It turned out to be a large female eastern painted turtle (Chrysemys picta), with her back end encrusted in mud - I surmised from laying eggs. We could not find evidence of a nesting site nearby, but I was excited at the prospect of baby turtles in the pond soon.
Later on, conducting my own walk around the pond, I paused to look at insects swimming near the water's edge. Just a few inches in front of me, through the leaves popped up a tiny head. Apparently some baby painted turtles have already hatched!
Raaaaawwwwrrrr I'm a ferocious turtle
I have raised hatchling painted turtles from the backyard before, they are a LOT of work (put them back after a year). I know the laws in the US against selling turtles under 4" are for salmonella fears, but it's a good idea for the safety of the turtles as well. So many people think painted turtles and red eared sliders are adorable, but don't realize they can live 20+ years and get to be the size of a dinner plate! When I worked at a pet store, we got sliders of various sizes dumped on us every week.
Here is a clip of this little guy making his way back into the pond (I think it's a male due to the very long tail).