Climate in the 1000 Islands is moderated by the Great Lakes waters and the fact that the valley channels the moderating influences along the river. Mean temperatures for January through mid-February are around -8° Celsius, with the greatest amount of precipitation, much of it in the form of snow, falling during these months. The river begins to ice in early December, and freezes over, except where swift water prevents it, by the end of the month.
The ice generally melts in late March/early April and by May, the daily mean air and water temperatures register 12° or 13° Celsius. The island land masses warm more slowly due to the influence of the river and remain more moderate throughout the summer's heat, when average temperatures are 21° Celsius. Throughout July and August the mean maximum air temperatures recorded during the day in the 1000 Islands are cooler than the inland areas, while the mean minimum air temperatures recorded at night are warmer than those inland from the river.
The average frost-free period on the islands is 160 days per year.