SOROTI, Uganda — Fish swam alongside the dugout canoes residents were using to flee their flooded homes, riding the water gushing through the streets of this town in eastern Uganda.

Across Africa, torrential downpours and flash floods have submerged whole towns and washed away bridges, farms and schools. More than a million people have been affected by the rains since the summer, according to the United Nations. At least 200 people have been killed, and hundreds of thousands displaced in 17 countries.

In Uganda, one of the hardest-hit, humanitarian workers were trying to reach villages that have been cut off by water amid warnings of food crises and a rising risk of disease outbreaks.