The Ulanga Valley is an intact natural wetland ecosystem comprising myriad rivers, which make up the largest seasonally freshwater lowland floodplain in East Africa.
From south the Ruhudji River winds eastward, losing height quite rapidly, to the head of the great floodplain of the Ulanga Valley.
Other affluents draining the mountains on opposing sides of the valley join the network so that in the central part there are ten major channels flowing roughly in parallel.
East of Ifakara the Ulanga flows through a delta of oxbow lakes and is joined on its left bank by the Msolwa River.
This stream comes from the high escarpment of the Udzungwas and traverses the northern part of the floodplain, skirting another zone of permanent swampland to the west.
From the point of confluence the Ulanga River swings sharply southeast and leaves the floodplain (and the Ramsar Site) on the border of the Selous Game Reserve.
The Tanganyikan coast proved relatively easy, but conquest of the inland areas of the colony—right up to the Belgian Congo—was more difficult as large parts were still unexplored.
Forester's novel The African Queen (1935) and its subsequent film adaptation (1951) was a steam-powered launch, owned by a Belgian mining corporation, that plied the upper reaches of the Ulanga River.