i imported a heightmap and put a material on it and i cant make a river when using Shift + Left-Click (subtractive brush)
are you trying to subtract from a section of the heightmap with a full black value? if so thats the reason. from my experience the add and subtract brushes can only go between certain levels and the full black and white are the limits. try changing the color levels in your heightmap to be more gray than near the limits.