When the system does the incise flow operation, it computes the result in a temporary surface and the blends that effect back into the main surface. Effect Blend specifies how strongly that blend occurs (0 means no effect, 1 means full effect, 2 means 2 times the effect, 3 means three times the effect, and so on). Setting an effect larger than 1 will carve channels below sea level, a decidedly unrealistic effect. The Amount field has a similar effect, but it won't let the incise operation go below sea level (where the altitude is 0). Setting the Effect Blend to 0.8 lets 80% of the erosion result through.

Exponent controls how much of the flow map affects the final result. The flow map is a function of the number of pixels upstream of the current pixel. It's an ugly exponential result. The exponent allows you to control the relative intensity of the mouth of the river to the highest upstream element.