The Da River (Sông Đà), known in older sources as the Black River, is the principal waterway of northwestern Vietnam and the river that the Hoa Binh Dam harnesses. Below the dam it runs fast and clear through limestone gorges; above it, it has been converted into a 200-kilometre reservoir of remarkable beauty. Boat trips from near the dam let you travel through the drowned landscape of the old valley, past forested karst islands and stilted fishing villages.
A full-day circuit from the city covers the main sections of the reservoir — including the stretch around Thung Nai — and can include stops at cave systems and village wharfs. Shorter morning trips focus on the islands closest to town. Wooden longboats are the standard vessel; in calm conditions the ride is smooth and the views are continuously good. This is one of the most rewarding ways to spend a day in Hoa Binh province, and the fact that it remains far less crowded than Ha Long Bay is itself an argument for coming here.