Two flood warnings are in place following heavy rainfall overnight.
The Government flood warning agency has issued an amber warning for the Upper River Colne and Radlett Brook running through London Colney, Radlett and Borehamwood.
Another amber warning has also been issued for the River Gade, running through Kings Langley and Hunton Bridge.
Throughout Wednesday night, heavy rainfall has caused the River Colne and River Gade to rise, the flood warning agency said.
The main area of concern along the River Colne is the Colney Heath area where river levels have risen the most.
The Government flooding agency warned: "There is likely to be flooding to low lying land and roads close to the river today on Thursday 28 January.
“Flooding from surface water, where water gathers and flows off road surfaces, could also occur. We expect the river to remain high on Thursday.
“Some further rainfall is forecast overnight tonight. We are constantly monitoring river levels and have staff on standby to clear any blockages."
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