The leaves should be netted out before you start your pump in spring.
Window screen is too fine if there's a lot of debris in your pond. What
works for us is to use two of those black plant baskets wired rim to rim
like a big clam - with the pump inside. Cut a hole in the bottom of one for
the hose and electrical cord to emerge. These will not clog for a long,
long time unless, as I said your pond is full of debris. If your pond had a
load of partly rotted plant debris and leaves you may want to just go ahead
and drain it down, clean out the mass of trash and start again. Other
people may suggest other ways to deal with it.

I live in Florida so this pump runs year-round. I actually haven't
had that many problems this winter and I'm very thankful for that.
February was extremely cold down here and fixing these pump problems
always requires getting into the water. I was thankful I only had to
do it once or twice that month.

The problem got worse recently and I think it was influenced by the
Live Oak in neighbor's yard. These trees are covered year-round with
small oval leaves about an inch in length. They kept sneaking through
my latest pre-filter screening and jammed up my impeller. I think
these trees shed a lot on the spring, but this one looks to have lots
of yellow in it. I wonder if it's dying.

Other debris isn't that bad because I have moved away from putting
every pond plant known to man in the pond and have instead limited
myself to just a few. Hornwort used to be a ******! But I have a
poweful pump and anything that's in there will eventually get pulled
towards that pump.