The Ganaraska River is a river that bisects the (original) Town of Port Hope, Ontario, Canada. From its headwaters in the Oak Ridges Moraine, it flows into the north shore of Lake Ontario. It is located in the western end of Northumberland County, Ontario.
It was the source of a significant flood in 1980, that caused considerable damage to the Port Hope downtown area. This event is commemorated with the Float Your Fanny Down the Ganny race, held annually in April. Contestants can enter the 10 kilometres (6.2 mi) race in one of three categories: canoes, kayaks, and crazy crafts (homemade crafts).
There have been major conservation efforts in recent years.
It attracts anglers for salmon and trout. The Ganaraska River Fishway, a fish ladder, allows rainbow trout to travel up river to spawn.
The river's name is thought to originate from "Ganaraske", the name for the Cayuga village which was located at the current site of Port Hope.