Following floods upriver, the Oder River and its tributary the Neiße in the state of Brandenburg began swelling on Monday. By 8 am on Wednesday, the high water alert centre in Frankfurt an der Oder reported that river levels had increased some 10 hours before expected.
The highest alert level could be breached by Thursday, the centre said.
Safety protocol for the current level dictates that dikes, watercourses and dams must be observed constantly for damage.
Ratzdorf, a town on the Oder river near the Polish border is in a particularly critical position, Brandenburg's Environment Ministry officials said. There water levels increased by 60 centimetres in just 24 hours by early Wednesday, and could reach the highest warning level on Thursday morning, when flooding will become a serious danger.
But any flooding will not be as severe as the devastating high waters of 1997, because they will be short-lived, the ministry predicted.
Many regions in neighbouring Poland have been evacuated following more than a dozen deaths due to flooding. The head of the Brandenburg environmental authority Matthias Freude told broadcaster ARD on Tuesday that the coming days will “with certainty not be easy” in the state.
Over the weekend in Poland waters reportedly rose by a metre in just 24 hours as streets and homes in several cities flooded. On Saturday a provisional dam broke in Wrocław.
Meanwhile dikes along the Vistula River, which goes through the Polish capital Warsaw, have burst. Police in the country reported on Tuesday that at least 14 people have been killed by the floods.