1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 043 
The most interesting swamps of the Belgian Congo are those of the 
valley of the upper Lualaba, between the confluence of the Lubudi 
and that of the Luvua. The Lualaba flows herein a wide, rift-like depres- 
sion, known as the ‘Graben of Upemba,” which is at present filled with 
deep, alluvial deposits. Apparently the whole depression was covered by 
a lake at some rather recent period. Nowadays the valley, some 40 
kilometers wide and some 200 kilometers long, forms a low, swampy 
plain almost completely under water toward the end of the rainy season. 
The Lualaba winds its way through this plain between very low banks, 
which either form narrow levees or in many places are hardly defined. 
On either side, beyond the levees, there are extensive papyrus swamps 
surrounding numerous lagoons of open water of all sizes and shapes, the 
largest being Lakes Upemba and Kabamba. These lagoons are usually 
connected with the Lualaba by means of narrow channels and are partly 
obstructed by floating islands of papyrus. All the lakes are very shallow, 
so that some of them can be hardly crossed with a canoe, and they have a 
bottom of oozy mud many feet thick. Lake Kisale differs from the 
others not only in its larger size, but also because it is traversed by the 
main current of the Lualaba, in one or more channels, which, however, are 
frequently choked by floating water plants. Near Kisale, the Lualaba 
receives the Lufira and Lovoi Rivers and it has been shown that when 
these two tributaries swell, the volume of water they pour into Lake 
Kisale is such that the Lualaba upstream is forced back through the side 
channels into the many lateral lakes and swamps. At the height of the 
rainy season the surrounding alluvial plain itself is for the most part 
inundated. When the level of Kisale begins to drop at the beginning of the 
dry season, the water gradually recedes and during the succeeding months 
the smaller lakes flow into the Lualaba, thus regulating the water supply 
of that river.1. The whole alluvial area of shallow lakes near Kisale is 
the counterpart on a smaller scale of the ‘“‘sudd’’ swamps of the upper 
Nile, near the junction of the Bahr-el-Ghazal and the Bahr-el-Jebel. 
Ecological conditions are much the same in both regions, which accounts 
for many points of similarity in the fauna and flora. 
The shallow, muddy lakes and papyrus swamps offer an excellent 
habitat for certain fresh-water snails which sometimes occur in consider- 
able numbers, although they represent but few species. The follow- 
ing were obtained by the junior author in Lake Kisale: Plancrbis 
gibbonst, Segmentina angusta, Bulinus forskalit, B. lamellosus, Physopsis 
iSee Mauritzen, 1912. ‘Le probléme du lac Kisalé. Rectification du cours du Lualaba dans les 
marais du lac Kisalé.’ Revue Congolaise, II, pp. 388-391, Pl. xxxvi (map). 
