The environment agency has issued a flood alert for The Upper River Colne and Radlett Brook at Radlett, Colney Heath and London Colney.
A statement from the environment agency said: "The River Colne has responded to rain that has fallen overnight and this morning.
"This is likely to result in flooding to lower lying land, road and gardens next to the river.
"Surface water flooding can also be expected in some locations.
"Showery rain is expected later today, with some possibly wintry showers tomorrow. We will continue to monitor the situation."
The alert was issued at 14.28 this afternoon.