Average annual precipitation in the upper stretches of the Ottawa River totals 819 millimetres and is comprised of 625 millimetres of rain and 195 centimetres of snow. There are on average each year 170 frost-free days, 156 days of precipitation, and 144 days with snow cover.

In the southern stretches of the river, the climate is generally milder and more humid. Average annual precipitation totals 965 millimetres, and is comprised of 187 centimetres of snow and 779 millimetres of rain.

Rideau WatershedOn average 926mm of precipitation falls in the Rideau River Watershed each year. 60% of this precipitation is used by plants and trees or goes back to the atmosphere. The remaining 40% of the precipitation either flows into the rivers and streams or infiltrates into the ground and may eventually reach the groundwater table and recharge the aquifers or moves underground downstream to eventually discharge back to streams and rivers.