I know you don't want to do it but taking apart the waterfall is about your
only option. You can rebuild the waterfall and fill in the den areas with foam
or rocks or some sort of 'stuffing'. You can try harrassing it, making his den
area uncomfortable and not safe, though it seems a bit 'mean' and will take
time and patience and a certain amount of bravery that not all of us have when
faced with a
!!snake!!. You've provided a food source and unless you remove that food or its
hidey hole it will probably come back or be replaced by another hungry snake.

kathy :-)
zone 7, SE WA state