a good amount of "weeds" errr water plants and mosses. will grow from within 4 feet depth. 4 feet and deeper the vegetation normally stops. if you have "shallow sides" then any sort of swimming, or fishing, or watching fish is going to be problematic.
if your soil can handle steeper sides, then go with steeper sides. so you can reduce amount of water weeds that will grow around the shore line.
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compound problems. with a clay bottom or mud bottom pond, or mud line pond. (take your pick in how ya describe it)
water will fluctuate throughout the year. and also along the years between drought years, to huge amounts of rain years.
for the 2 lake / ponds i have. these can by any were from right at full going over the "over flow pipe" and if overflow pipe gets clogged, then another 6 to 12 inches for water to go over spillway.
but during drought years or mid to late summer, water may go down 5 plus feet easily.
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for wintering fish in pond, ya looking at most likely wanting 6 feet depth or deeper. (6 feet dept = actual water depth, and accounting for fluctuation of water level in pond/lake). 6 feet is ball park. but getting a couple feet below your "frost line", might be better way to judge things. say if frost line is 3 feet depth for your area, then dig down 5 feet. and then from that point you would want 6 feet of water depth above the bottom of the pond. for fish to overwinter much better in the pond.
getting past the frost depth line for your area, gets the pond into more better 55 F degree zone of dirt. and helps keeps the fish from turning into popsicles. further you go down below the frost line, the better off the fish will over winter in regards to just temperature, there are many other factors.