to the Mississippi.

184, Europe forms an exception to the other
continents as regards its drainage. Though some
of its large rivers rise in its predominant moun-
tain-system, yet the majority rise in the incon-
siderable elevations of the Valdai Hills. The
Alps are drained by four large rivers—the Rhone,
the Rhine, the Danube, and the Po. These all
have large deltas.

Although in this part of the continent the frequent in-
tersection of the two lines of trend produces numerous
basin-shaped valleys, yet, owing to breaks in the enclosing
mountains, none of any size have an inland drainage, but
discharge their waters through numerous tributaries into
one or another of the principal river-systems.

The Great Low Plain of Europe is drained
toward the north and west by the Petchora and
Dwina into the Arctic; by the Duna, the Mie-
men, the Vistula, and the Oder into the Baltic;