Freshwater clams can be eaten, but it is not advisable to eat them as they can be toxic. Many different species of clam inhabit Ontario's lakes and rivers and were eaten for centuries by Native Americans, especially when game was scarce.
The difference between then and now is that when fresh clams were eaten as a survival food, there was less pollution. Fresh clams are filter feeders and thus ingest a variety of substances, such as pollutants and toxins. Their tissues can accumulate these toxins. Aside from their tendency to be toxic, they are generally considered not to taste good.