Old ponds that need cleaning and other wonderful stuff

Hi all, I just found this newsgroup and had a few questions. I guess I
should attempt to describe the ponds on my property: there's an upper and
lower pond, seperated by a concrete dam (which I'm interested in
stonefacing, but more on that some other time) and fed by a stream. The
whole thing outlets in a drain in the lower pond to a stream that heads out
to the Hudson River so the water keeps moving through, even in the winter.
A LOT of sediment and sludge has built up in both ponds over the years as
the guy who owned this place before me never took care of them. Does anyone
know the most effective way to clean out the years of built up goop? I'm
also interested in raising the bottom up closer to the top as they're both
fairly deep in the middle and I worry about kids in the neighborhood falling
in. Has anyone done that and have any advice? Ideally, I'd like to be able
to see round river rock and boulders through the surface from the deck that
overlooks all this. Finally, I'd ultimately like to build a bridge across
the upper pond. This would be a fairly long bridge, about 25 feet or so -
has anyone seen any plans for something like that or know where I could find
them?

Thanks for any help,
Mike O'Connor