1. In a large pot, mix the rhubarb, berries, and the water or apple juice. Cook, covered, stirring frequently over moderate heat, until the rhubarb is cooked through and thoroughly tender. It should take about 15 minutes.
Put a small plate in the freezer.
2. Add the sugar, lemon juice, and salt, and cook, uncovered, skimming off and discarding any foam that rises to the surface, until the jam is thick and passes the wrinkle test.
To do so, place a small spoonful of the jam on the frozen plate. Return it to the freezer and check it a few minutes later; if the jam wrinkles when nudged, it’s done. You can also use a candy thermometer; jam jells at approximately 220ºF (104ºC).
3. Stir in the kirsch, if using, then ladle the jam into clean jars, cover, and store in the refrigerator.