In a large saucepan, layer the rhubarb and 1 cup of sugar. Pour in 1/2 cup of the water, and bring to a boil. Simmer over low heat, stirring occasionally, for 30 to 45 minutes, until rhubarb is tender. Smash the rhubarb into smaller strands if you like.

Stir in the mixed berries. Mix cornstarch with a small amount of the water, and pour the rest of the water in the pan. More or less water can be used to achieve your desired thickness. Bring to a boil, then remove from the heat. Wait for it to stop bubbling, then stir in the cornstarch slowly. Return to a simmer until the cornstarch is no longer cloudy. Remove from heat, cool, and serve. Sprinkle additional sugar over the top to keep a skin from forming.