Merging Raster layers

Discussion created by digger8169 on Dec 19, 2010
I have a series of raster data that I need to combine and get as sum of the values. One is an overlay of slope and another is the river data. The slope has a series of positive values and the river has a series of negative values. The issue I am having is that when I use the combine function, or the raster calculator it adds the values together fine, but the end layer is only of the parts that were added. In otherwords instead of the slope raster with negative values where the water is, I end up with only the water area (with its new values when combined with a slope). Does anyone know how I can combine the two layers, where if there is nothing to combine it maintains its original value while if it is combines where there is the river it has the new value.