WATERFALL

This article appears in Volume V28, Page 368 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: VIR-WAT

WATERFALL , a point in the course of a stream or river where the water descends perpendicularly or nearly so. Even a very small stream of water falling from any considerable height is a striking object in scenery. Such falls, of small volume though often of immense depth, are common, for a small stream has not the power to

erode

ERODE

a steady slope, and thus at any considerable irregularity of level in its course it forms a fall. In many mountainous districts a stream may descend into the valley of the larger river to which it is tributary by way of a fall, its own valley having been eroded more slowly and less deeply than the main valley. Mechanical considerations apart, the usual cause of the occurrence of a waterfall is a sudden

change

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

geological

GEOLOGICAL

structure. For example, if there be three horizontarl strata, so laid down that a hard stratum occurs between twosoft ones, a river will be able to grade its course through the upper or lower soft strata, but not at the same rate through the intermediate hard stratum, over a ledge of which it will consequently fall. The same will occur if the course of the river has been interrupted by a hard barrier, such as an intrusive dyke of basalt, or by glacial or other deposits. Where a river falls over an escarpment of hard

rock

overlying softer strata, it powerfully erodes the soft

rock

at the base of the fall and may undermine the hard rock above so that this is broken away. In this way the river gradually cuts back the point of fall, and a gorge is left below the fall. The classic example of this process is provided by the most famous falls in the worldNiagara.
WATER-FLEA, a name given by the earlier microscopists (Swammerdam, 1669) to certain minute aquatic Crustacea of the order Cladocera, but often applied also to other members of the division Entomostraca (q.v.). The Cladocera are abundant everywhere in fresh water. One of the commonest species, Daphnia pulex, found in ponds and ditches, is less than one-tenth of an inch in length and has the

body

BODY

enclosed in a trans-parent bivalved shell. The
head, projecting in front of the shell, bears a pair of branched feathery antennae which are the

chief

CHIEF (from Fr. chef, head, Lat. caput)

swimming

SWIMMING (from " swim," A.S. swimman, the root being common in Teutonic languages)

organs and propel the animal, in a succession of rapid bounds, through the water. There is a single large black eye. In the living animal five pairs of

leaf

LEAF (O. Eng. leaf, cf. Dutch loof, Ger. Laub, Swed. lof, &c.; possibly to be referred to the root seen in Gr. Ma-eta, to peel, strip)

-like limbs acting as gills can be observed in constant motion between the valves of the shell, and the pulsating heart may be seen near the dorsal surface, a little way behind the head. The

body

BODY

ends behind in a kind of tail with a

double

curved claw which can be protruded from the dell. The female carries the eggs in a brood-chamber between the back
of the body and the shell until hatching takes place. Through-out the greater

part

PART

of the year only females occur and the eggs develop " parthenogenetically," without fertilization. When the small males appear, generally in the autumn, fertilized " winter " or " resting eggs " are produced which are cast adrift in a case of " ephippium " formed by a specially modified

part

PART

of the shell. These resting eggs enable the race to survive the cold of winter or the drying up of the water.
For a fuller account of the Cladocera and of other organisms which sometimes share with them the name of " water-fleas," see the article ENTOMOSTRACA. (W. T. CA.)

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