The erosive cutting power of water is very approximately equal to the amount of flow multiplied by a power of the slope angle divided by the hardness of the underlying material. Steep mountains get eroded fairly quickly because the slope is high, even though they are made of fairly hard stuff. On flat lands, the slope is close to 0, meaning that the river won't erode downward. If your equations keep track of the sediment being cut and carried, then terrain will build up where the land flattens out and can't carry much stuff.