Start here: Quality Farm Dugouts
When doing this for a farm, the usual calculation (Central Alberta Climate) is to plan for 2 years use, plus 2 years evaporation.
Dugouts should be as deep as practical to excavate to minimize surface area. That usually translates into 15 to 18 feet which is what most trackhoes can handle. Here a trackhoe costs you ...
You can seal a pond by putting pigs in it. They'll compact the bottom so much that it stops leaking. It may take a few years though. See http://www.makeitmissoula.com/2012/07/paul-wheaton-can-pigs-build-ponds/
Currently, if I understand correctly, the pool overflows into the brook with sufficient flow for fish to escape and to maintain water quality in the pond.
Raising the level of the pond will only affect that outflow during the time required to fill to the desired level. Once achieved the original outflow volume will need to be maintained. Whether this is ...
One method that has been used to aerate water in larger water treatment plants is to cascade the water.
Depending on the amount of land available, you might be able to install a stepped cascade, prior to the water entering the main pond - see page 43 in the link.
Cedar fence posts work about as well as pressure treated ones, 25 years or so. I think I would derate that somewhat due to constant circulation of water around them.
Estimate of 15 years.
Warmer temps will shorten life.
Fungi are aerobic: They are typically most active on a fence post in the 3" just below the top of the soil. Moisture + air. A ...