found the largest islands of the Congo—Esumba, 30 m. long,
and Nsumba, 50 m. long, and over 5 across at its broadest part.
At this point the river from bank to bank is 9 m. wide. Opposite
Nsumba, the Mongala, a northern affluent, enters the main
stream, whilst lower down (just north of the equator) the Lulanga,
Ikelemba and Ruki rivers, southern tributaries, mingle their
black waters with the dark current of the Congo. Thirty miles
south of the equator the river is joined by the Ubangi (q.v.), its
greatest northern affluent. Here the Congo is fully 8 m. wide.
Opposite the Ubangi confluence is the mouth of a narrow channel,
some 10 m. long, which connects the Congo with Lake Ntomba,
a sheet of water about 23 m. long by 8 to 12 broad. In flood
time the water flows from the Congo into the lake. Immediately
below ferruginous conglomerate hills of slight eminence reduce
the river to a width of less than 2 m., and in comparatively close
succession are two or three other narrows. With these exceptions
the Congo continues at a width of 5 to 6 m. until at 2° 36′ S. it
abruptly contracts, being confined between steep-faced hills
rising to 800 ft. This stretch of the river, known as the “Chenal,”
is 125 m. long and is free from islands, though long reefs jut into
the stream. Its width here varies from 2 m. to less than 1 m.
About 40 m. after the Chenal is entered the Kasai (q.v.) coming
from the south empties its brick-coloured waters at right angles
into the Congo through a chasm in the hills 700 yds. wide. The
confluence is known as the Kwa mouth. The Chenal ends in
the lake-like expansion of Stanley Pool, 20 m. long by 14 broad.
The middle of the pool is occupied by an island (Bamu) and
numerous sandbanks. Its rim is “formed by sierras of peaked
and picturesque mountains, ranging on the southern side from
1000 ft. to 3000 ft. in height.” The banks offer considerable
variety in character. On the north bank are the Dover Cliffs,
so named by H. M. Stanley from their white and glistening
appearance, produced, however, not by chalk but by silver sand,
the subsidence of which into the water renders approach to the
bank sometimes dangerous. The banks of the lower end of the
pool are comparatively flat. On the south side, however, stands
the great red cliff of Kallina Point (about 50 ft. high), named
after an Austrian lieutenant drowned there in 1882. Round the
point rushes a strong current 712 knots an hour, difficult to stem
even for a steamer. On the northern bank of the river at the
western end of the pool is the French port of Brazzaville. South
of the pool hills, low but steep, reappear, and 4 m. lower down
begin the cataracts which cut off the middle Congo from the sea.
Some 300 yds. above the first of these cataracts is the Belgian
port of Leopoldville, connected with the navigable waters of the
lower river by railway. At Stanley Pool the elevation of the
river above the sea is about 800 ft., a fall of over 500 ft. in the
980 m. from Stanley Falls. The banks of the river throughout
this long stretch of country are very sparsely populated. The
number of inhabitants in 1902 did not exceed 125,000.
The velocity of the stream in the middle Congo varies considerably. At the Aruwimi confluence the rate is from 300 to 350 ft. a minute; in the broader stretches lower down the current is not more than 200 ft. a minute. Through the Chenal the pace is greatly accelerated, and as it flows out of Stanley Pool the current is not less than 600 ft. a minute.
The Lower Congo.—The cataracts below Stanley Pool are caused by the river forcing its way through the mountains which run parallel to the western coast of the continent. The highlands (known as the Serro do Crystal) consist of two principal mountain zones with an intermediate zone of lower elevation. The passage of this intermediate zone is marked by a fairly navigable stretch of river extending from Manyanga to Isangila, a distance of 70 m., during which the only serious rapids are those of Chumbo and Itunzima, the latter in 13° 54′ E.; while above and below, rapids succeed each other at short intervals. Some eighteen main rapids or falls occur during the upper section (87 m.), in the course of which the level drops about 500 ft.; and about ten in the lower section (56 m.), during which the fall is about 300. The last rapid is a little above the port of Matadi, beyond which the river is navigable for large vessels to the sea, a distance of about 85 m. At Matadi the tall cliffs on either side sink away and the river widens out into an estuary with many mangrove-bordered creeks and forest-clad islands of a deltaic character. This estuary is traversed by a deep cañon, in which soundings of 900 ft. have been obtained. The mouth of the river is in 6° S. and 12° 20′ E. The cañon or gully is continued into the open sea for over 100 m., with depths as much as 4000 ft. below the general level of the sea floor. Just below Matadi, where the width of the river is about half a mile, depths of 276 and 360 ft. have been found, the current here running at from 4 to 8 knots, according to the season; while the difference in level between high and low water is 20-25 ft. The difference in level is not due to tidal action but is caused by the rainy or dry seasons, of which there are two each during the year. In the middle Congo May and November are the times of greatest flood; in the lower river the floods are somewhat later. At Stanley Pool the maximum rise of water is about 15 ft. The tides are felt as far as Boma, 49 m. from the mouth of the river, but the rise is there less than a foot; while at the mouth it is 6 ft. The cañon above mentioned is occupied by salt water, which is nearly motionless. Above it the fresh water runs with increasing velocity, but decreasing depth, so that just within the mouth of the river it is only a few feet deep.
The river at its mouth between Banana Point on the north and Sharks Point on the south is over 7 m. across. Banana Point (which grows no bananas) is the end of a long sandy peninsula, its highest spot not more than 6 ft. above high water; Sharks Point is bolder and shaped somewhat like a reaping-hook with the point turned inward, thus enfolding Diegos Bay. The current of the river is perceptible fully 30 m. out to sea, the brown waters of the Congo being distinguishable from the blue of the ocean.
Northern Tributary Rivers.—The various head-streams and affluents of the upper Congo have been already described. Below Stanley Pool numerous streams with courses of 100 or more miles drain the Crystal Mountains and join the Congo. They are unnavigable and comparatively unimportant. There remain to consider the affluents of the middle river. Of these the most important, the Ubangi on the north and the Kasai on the south, with their tributary streams, are noticed separately. In dealing with the other affluents of the Congo those entering the river on the right bank will be considered first.
The Lindi enters the Congo about 15 m. below Stanley Falls in 25° 4′ E. It rises in 1° N., 28° 30′ E., and flows W. in a tortuous course. Below the Lindi Falls in 1° 20′ N., 26° E. it is navigable, a distance of over 100 m. A mile or two above its confluence with the Congo it is joined by the Chopo, a more southerly and less important stream. The basins of these two rivers do not extend to the outer Congo watershed, but the next feeder, the great Aruwimi, rises, as the Ituri, in close proximity to Albert Nyanza, flowing generally from east to west. It is formed of many branches, including the Nepoko from the north, and its upper basin extends over 212° of latitude. The upper river, to about 27° E., is much broken by rapids, but apart from those of Yambuya in 24° 47′ the lower river is nearly free from obstructions. To Yambuya, the limit of navigation from the mouth of the Aruwimi, is a distance of over 90 m. The Aruwimi flows almost entirely through the great equatorial forest, which here seems to reach its maximum density. Its confluence with the Congo is in 1° 12′ N., 23° 38′ E. On its north bank just above the mouth is the station of Basoko. The next tributary, known as the Loika, Itimbiri or Lubi river, rises in about 26° E., and, flowing generally west, joins the Congo by two mouths, 22° 35′–46′ E. The Loika is navigable by steamers as far as the Lubi Falls, a distance of 150 m. The Mongala, the next great tributary to join the Congo, drains the country between the Loika to the east and the Ubangi to the west. It rises in about 3° N., 23° 20′ E., and flows in a somewhat similar curve (on a smaller scale) to that of the Ubangi. The Mongala is navigable for over 300 m., and gives access to a fertile rubber-producing region. The Mongala confluence is in 1° 53′ N., 19° 49′ E. Below the Ubangi confluence the Sanga, in 1° 12′ S., 16° 53′ E., joins the Congo. The Sanga rises in the north-west verge of the Congo
