The Hebrus, together with its tributaries which flow into it from the north, east and west, drains almost the whole of Thrace.
The Kaskaskia, in the S., notable for its variations in volume, and the Rock, in the N., are the other important rivers emptying into the Mississippi; the Embarrass and Little Wabash, the Saline and Cache in the E., are the important tributaries of the Wabash and Ohio rivers.
The Nera, which rises in the lofty group of the Monte della Sibilla, is a considerable stream, and brings with it the waters of the Velino (with its tributaries the Turano and the Salto), which joins it a few miles below its celebrated waterfall at Terni.
Its more important tributaries are the Enz, Eschach and Glatt (left), and the Fils, Rems, Kocher and Jagst (right).
The basin thus presents interesting problems. The existence of wide valleys where the small upper waters of the Cherwell, Evenlode and Coln now flow, the occurrence of waterborne deposits in their beds from the northwest of England and from Wales, and the fact that the Thames, like its lower southern tributaries which pierce the North Downs, has been able to maintain a deep valley through the chalk elevation at Goring, are considered to point to the former existence of a much larger river, in the system of which were included the upper waters of the present Severn, Dee and other rivers of the west.
The Oder and some of the tributaries of the Elbe abound in crayfish, and in the stagnant lakes of East Prussia leeches are bred.
Former tributaries have given place to others developed with reference to the distribution of more or less easily eroded strata, the present longitudinal valleys being determined by the out-crop of soft shales or soluble limestones, and the parallel ridges upheld by hard sandstones or schists.
None of the tributaries of the La Plata system thus far mentioned is navigable except the lower Pilcomayo and Bermejo for a few miles.
The central and southern districts are drained by the Derwent from Lake St Clair - its tributaries being the Nive, Dee, Clyde, Ouse and Jordan.
The southern districts and the Amazon and its tributaries were often raided by slave-hunting expeditions, and their Indian populations were either decimated, or driven farther into the inaccessible forests.