Hello Ponders and fellow Country-Folk!

I bought a house two years ago with an backyard earth pond that looked
like it needed some work. I believe it was dugout in the 1950's as a
sauna pond for the prior Finnish inhabitants. It has a small creek as
an inlet and a culvert as an outlet. It is about 40 feet across and
is pretty much tear dropped in shape. The depth is 15 feet and I
believe it is spring fed because the water stays very cold even in the
warmest of northern Wisconsin's summer days. I planted some trout and
they have survived well. The pond is teaming with insect and frog
life. Unfortunately it is pretty mucky from years of service.

When I began work the pond had several tag alders (some submerged)
along the inlet bank and grass from the embankment was growing out
into the pond. I cut the tag alders and mowed as much of the grass as
possible, but the pond looked as though it was half empty, like a
partially eaten bowl of soup. I thought I could raise the water level
by building a small box out of treated lumber and driving it into the
bank in front of the culvert. This actually worked quite well, and
raised the water up four feet last summer. The problem is it is
impossible to stop 100% of the leaks, and it looks like water still is
finding a way under my clay covered box and into the outgoing culvert.
Also, the appearance is unatural, even with the rocks I've covered
the box with. I just hate the culvert. I think they used it so they
could drive a vehicle across the dam, but I have no need to do so.

Now, with the water level down and the pond frozen, I'm thinking of
trying to block off the culvert completely by filling my dam box with
concrete in early spring and trenching a natural earth spillway above
the embankment that runs about 15 feet. I would probably line the
spillway with red clay and then put down landscaping fabric topped
with pea gravel and fieldstones. The hope would be to make something
that looks like a natural cascading creek with a waterfall at the end
that flows off the far side of the embankment. I'm concerned that it
would still find a way to leak into the culvert however, so I'm
debating spending the money to get a backhoe to dig out the culvert
completely.

Questions:

1) Would my concrete idea work? Please, take a guess. Any other
ideas? Should I just spend the money on the backhoe and do it right?

2) Are there any tips for good erosion-proof spillway designs? How
about getting the look I'm after, any thoughts on that?

3) How do I fight the thick underwater grass growing out into my pond?
I've raked, shoveled and pulled up some of it, but it's really heavy
because of the huge root pile that are soaked with sludge water.
Should I just shut up and keep working or is there a smarter way?

4) Is there a cheap way to remove some of the sludge and muck from my
pond without draining it? Would adding sand or gravel help? I just
want swimmers to be able to walk out into the pond without feeling
like swamp thing.

5) Any landscaping ideas to minimize the empty soup-bowl appearance?
Small shrubs that look good on pond edges? Perennial plants/flowers?

Sorry if my questions are vague, I'm new to this exciting hobby and
I'm really just searching for ideas and tips. I'll be watching this
thread and will provide follow ups to any questions and/or comments.
Thank you for taking the time to read my long-winded post.