, being composed of sand and clay, and lie from 5 to 20 m. from the
shore, which throughout its eastern side nowhere faces open water. The
channels between the islands do not exceed 2 m. in width. Two principal
groups are distinguished, the Kuri archipelago in the south, and the
Buduma in the north. The inhabitants of the last-named islands were
noted pirates until reduced to order by the French. The coast-line is,
in general, undefined and marshy, and broken into numerous bays and
peninsulas. It is also, especially on the east, lined by lagoons which
communicate with the lake by intricate channels. The lake is nowhere of
great depth, and about midway numerous mud-banks, marshes, islands and
dense growths of aqueous plants stretch across its surface. Another
stretch of marsh usually cuts off the northernmost part of the lake from
the central sections. The open water varies in depth from 3 ft. in the
north-west to over 20 in the south, where desiccation is less apparent.
Fed by the Shari (q.v.) and other rivers, the lake has no outlet and its
area varies according to the season. The flood water brought down by the
Shari in December and January causes the lake to rise to a maximum of 24
ft., the water spreading over low-lying ground, left dry again in May or
June. But after several seasons of heavy rainfall the waters have
remained for years beyond their low-water level. Nevertheless the
secular shrinking goes on, the loss by evaporation and percolation
exceeding the amount of water received; whilst, on the average, the
rainfall is diminishing. In 1870 the lake rose to an exceptional height,
but since then, save in 1897, there has been only the normal seasonal
rise. The prevalent north-east wind causes at times a heavy swell on the
lake. Fish abound in its waters, which are sweet, save at low-level,
when they become brackish. The lagoons are believed to act as purifying
pans in which the greater part of the salt in the water is precipitated.
In the south-west end of the lake the water is yellow, caused by banks
of clay; elsewhere it is clear.

[Illustration: Lake Chad]

The southern basin of Chad is described under the Shari, which empties
its waters into the lake about the middle of the southern shore, forming
a delta of considerable extent. Beyond the south-east corner of the lake
is a depression known as the Bahr-el-Ghazal (not to be confounded with
the Nile affluent of the same name). This depression is the terminatio