Combine 1 3/4 cups of water, the sugar and the Saskatoon berries and boil for 15-20 minutes, until the juice is very dark and rich with flavor. Some of the berries will start to break down and this is a good thing. Saskatoon's aren't the most sauce friendly berries, they can be seedy and tough.
Stir the cornstarch into your remaining water add the lemon and then cook 5 minutes more or until the Saskatoon's are the texture you want.
You can can these in a water bath, freeze the sauce in the freezer for up to 6 months or it will keep in the fridge for a few weeks.