I often wondered when I went hiking in the tropics why many areas had only small creeks, given the high rainfall. The answer seems to be that the soil and the trees absorb the water, which goes on a cycle of evaporation and precipatation without ever reaching a river.
New Zealand is the opposite. Rather than being incessant, the rain tends to come all at once, and the slopes are steep and non-porous. The result is spectacular waterfall displays and roaring rivers.
This video is from when I did the Rob Roy Glacier walk in the rain.
