On leaving Kumai, boats enter the mouth of small Sungai Sekonyer, where an avenue of trunkless nipa palms gradually cedes to patchy forest at Tanjung Harapan ranger post. An information hut here has a good rundown on the park's ecology; a few orang-utans move in for feeding at about 3pm; and there's a muddy five-kilometre circuit track into surrounding thickets.

The jungle closes in after Tanjung Harapan - lining the banks are giant lilies, pandanus and gardenia bushes with yellow flowers. Behind them grow dense stands of tall, thin trees with buttressed roots, providing perches for vines, orchids and ferns. The next stop is Pondok Tanggui , a good place to moor for the night and watch families of proboscis monkeys. A two-kilometre track leads past the ranger's office to an orang-utan feeding area (milk and bananas at 8am and 4pm).

Past Pondok Tanggui, you veer east off the Sekonyer up Leakey Creek. The water immediately turns clear and deep, and trees eventually give way to grassy marshland, home of triangular-headed false ghavials. At Camp Leakey , the local orang-utan population includes two mature males whose overgrown cheek pouches and sheer size - one weighs 100kg - are most imposing. There's another feeding area here, and the camp's rangers can take you on a short hike into the forest proper.

Back on the Sekonyer, a final hour brings you to Natai Lengkuas , a research post investigating proboscis monkeys. In fact, this is the last animal you're likely to see here, as trails take you well away from the riverside study areas, but following the orange tags of the ninety-minute "Habitat Walk" through the forest you may see mahogany trees, lizards, bears, wild pigs, scorpions and roosting colonies of short-nosed fruit bats. There are no rangers to act as guides, so read the information notice above the dock and follow all of its instructions.