Officials and residents of the town of Dire Dawa said on Tuesday that although frantic rescue efforts were continuing, most of the 300 people who are still missing are probably dead.
Officials said they had recovered 206 bodies in and around the flooded town but expected the toll to climb as 300 people were still unaccounted for.
At least 39 of the confirmed dead are children under the age of seven, according to police, who said many of the casualties were women, children and the elderly who were sleeping and unable to escape when the floods hit.
Aid workers said some 10,000 had been left homeless by the waters from the swollen Dechatu and Dire Dawa rivers which broke their banks, sweeping through the town and nearby villages.
In addition, the waters had brought down electricity and telephone lines, further complicating rescue efforts in an area of Ethiopia's lowlands that is prone to flooding during the June-to-September rainy season, they said.
Ethiopia, a nation of about 70 million people, has frequently been ravaged by natural calamities, notably drought and famine.