extraordinarily shallow a basin as that of Lake Chad, alterations are
inevitable owing to the deposit of the silt brought down by the rivers.
The question is, what becomes of the quantities of water from the Shari
and other rivers when the basin of the lake has once been filled up with
the silt? This leads to another interesting problem, viz. the connexion
between Lake Chad and the Bahr-el-Ghazal, which in a way is a
continuation of the lake towards the east. The views on this question,
even of recent geographers, differ widely as to whether the Bahr-el-
Ghazal is a former affluent or an effluent of the lake. Although
Nachtigal tries to produce proof that this valley was formerly a channel
of effluence of Chad leading to the depression of Bodele, which lies 100
metres nearer sea-level,[94] yet some of the travellers who have
latterly examined these regions have set forth the opposite
assertion.[95] Freydenberg cites as a proof of this the delta-like
passage from the Bahr-el-Ghazal to the lake, as well as the appearance
of stones in this channel, which can only have originated from the
highlands of Tibesti in the Sahara.[96] The delta-like formation at the
mouth of the Bahr-el-Ghazal is at least not a sufficient argument, for
it resembles in many respects the mass of islands stretching north-
westwards along the Kanem shore of Chad.[97] This archipelago is of
enormous extent, and the number of its islands and islets is legion. All
these islands are of sandy formation and run longitudinally from north-
west to south-east, decreasing in height towards the middle of the
lake.[98] Where these rows of islands cut the Kanem shore they are
continued by sand-dunes; when the level of the lake is low, those dunes
which are nearest to the land assume the form of peninsulas; on the
other hand, canals in the form of countless lagoons running between the
islands penetrate the mainland and lend an extraordinarily jagged
appearance to the coast there.
Even the island, or more correctly the peninsula, of Seyorum on the
Bornu side has the same longitudinal direction from north-west to south-
east; the same applies to the dune formations which continue along the
Bornu shore roughly parallel with it. This remarkable identity of
direction between the dunes and the islands on the Kanem shore on the
one hand, and the dunes on the Bornu shore on the other, is at the root
of the conjecture in which Marquardsen indulges when he says, ‘the proof
that the islands belong geologically to the mainland would as a
consequence lead us to the conclusion that before the lake was here
there existed a desert which created the dunes.’[99] As a matter of
fact, such a conclusion would not be incorrect. How otherwise could the
existence of dunes on the Bornu side, in spite of the thickly overgrown
bank, be explained, unless in earlier times there was dry land where now
is the bed of the lake? And the whole behaviour of the lake speaks in
favour of such a theory. At all events, there can to-day be scarcely any
doubt that the islands are the ridge-tops of dunes which were cut off
from the land by water, but which formerly originated, like the still
existing dunes, under the influence of the strong north-east trade wind
prevailing in Bornu.[100] In what manner this collection of water, to
which the Chad owes its origin, has found room for itself, whether
through the blocking up of the Bahr-el-Ghazal, as Nachtigal
assumes,[101] or through tectonic movement, this indeed has still to be
definitely settled. Nachtigal’s assumption has in its favour the greater
probability; if one accepts his theory, one can also assume as causes of
the blocking up, owing to the slightness in depth of the depression, the
formation of dunes as well as the copious deposit of sediment by the
Shari. Audoin’s opinion, that volcanic disturbance has had a say in the
matter, would presuppose a high antiquity for the lake, which is
scarcely to be assumed, since neither the configuration of the shore nor
its vegetation warrants such a conclusion.[102]
A further point, which has called forth much discussion, concerns the
saltness of the lake. Barth, as well as Nachtigal, lays special stress
on the fact that Chad is a fresh-water lake;[103] Barth does so,
although he adds that the soda-impregnated soil on the north shore of
the lake ‘communicates this quality to the water’. Nay, he even says
further on (German edition, iii, p. 237): ‘It is very remarkable that,
while the water of Chad itself is fresh, the greater part of the water
found in the region quite close to the edge of the shore of the lake
contains natron.’ It was at this place that ‘the water was so strongly
impregnated with that mineral as to be scarcely drinkable’. Nowadays it
can hardly be any longer disputed that the water of Lake Chad is
salt—though certainly in a different sense to that of other well-known
salt inland seas—even if the actual quantity of salt present is so small
as to be generally not perceptible to the taste, at least at the period
of high water.[104] The saltness of the water varies not only according
to the time of year, but also according to its situation. The places
which are least salt are those which lie at the mouths of the rivers and
which are distinguished by the free flow of water. One may well
distinguish the water here at all times of the year as fresh or quite
fresh.[105] According to Tilho’s map of Lake Chad,[106] the saltness is
greater to the north of the two stretches of open water than to the
south of them. This tallies with the fact that those pools which are
partly separated from the lake, and which contain a more or less strong
solution of salt according to their volume of water, leaving behind when
evaporated a visible layer of this mineral, are also to be found along
the banks of the northern half of the lake, just where Denham was struck
by the taste of natron in the water.[107] These salt pools form the
continuation of that strip of country already mentioned, dotted with
salt lakes, which extends from Munio to the north-west corner of Lake
Chad and forms the southern boundary of the Sahara. The soil here
contains everywhere a considerable amount of natron, which gradually
disappears as one proceeds southwards. It is only natural that the water
of Chad should penetrate the natron-impregnated soil and communicate
traces of this mineral to the whole basin of the lake, traces which will
be perceptible in a greater or less degree according to the hydrographic
relations already described.
Of the rivers which mingle their waters in Lake Chad, the Shari with its
affluents and side streams is by far the most considerable and also the
most important for Bornu, although it is only in its lower reaches that
it flows through this country. Rising outside the Sudan proper, with its
furthest southerly sources lying somewhat below 6° of north latitude, in
a district whose climatic conditions show a strong resemblance to the
damp tropical basin of the Congo, it conveys a powerful volume of water,
and, including the Logone, which debouches into it, is the only affluent
of Lake Chad, which contains continuously flowing water at all times of
the year. In its lower reaches, which form the eastern boundary of
Bornu, the river has an average breadth of from 2,000 to 4,000 metres,
but below Kusseri it has dug itself a bed contracted to 1,000 metres,
and its banks have often a sheer drop of as much as ten metres. This bed
is, of course, not always full. Towards the end of the dry season it is
beset or constricted by multitudinous sand-banks, and the river itself
is generally not more than 400 metres broad and often less, but,
nevertheless, even then retains its navigability, and, with the
exception of a few fords, cannot be crossed except in canoes.[108] In
June the river begins to rise and reaches its highest point in October,
when its level has risen from four to seven metres; it then completely
fills its bed, and under certain circumstances even overflows its banks
far and wide. Conditions similar to those of the Shari prevail with its
most important affluent, the Logone. This river also takes its rise in
districts which are lower-lying than Bornu itself, though its sources do
not reach quite so far south; it also conveys a constant stream of
water, and in the rains shows a still greater inclination to overflow
its banks even than the Shari. Besides, in the rains it conveys to its
lower reaches, through numerous streams and rivulets, a large part of
the heavy rainfall from the Mandara highlands. The Logone, which rises
from three to four metres in the rains, shows in places a canal-like
character; its course is very tortuous, for with a very small fall it
apparently has great difficulty in forcing its way through the clayey
soil of the absolutely level plain. To the same reason is attributed the
endeavour of the river to build up a system of steep-banked side
streams, such as the deep and canal-like Lage-Matia (Laho-Matia) below
Musgu—a sort of discharge-valve for the pent-up masses of water. To this
also may be traced the origin of the numerous natural canals which unite
the Logone and the Shari, and which caused Barth to call this region
‘the African Holland’.[109] Many of these canals appear to have no
current whatever, and are designated in the language of the country
‘ngaldjam’[110] according to Barth, and are called by the same traveller
‘meadow-water’ or ‘flooded meadows’. This extensive network of natural
canals is nevertheless not sufficient to drain off the volume of water
brought down by the river, and so from about the end of September to a
time when the waters of Lake Chad are still far from being at their
highest level, widespread floods occur, which place a large part of
Bornu under water. Every year from August to January the whole country
between Logone and Matia is one huge lake which, during the time when
the water reaches its highest level, may even extend to Mandara, and out
of which the towns of the Musgu stick up like islands; the right bank of
the Logone till it reaches the Shari is also then far under water.
Communication between village and village is then only possible in
canoes, and even the harvest has often to be brought in in this
fashion.[111] At the beginning of the dry season the water in the canals
very quickly runs off and only remains in isolated spots—in fact, it
often dries up so completely that the canals appear as hollow roads, and
can be used as avenues of communication by people on foot.[112] To the
Shari-Logone system belongs also the Ba-Ili, the ‘second great artery of
the Musgu country’,[113] which has just the same characteristics as the
Logone, with which it is also connected through the various ‘ngaldjam’.
It has this difference, however, from the latter, apart from its size,
in that during the dry season it contains no flowing water, but only
exhibits larger or smaller stagnant lagoons. Moreover, before the
Logone, reinforced by the Ba-Ili, discharges itself into the Shari, it
sends towards Lake Chad a regular network of waterways of its own, which
are again themselves connected with the Shari, and thus give the
impression of a wide-branching delta. The many-armed delta proper at the
mouth of the Shari is, however, not in connexion with the network of
canals. In their condition during the dry season these water-courses
resemble the canals connecting the Shari and Logone; only a part of them
then have water communication with the lake. Before the many-limbed
river and canal system of the Shari-Logone reaches Lake Chad, it
occasions in places the formation of a very remarkable kind of soil
especially characteristic of these regions, a formation which is common
also to the other rivers flowing into Chad from the south of Bornu. This
formation is known in the country under the name of ‘firki’, a peculiar
boggy loam—very expressively called ‘terre cassée’ by Lenfant[114]—which
is saturated with water in the rainy season and forms a gigantic sponge,
while the humidity thus absorbed is again returned to Chad in the form
of little rivulets. Though entirely covered with water in the rainy
season, the ‘firki’ dries up within a few days, the deposits disappear,
and the rivulets dwindle, so that the soil as far as the eye can reach
is split open by gaping cracks over a yard deep, which divide the
surface into larger and smaller many-cornered lumps, and render the
crossing of the marsh over these stretches extraordinarily arduous both
for man and beast. Only isolated spots still remain covered with water,
and form during the dry weather perennial pools. Marquardsen’s
theory[115] that the ‘firki’ is a loamy deposit above a sandy subsoil
has the most probability: the sand allows the water to sink through and
withdraws it from the upper layer of loam, which, receiving no new
accessions of humidity, dries up completely under the intensive
insolation and splits asunder into big lumps. It is only where the
subsoil also is loamy that it is possible for the water to lie and form
those lakes that continue even during the dry weather. The layer of
‘firki’, quite different in its strength in different localities, is
only interrupted by the higher dune formations, it submerges the more
insignificant sand outcrops.
In many more striking ways the presence of ‘firki’ soil on the lower
course of the Yedseram, the third largest of the rivers flowing towards
the Chad basin, makes itself felt, as also on the smaller rivers Ngadda,
Goma, and Ngua, which happen to flow parallel to it. All these do not
reach Chad itself, but are in communication with the lake only during
the rains by means of larger stretches of flood water, in which the
current of the river is still perceptible, whilst the ‘firki’ soil
underneath, probably of considerable thickness here, absorbs vast masses
of the water.[116] The ‘firki’ is here sometimes piled up into regular
dikes—probably built up on sand-dunes underneath—as one can clearly
perceive in the dry weather at certain spots between Ulugo and Bornuski.
At this time of the year there remain also channels leading from these
‘firki’ masses to Chad, such as the Mbulu, in the form of stagnant
shallow canals consisting of extremely dirty water, whilst the streams
ending in the ‘firki’ district have long ago dwindled away. The Yedseram
is remarkable in that its hard gravelly bed contains in its upper
courses even in March, i.e. at the end of the dry season, a continuous
flow of water; and above all, in that its sources springing from the
Mandara Highlands form even then mountain brooks of crystal clearness,
and in some cases of considerable size. But below Mutube the water
dwindles and becomes more and more exiguous among the pebbles, and then
enters the dried-up river-bed consisting of pools more or less deep,
whose water becomes more and more dull-coloured as the loamy low-lying
plain is reached. At the end of September it has reached its highest
level, and is then impassable except at the fords even above Issege, but
even by October the running water has quickly fallen again, and recedes
further and further towards the source.
Much more considerable than this river, both as regards its length[117]
as well as its volume of water, is the river of Yo or the Komadugu-Yobe.
It takes its rise from quite a number of streams, which unite into one
river below the ruins of the ancient capital, Ghasr Eggomo, after
forming large and numerous fertile islands in a rich low-lying plain.
The main stream proper comes from the Hausa States and is known as the
river of Katagum, that on the left, which is not much smaller, as the
river of Hadeija, and that on the right coming from southernmost Bornu
and rising south-east of Gujba is called Ansei. The streams coming from
the north-west, such as the river flowing from Yamia, are mere ‘wadis’,
as might be expected from the desert districts where they rise, and seem
to contain running water only intermittently. The bed of the Komadugu-
Yobe is scarcely fifty metres broad at the mouth, and the river
discharges itself into Lake Chad without any delta being formed; during
a great part of the year it contains no running water, and at the
beginning of the rains has nothing to show but a few larger or smaller
pools. At the beginning of July the river begins to contain running
water, and by the end of November, when it reaches its highest level and
overflows its banks, it can only be crossed by artificial means; but it
falls again very quickly, and by the end of January it is again
fordable, so that it can only fulfil the duty entrusted to it of forming
a bulwark on the border against the hordes of robber Tuaregs in a very
imperfect fashion and for a very short time.
[Footnote 70: Nachtigal, ii. 380.]
[Footnote 71: Denham gives the respective degrees as 10° and 15° N., and
12° and 18° E. (Denham, ii. 138). The difference is accounted for by the
fact that Denham regarded Kanem as still forming part of Bornu.]
[Footnote 72: Along the tenth degree of north latitude runs also a part
of the present boundary-line between the German Kameruns and the French
Congo.]
[Footnote 73: Cf. also the map annexed to Lugard’s ‘Northern Nigeria’ in
_Colonial Reports_, 1907. Even this boundary-line is somewhat arbitrary,
since the districts in question belonged sometimes to Bornu and
sometimes to the Hausa States on its western border. But the spheres of
influence of the adjoining Sultanates are not exactly determined.]
[Footnote 74: Nachtigal puts it at 150,000 square kilometres. Nachtigal,
ii. 382.]
[Footnote 75: Cf. also below.]
[Footnote 76: Chudeau, p. 334.]
[Footnote 77: Barth, iv. 68. One-half of the lake contains fresh water.]
[Footnote 78: Denham, i. 174.]
[Footnote 79: Barth, iii. 255.]
[Footnote 80: Even immediately to the south of Chad shore, but outside
Bornu, in Bagirmi country, there is a similar rock formation, Hadjer el
Hamis (called Hadjer Teous by Denham) composed, according to Foureau, of
rhyolite. Foureau, _Doc. sc._, p. 728. Cf. also Denham, ii. 52, and the
plate in Lenfant, p. 203. [Cf. p. 311 of the present work.]]
[Footnote 81: Rohlfs, ii. 120.]
[Footnote 82: _La Géographie_, xiii (1906), p. 203.]
[Footnote 83: _La Géographie_, xv (1907), p. 169.]
[Footnote 84: Barth, ii. 322; cf. v. 396. Even Denham mentions a
considerable rise of Chad (at the beginning of February), owing to which
the cotton plants at Wudi were under water. Denham, i. 73.]
[Footnote 85: Marquardsen attributes the high water mentioned by Rohlfs
(ii. 87) to the flooding of the River Wobe (_Mitteil. aus d. deutschen
Schutzgeb._ xviii. 328).]
[Footnote 86: Nachtigal, ii. 9.]
[Footnote 87: M. Audoin, _La Géographie_, xii. 308.]
[Footnote 88: Freydenberg, _La Géographie_, xv. (1907), 166.]
[Footnote 89: Tilho, _La Géographie_, xiii. 205; cf. also the map
annexed to his article.]
[Footnote 90: _La Géographie_, xii. 310.]
[Footnote 91: Nachtigal, ii, p. 357; Tilho, _La Géographie_, xiii. 204.]
[Footnote 92: Information obtained prior to this only concerns the banks
of the lake.]
[Footnote 93: D’Huart, _La Géographie_, ix. 164; M. Audoin, _idem_, xii.
318.]
[Footnote 94: Nachtigal, ii. 116 and 120. Cf. also Denham, ii. 57.]
[Footnote 95: Foureau, _D’Alger au Congo_, p. 670, and _Doc. sc._, p.
278.]
[Footnote 96: _La Géographie_, xv. 162.]
[Footnote 97: [Vid. Appendix IX and note [343].]]
[Footnote 98: Audoin, _La Géographie_, xii. 305.]
[Footnote 99: _Mitteilungen a. d. deutsch. Schutzgeb._, xviii. 339.]
[Footnote 100: Audoin, _La Géographie_, xii. 307.]
[Footnote 101: Nachtigal, ii. 359.]
[Footnote 102: _Mitteilungen a. d. deutsch. Schutzgeb._, xviii. 339.]
[Footnote 103: Barth, ii. 325; iii. 53.]
[Footnote 104: Tilho, _La Géographie_, xiii. 204.]
[Footnote 105: In February 1904 I camped at Ulugo on the southern shore
of Chad, and for days I had to use the lake water for cooking and
drinking. Although the pools, from which I took the water, were cut off
from circulation with the open water owing to thick ambach and papyrus
vegetation, yet I could not perceive the slightest taste of salt.]
[Footnote 106: _La Géographie_, xiii, no. 3.]
[Footnote 107: Denham, ii. 92; Nachtigal, ii. 328; Foureau, _Doc. sc._,
ii. 665; Destenave, _La Géographie_, vii. 425.]
[Footnote 108: Cf. Nachtigal, ii. 549; Foureau, _D’Alger au Congo_, p.
712.]
[Footnote 109: Barth, iii. 241.]
[Footnote 110: Kund, as well as Dominik, indeed affirms that ‘ngaldjam’
means a rhinoceros in the Musgu language, so there may be some
misunderstanding here. _Mitteilungen a. d. deutsch. Schutzgeb._, xix.
18; Dominik, p. 239. [‘Ngaljam’ is a Kanuri word equivalent to the Hausa
‘faddama’, meaning a shallow water-course.—P. A. B.]]
[Footnote 111: Stieber, _Kolonialblatt_, xvi. Jahrg., p. 83.]
[Footnote 112: Cf. plate, Dominik, p. 188.]
[Footnote 113: Stieber, _Kolonialblatt_, xvi. Jahrg., p. 117.]
[Footnote 114: Lenfant, _La grande route du Tchad_, chap. vi, viii.]
[Footnote 115: _Mitteilungen a. d. deutsch. Schutzgeb._, xviii. 342.]
[Footnote 116: Cf. Rohlfs, ii. 66 and 72.]
[Footnote 117: Nachtigal gives the length as about 600 kilometres.
Nachtigal, ii. 353.]
IV
CLIMATE
Climatically, as well as geographically, Bornu belongs completely to the
tropic zone; although cut off from the warmth of the Equator, its
average temperature throughout the year of 29° to 30° centigrade makes
it not only one of the hottest countries in Africa, but even of the
whole world.[118] Though the climate of Bornu may on the whole be called
regular, considerable fluctuations in temperature nevertheless take
place, as is proved by the records of the last sixty years.
Of all the travellers who have spent any length of time in Bornu, the
only observations that extend over a detailed period are those of Barth
and Nachtigal; moreover, those of later travellers, which embrace a few
months at the most, in the main and on the whole, in so far as they are
not of small value, confirm the former results. Barth’s observations
have the advantage of extending over several years, of embracing all
parts of the country which he personally visited, and of having been
manifestly taken at places whose situation could not influence the
result one way or other. But their reliability is somewhat prejudiced by
the fact that the necessary thermometrical readings are sometimes
discontinued for weeks at a time, and were resumed on odd days at quite
different times. These irregularities happen to be of especial
importance in the thermometrical readings, which were taken after midday
or during the afternoon, when according to experience quite considerable
increases of temperature may occur in a short time. Owing to the fact
that the traveller, especially at the beginning of his journey, was not
always in a position to be able to take observations during the hottest
part of the day, many high maximum temperatures no doubt escaped him.
Barth himself was well aware of the insufficiency of his data, for he
refers to them throughout only as ‘Fragments of a Meteorological
Register’.
Nachtigal’s observations are extraordinarily conscientious and arranged
with greater regularity and include also barometrical readings. But they
extend, as far as Bornu is concerned, only over seven months and omit
precisely the period when the highest temperatures occur. Moreover, the
place where the observations were taken, viz. the court-yard of
Nachtigal’s house in Kuka, was such that it must have influenced at any
rate the thermometrical readings. Inside the walls of the town, daily
baking in the sun, the nightly radiation from the soil does not nearly
reach the same intensity as in the open plain; this is strikingly
apparent in Nachtigal’s thermometrical readings. There is an absence
throughout the winter months of the low temperatures, shortly before or
after sunrise, which have struck every one travelling in Bornu at this
season.[119] Moreover it is not to be forgotten that the shade-tree,
under which Nachtigal took his observations, influenced the result both
of the maximum and minimum readings.[120]
The table given below makes it apparent that April, with an average
temperature of 34° centigrade, is the hottest month of the year and
December the coldest; further, that the chief rainy months, July,
August, and September, show the least fluctuations in daily temperature,
while the coolest months, November, December, and January, show the
greatest. The reasons are not far to seek, they depend on the position
of the sun together with the degree of intensity of its rays resulting
from its position. At the end of March the sun approaches its zenith for
the first time—i.e. as regards Central Bornu—the atmosphere begins to be
saturated with moisture and this lessens the fall of temperature at
night, which becomes still less marked in April, in which month prevails
the most oppressive heat, not yet mitigated by the rains.[121] The first
tornadoes now set in, and become more and more frequent, while their
ever-increasing volume lessens the nightly fall of temperature, though
it affects the daily temperature in the opposite direction, so that the
daily fluctuation of temperature in July and August only reaches 9°. At
the beginning of the month of October the tornadoes cease, and
consequently there is an ever-increasing rise of temperature to
correspond, which, however, very soon drops in the following months.