Your picture looks like a perfect set up to get the leaves out with a pool skimmer net.
Organic decomposition is strongly dependent on water temperature. As the water temperature cools, the leaves that sink to the bottom will barely decay over the course of the winter. The first spring rain that brings some silt into your pond will begin to cover the leaves and remove (bury) them from the oxygenated water/bottom interface where most of the decomposition occurs.
I have observed sunk leaves held up in shallow water within BG beds, cattle hoof prints, etc. If your pond has relatively uniform slopes, then I suspect the leaves that make it into your pond work down to the deepest portions of your pond.
