Bosnia, Serbia and Montenegro have declared flood emergencies after heavy rain pushed the River Drina to its highest level in 100 years.
Hundreds of people have had to be evacuated from areas close to the river, which runs along the three states' borders.
In Foca, Bosnia, floodwater reportedly reached the second floor of buildings.
An eyewitness in nearby Gorazde compared the river to a lake.
"This area is famous for rafting but if this goes on, big ships will be able to dock in Gorazde, it's really unbelievable," Muris Razanica told the Associated Press news agency.
On the opposite bank, rescue workers in Serbia and Montenegro were evacuating hundreds of people from their homes and the Drina flooded farms and roads.
The entire area was hit by power outages and lacked drinking water.
Officials warned that more rivers, especially in southern Bosnia, were threatening to burst their banks.
In southern Croatia, the River Neretva hit its highest level in 50 years, flooding 700 houses in Metkovic.