Didier Mbula, provincial health minister for Equateur Province, said that 189 people have been rescued and that the search is still going on.
"According to the numbers we have so far at the provincial government, 30 corpses have been recovered, 167 are still missing," Mbula said.
Deadly boat accidents are frequent in Congolese waters, where vessels are frequently loaded well beyond their capacity.
The country has few tarred roads across its vast, forested territory, and travel by river is common.
The locally made boat that capsized near the town of Mbandaka was overloaded, carrying more than 300 people, and it was sailing at night, Mbula said.
"We think that the toll could climb because we have just removed the 30th corpse," he said.