The Lower Thames Valley Conservation Authority has issued a flood warning for low-lying areas along the Thames River.
The river has already spilled over its banks in downtown Chatham with minor flooding.
The water level is expected to peak in Thamesville today with water rising downstream in Chatham tonight and Wednesday morning.
More than 50 millimetres of rain fell during the past few days and more rain is forecast today and tomorrow.
The LTVCA is operating the Sixth Street Dam in downtown Chatham to prevent flooding in south Chatham.
Flooding is expected later today in basements of the businesses along King Street that back onto the river.