tributary of the Vistula River, rising in western Ukraine on the slopes of the Volyn-Podolsk Upland in Lviv oblast (province).

The river has a length of 516 miles (830 km) and a drainage area of 28,367 square miles (73,470 square km). Excepting its extreme upper course, the Bug flows across a level plain through alternating forest and farming land. For about 125 miles (200 km) of its course the Bug forms the international frontier (since 1945) between Poland (west) and Ukraine and then Belarus (east). Near Brest it swings west into Poland to join the Narew River, which flows into the Vistula 23 miles (37 km) below Warsaw and is linked to Warsaw by a canal that avoids difficult currents near the Bug-Narew confluence. The Bug is linked by river and canal to the Neman and Dnieper river systems. It is navigable for 195 miles (314 km) above its confluence with the Narew.