The St. Clair Region Conservation Authority (SCRCA) has issued a flood watch after the watershed received between 50 and 100 millimetres of rain on Wednesday.
The conservation authority said the rainfall event caused water levels to be very high.
Localized flooding has been reported in low lying areas, such as floodplains and parks.
Watershed conditions will continue to be monitored as another weather system is possible Thursday evening and Friday morning.
Residents are advised to be careful near area waterways as riverbanks are expected to be slippery. The SCRCA said children and pets should be kept away from swift moving water.
The flood watch is expected to remain in effect until 4 p.m. Friday.
Wednesday's heavy rainfall also washed out several roadways in the Warwick area.