Si Phan Don — "four thousand islands" in Lao — is where the Mekong, approaching the Cambodian border, spreads into a vast delta of islands, channels, and rapids. At the peak of the dry season the Mekong is 14 kilometres wide here. Most travellers base themselves on Don Det or Don Khone, the main islands, which are connected to each other by a French colonial-era railway bridge (the locomotive is still there, rusting).
The area's headline natural features are the Khon Phapheng Falls — the widest waterfall in Southeast Asia by volume — and the rare Irrawaddy dolphins that survive in a small section of the river near Don Khone. Dolphin sightings are not guaranteed but still possible. The pace of life on the islands is extremely slow, the sunsets over the Mekong are long and golden, and the hammock-and-river lifestyle has kept travellers here far longer than they planned. From Pakse, buses and minivans reach the islands in about three hours.