Ottawa Mayor Jim Watson has declared a state of emergency over flooding along the Ottawa River and other waterways.

In the declaration, the mayor's office says the water levels pose "an ongoing threat" to property, health and safety of nearby residents.

Environment Canada has issued a special weather statement forecasting up to 35 millimetres of rain in the capital region by Saturday.

The mayor's office says the combination of already high water levels and further rainfall give rise to a situation that "constitutes a danger of major proportions that could result in serious harm."

Watson says about 400 soldiers with the Canadian Armed Forces are expected to assist with flooding efforts in the city.

He says the declaration will help city crews, volunteers and residents in affected areas, which are mainly in outlying suburbs.