Biskra lies immediately south of the Atlas Mountains, in the
Department of Constantine, 220 kilometers from the Mediterranean
and about 400 kilometers from Ouargla. To the northeast of the oasis
lie the Aurés and to the west the beginning of the Saharan Atlas,
which run south of west across Algeria into Morocco. Just north
of the place are small hills and low, jagged mountains—detached
spurs from the main ranges. These are the Djebel Bou Rhezal, running
nearly northeast and southwest. The highest of them, directly west
of Biskra and about 8 kilometers distant, has an altitude of 463
meters. The Bou Rhezal Mountains have a precipitous southern face,
but fall away more gradually to the north, where the slope joins
a wide and undulating plain. The latter extends to the base of the
main Atlas ranges. Southwest of the oasis, and about 2 kilometers
distant, a range of rocky hills extends for a distance of about 6
kilometers, or until they join the mountain range of the Saharan
Atlas. These hills are called Ed Delouatt. To the south of Biskra,
as has already been stated, there extends a vast plain, the reg,
which dips gently to the south and drains into the Chott Melrirh,
50 kilometers or more distant. The situation of Biskra relative to
the mountains on the one side and the desert on the other, together
with its altitude, governs the climate of the place. Except Laghouat
no vicinity in the desert proper was seen with so great an amount of
precipitation (about 200 mm.) as Biskra, which is indicated by the
relatively rich flora. One who has seen only Biskra can not draw
conclusions regarding the vegetation or the conditions of plant
life of those portions of the Sahara that lie farther to the south,
where much more intense conditions of aridity obtain.
The soils of the vicinity of Biskra are various. That of the low
hills between the town and the Djebel Bou Rhezal is only a few
centimeters in thickness, but in the washes from the hills it is
a meter or more. Here the soil is a sandy loam with an admixture
of small stones and pebbles. On the flat ground to the north and
to the south of these hills it is of a finer texture, approaching
the adobe of the southwestern United States. On the reg to the
south of the oasis the soil is also fine, and in some places, if
not underlying the plain as a whole, there are strata of gravel
at varying depths beneath the surface. This soil in places carries
considerable salts. It dries to a powder during the long dry seasons
and is easily blown by the winds. Owing to outcropping rock, the
south face of the Bou Rhezal Mountains has but scant soil, but that
of the northern side resembles the soil of the low hills to the
south, which has already been characterized. Near the town are dunes
of good size. Especially to the southwest the sand banks against
Ed Delouatt hills is in large amount. In the opposite direction,
but farther from the oasis, the dunes are fairly extensive.
The Oued Biskra is of great importance to the oasis, since it
carries water for several weeks of the year and furnishes water for
irrigation. Its channel lies about 3 meters, or possibly more, below
the general level of the oasis, and possibly in earlier times may not
have been so well defined as at present, spreading its waters over
its flood-plain during high water. The oued is made up of several
tributary oueds which cross the plain north of Djebel Bou Rhezal,
unite where there is a pass in these mountains, and finally debouch
on the reg to the south of the town, where the channel becomes
continually less well defined. Another oued takes its origin in the
Bou Rhezal Mountains in several independent branches which unite at a
pass in Ed Delouatt hills and extend for a distance of 15 kilometers
or more into the reg. One of the feeders of this oued is from hot
springs, Hamman es Salahine, about 8 kilometers northwest of Biskra.