66 PHYSICAL GEOGRAPHY.

 

cumulating lower down the river, in the bed of
the stream, where the velocity issmall. The river
is thus continually damming
up portions of its old chan-
nel and cutting new ones.
The rapid excavation of
these portions of the alluvial

materials which compose it.
Sometimes the river cuts a
new channel across the nar-
row neck of a bend, part of
its waters running through
the old channel and part
through the new.. In this
way fluviatile islands are
formed. One of the chan-
nels is sometimes separated
from the other by a deposi-
tion of mud or sand. The
water fills the old channel
by soaking through the soil,
and thus fluviatile lakes are
formed. Numerous fluviatile
lakes occur near the banks of the Lower Missis-
sippi and the Red River.

 

Fig, 64. Formation of
Fluviatile Islands and
Lakes,

plain is favored by the loose

 

Thus, suppose the river flows in the direction of the
arrow at S, Fig. 64, and its channel has the bends shown.
A new channel may be formed at a,b, the river either
flowing through both channels, thus converting the neck
of land I, into a fluviatile. island, or the old channel may
fill up and form a fluviatile lake, L, by bars forming in
the old channel at a and 0.

“179. At the Mouth—Delta Formations—In

sheltered parts of the ocean, where the tides are
weak and the ocean-currents feeble, or in inland
seas and lakes, where they are entirely absent, the
eroded material accumulates at the mouth of the
river in large, triangular-shaped deposits, called
delias, from their resemblance to the Greek letter
(4).of that name.

The Delta of the Mississippi is the largest in the
Western Continent. Its entire area is about 12,300 square
miles, though but two-thirds of it are permanently above
the water, the remainder being a sea-marsh. It begins a
little below the mouth of the Red River. The stream cuts
through the delta in one main channel, but near the ex-
treme end of the delta forms several mouths. On all sides
of the main stream, numerous smaller streams force their -
way into the Gulf through the soft material.

The Delta of the Nile, at its outlet into the Mediter-
ranean, occupies an area of nearly 9000 square miles. A
large portion of the sediment of the river is deposited over
the flood-grounds during inundations. The fertility of the
land is largely dependent on these deposits.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 65. Delta of the Mississippi, (After Dana.)

The Delta of the Ganges and the Brahmapootra,
jn the Bay of Bengal, is considerably larger than the Delta
of the Nile. Between the Hoogly and the main branch
of the Ganges, numerous streams force their way between
countless islands, called the Sunderbunds, inhabited by
tigers and crocodiles. The Po, the Rhone, the Rhine, and the
Danube in Europe, the Tigris, the Euphrates, the Yang-tse-
Kiang and Hoang-Ho in Asia, and the Senegal and the Zam-
bexi in Africa, have extensive deltas.

180. Along the Coast, near the Mouth.—Fluvio-
Marine Formations are deposits of silt that form
along the coast near and opposite the mouths of
rivers, under the combined action of the river-
current and the tides of the ocean. A sand-bar
is formed at some little distance from the mouth
of the river, where the outflowing river-current