The island is probably the best watered in the world. On every side are numerous rivers, mainly rising in the central highlands, at first dancing down the mountain ledges in cascades, then, forming dangerous rapids, enter the plain, and there swelled by affluents and widening out advance with no strong current to the sea. Owing to the width of the river-mouths, and to the configuration of the coast, some of them, as the Batang Lupar, the Sadong, and Saribas, have tidal bores, as is the case with our River Severn, that run up as many as seventy miles into the interior, and most have deposited troublesome bars at their mouths, and have embouchures clogged by shoals. To the slight fall is largely due the remarkable way in which several of these rivers descend into the ocean through plural mouths, thus forming a network of lateral waterways, called Loba and Trusan, whereby they mix and mingle with other rivers, and, very much like the Rhine after entering Holland, lose their identity and are frittered away in many channels. The Rejang, for instance, finds issue through five mouths, and the land between the Rejang and Igan entrances, which meet at Sibu, the apex of the delta, is a vast unbroken swamp, 1200 square miles in area. The same phenomenon is noticed in the Sarawak river, and in the Limbang to a smaller degree.
The rainfall in Borneo is so great, the rainy season lasting from October to April,[8] that the rivers are very numerous and copious, rolling down large volumes of water. Severe droughts are, however, not uncommon during the fine season of the S.W. monsoon.
Between Kuching and Bruni are the Sadong, Batang Lupar, Saribas, Kalaka, Rejang, Bintulu, and the Baram rivers, all available as waterways for trade with the interior. For fifteen miles only from its mouth is the Batang Lupar navigable by steamers, above that, though a fine broad river, it is obstructed by dangerous shoals. The Rejang is navigable by steamers for 170 miles, nearly as far as the first rapids. This noble river descends many stages by as many plunges from terraces. Between the rapids the river is deep, sluggish and broad for many miles. Boats that can be hauled up past the rapids can ascend a distance of 650 miles from the mouth. The Baram river is navigable by steamers for some twenty miles above Claude Town, that is, eighty miles from the mouth, but owing to the exposed position of the bar and to the heavy seas breaking over it, and also to the silting up of the mouth during the N.E. monsoon, only very small craft can then enter, but during the S.W. monsoon it can be entered by steamers of light draught.