The police found the unexploded ordnance on Wednesday evening in the river, near the steel-suspension Rodenkirchen bridge in the western German city, reports RP Online.
According to initial estimates, the explosive device weighs 50 to 125 kg and is an American air bomb from the Second World War, Cologne city authorities said.
A bomb disposal squad was on site Thursday checking whether there was a detonator on the device.
Cologne authorities issued a statement saying the bomb had been damaged so it could not be defused, according to bomb experts. A controlled explosion will now be planned.
The statement said: “Preparations for a controlled explosion are currently underway. Further details about the bomb are not yet known.”
— Stadt Köln (@Koeln) October 18, 2018
An update from Cologne authorities on Twitter. 'The bomb in the Cologne-Poll area has to be blown up'
When this happens shipping traffic will be banned from the river. It's also possible that the residential areas and businesses in the surrounding area will have to be evacuated so that the bomb can be defused safely.
Unexploded bombs are scattered throughout Germany more than 70 years after the end of World War II.
Just over a week ago in Potsdam, thousands of people were removed from their homes while bomb disposal teams exploded and defused an American World War II bomb.
The Rhine currently has its lowest ever amount of water. In Emmerich, the water level has reached a record low of 22 centimeters – six centimeters below the lowest known water level on October 1st, 2003.
In Cologne, authorities expects the Rhine level to fall below the previous low water record of 0.81 meters from 2003 this week. For Thursday, Cologne's municipal sewerage companies expect a level of 0.80 metres.
The low tide is already causing massive problems to freight traffic on the river, which is the second-longest river in Central and Western Europe – after the Danube – at about 1,230 km
Freight firms have raised concerns about losses because of the disruption the low water levels have caused. There are also problems with passenger transport. The Cologne-Düsseldorf shipping company has discontinued its scheduled services on the Rhine.
The energy company RWE is still unable to supply the coal-fired power plant in Hamm with full coal freighters, RP Online said.
A company spokesman said that due to the low water level, the ships could only transport just over two thirds of cargo to the power plant. In the summer the plant had been taken off the grid for a few days because of problems with the coal supply.
According to German weather reports, the Cologne area is expected to remain dry but there is a chance of some rain showers on Thursday.
As The Local reported, Germany has been experiencing mild weather in a so-called 'Golden October', but after a dry and hot summer the drought conditions have exacerbated problems for many people, including farmers.