I should have clarified two things:

(1) The filter is gravity-fed, with the pump in the final chamber.
That means that with the pump off, the water level in the filter will
be the same as the water level in the pond, so I can't backwash without
using a pump.

(2) I'm building the pond with both a bottom drain and a skimmer, since
it will have a relatively heavy load of leaves from surrounding trees.
I don't want to mess with nets to keep the leaves out.

Chris