TRANSPORTING POWER OF RIVERS. 65

 

 

(3.) On the nature of the bed or channel, whether leaky
or not.

It will be noticed that these three circumstances are
connected with the two additional river-mouths already
alluded to: the air-surface of the river, and the channel-
surface. :

Keith Johnston estimates the daily discharge of all the
rivers of the world at 229,000,000,000 cubic yards, or over
2,620,000 cubic yards per second.

——20ia30e—_

na CHAPTER IV.

Transporting Power of Rivers.

174. Silt or Detritus—Rivers are ceaselessly
at work carrying the eroded materials, called silé
or detritus, from their upper to their lower courses.
Valleys are thus formed, miles in width and thou-
sands of feet in depth, and lofty mountains greatly
reduced in height.

The amount of silt transported by rivers is almost in-
credible. According to the careful estimates of Hum-
phreys and Abbot, the silt brought down every year by
the Mississippi and thrown into the Mexican Gulf, if
collected in one place, would cover a field one square mile
in area to the depth of 268 feet. According to Lyell, the
deposits, in the Bay of Bengal, of the Ganges and the
Brahmapootra, are nearly as great.

The rivers are carrying the mountains seaward,
and the continents are thus decreasing im mean
height and increasing in mean breadth.

175. Deposition of Silt—Since the silt or
eroded mineral matter is-heavier than water, it
will settle in all parts of the river-course. It. will,
however, remain in those places only where the
velocity of the river is comparatively small.
These places are as follows:

(1.) In the channel of the river;

(2.) On the banks, over the alluvial flats or
flood-grounds ;

(3.) At the mouth ;

(4.) Along the coast near the mouth.

176. In the Channel—lIn rivers that traverse
great plains, the inclination near the mouth is
slight, and the diminished velocity allows the ma-
terial to accumulate in the channel, thus raising
the general level of the stream. When the rivers
traverse settled districts, the inhabitants are com-
pelled to erect huge river-walls to prevent the
flooding of the adjacent lands; and, in some places,
the channel has been filled to such an extent that
the ordinary level of the river is higher than that
of the plains along its banks.

The levees or banks of the Mississippi are of this nature.
On the level plain of Lombardy the surface of the Po, in

 

some places, is higher than the tops of the neighboring

_ houses. When floods occur in such districts, the breaking

of a levee or river-wall is generally attended by much
loss.

177. Rafts.—Drift timber, thrown into the stream by
the undermining of the banks, is common in rivers that
traverse wooded districts. Portions of such’ timber, be-
coming imbedded in shallow parts of the channel, form
obstructions which prevent the passage of subsequent
masses. The impediment so formed checks the velocity
of the stream, and mud deposits occur between the trees.
Such accumulations are called rafts. The raft of the Red
River, previous to its removal, was thirteen miles in length.
A large raft exists near the mouth of the Mackenzie River
in British America.

178. On the Alluvial Flats or Flood-grounds.
—The low flat plains on the sides of the river,
which are formed by the erosion of the banks in
the middle and lower courses, are covered by the
water when the river overflows its banks. In the
shallow water over these parts the velocity of the
water is slight, and the silt is deposited, thus
forming rich alluvial plains.

In large rivers the flood-grounds often attain consider-

able size. In. the Mississippi at Vicksburg the width of
the alluvial plain is over 60 miles.

In the lower courses of a river, the velocity
being small, comparatively slight obstacles suffice
to turn the waters from their course. The river-
channel is therefore characterized by wide bends

 

 

 

 

 

 

 

 

Fig. 68, Alluvial Flats of the Mississippi.
(Showing deserted courses and fluviatile islands and lakes.)

or curves. At the bend of a river the main cur-
rent is directed against one of the banks, where
rapid erosion takes place, the eroded material ac-