After the collision of two barges on the Weser near Balge in the Lower Saxony district of Nienburg, one of the ships sank.
One goes from an "immense damage", said a spokesman for the water police.
How the accident happened on Sunday afternoon was still unclear.
According to initial findings, no one was injured.
However, the crew of the sunk ship had to be rescued by the fire brigade and brought ashore.
The ship, which is around 67 meters long and eight meters wide, had several tons of maize on board.
As a result of the accident, operating materials leaked from the ship, which should be stopped with the help of oil barriers on the water.
Rescue workers from the technical relief organization secured the ship so that it could not drift away.
According to the water police, the depth of the river in the area is between three and four meters.
The second ship was also damaged in the accident.
The Weser was closed to shipping in the area.
How long the lockdown will last was still unclear.
According to the spokesman for the water police, more detailed investigations of the wreck will not begin until Monday.
Divers should then clarify, for example, how the cargo can be removed from the ship and how the ship can be recovered.