Water levels at the town of Kaub, a bottleneck point in west Germany, have fallen beneath one metre.
According to The Telegraph, forecasts suggest levels could reach 80cm, which would trigger an "extreme" alert and mean some vessels can no longer pass through.
Warnings over the state of the river came early this year, with water levels at Kaub previously falling below one metre in March following a dry winter. The river runs for 766 miles and is used by around 7,000 vessels.