This may seem contradictory, but you want to leave a little bit of debris in the pond when preparing it for winter.
Some water gardeners net out the fish, completely drain the water and scrub out the pond, refilling it with fresh water. Frogs, tadpoles, snails and microscopic pond life need to burrow down into mud and leaves to survive the winter.
Fish also hibernate on the bottom, settling in around a bed of leaves and mud. I remove about 90% of the leaves and silt that have accumulated over the summer.
Leave the rest as "bedding material." You’ll be amazed at the diversity of pond life that emerges in spring. Keep in mind that tree leaves will continually blow into the pond as long as the water isn’t frozen. I recommend covering the pond with bird netting. This black plastic netting is almost invisible and prevents tree leaves and debris from getting into the pond.