Right now the river is covered with ice. It does tend to create an ammonia problem as it moves downstream. The city of Brantford takes its water from the Grand River, and when it's entirely covered with ice it tends to cause an ammonia problem that requires changes in how they treat their drinking water.
I'm not a drinking water specialist, but I know that the ice cover does actually not allow the ammonia to escape and volatilize, or go off into the atmosphere. It does tend to be a bit of a problem in the winter.