64 PHYSICAL

GEOGRAPHY.

 

 

 

Fig, 62, The Falls of Niagara.

The grandest falls in the world are those of the Niagara,
160 feet high. Though greatly inferior to many others in
height, yet their volume of water is so great that they
surpass all others in grandeur. The Victoria Falls of the
Zambezi in Africa nearly equal in volume those of the
Niagara. Their height is 360 feet.

The highest falls in the world are those of the Yosemite,
in California. Two projecting ledges break the sheet into
three falls, whose total height exceeds 2000 feet. One of
the highest falls in Europe is the Staubbach or Dust-brook,
in the valley of the Lauterbriinnen in Switzerland. The
water makes one sheer fall of 959 feet, and is lost in a
sheet of mist before it reaches the ground.

The Middle Course extends from where the
river emerges from the mountainous or hilly dis-
tricts to the low plains near the mouth. The
descent is comparatively slight, and the velocity
small. The erosion of the bottom of the channel
is insignificant, but at the sides, especially during
freshets, the river undermines its banks and thus
widens its valley. Here the river is divided into
two distinct portions: the channel proper and the
alluvial flats or flood-grounds.

The Lower Course extends from the middle
course to the mouth. The fall is slight, and the
velocity small.

170. Changes in River-courses.—During floods, when
the velocity and eroding power are greatly increased, ex-
tensive changes often occur in river-courses. After the
floods have subsided the water is found running through
new channels, its old ones being either completely filled
with deposits of mud, or occupied by slender streams.

Along the Mississippi these partially deserted channels
are called bayous, and, in places, widen out into large lakes.

 

 

(See Fig. 63.) The Red River appears to have formerly
emptied into the Mexican Gulf through a separate chan-
nel. In the basins of the Amazon, the Ganges, and the
Po, the old deserted channels are numerous on both banks
of the streams.

~ 171, River Mouths—A wide, open river-mouth
is called an Estuary; the accumulation of mud
or sand which occurs in the mouths of certain
rivers is called a Delta.

172, Inundations.—During certain seasons of
the year, the amount of water drained into the
river-channel is greater than it can discharge; it
then overflows its banks and inundates the sur-
rounding country.

Inundations of rivers are caused—

(1.) By excessive rainfall ;

(2.) By periodical rains ;

(3.) By the melting of ice and snow.

In the tropics, where the rainfall is more or

- less periodical, the inundations of the rivers are -

also periodical. The melting of the ice and snow,
which occurs regularly at the beginning of the
warm weather, also causes periodical inundations.
The Nile rises annually on account of the period-
ical rainfall of its upper sources; the Mississippi
semi-annually, once from the melting of snow,
and once from the winter rainfall.

When both the area of the river-basin and the rainfall
in inches are known, experience permits of a calculation,
by means of which the probable time and extent of rise of
water in a river can be approximately predicted. In times
of heavy rainfall, the Weather Bureau of the United
States is enabled to predict the probable rise of the im-
portant rivers.

Influence of the Destruction of the Forests on In-
undations.—When the forests are removed from a large
portion of a river-basin, the rains are no longer absorbed
quietly by the ground, but drain rapidly off its surface into
the river-channels, and thus in a short time the entire
precipitation is poured into the main channel, causing an
overflow. It is from this cause that the disastrous effects
of otherwise harmless storms are produced. The inunda-
tions are most intensified by this cause in the early spring,
when the ice and snow begin to melt. The destructive
effects of the floods are increased by masses of floating ice,
which, becoming gorged in shallow places in the stream,
back up the waters above. The increased frequency of
jinundations in the United States is, to a great extent, to
he attributed to the rapid destruction of the forests.

173. The Quantity of Water Discharged by a
River depends principally—

(1.) On the size of the basin ;

(2.) On the amount of the rainfall.

The quantity of water in a river also depends—

(1.) On the climate of the basin, a dry, hot air diminish-
ing the quantity by evaporation ;

(2.) On the physical features of the basin, whether wooded
or open;