Not sure about the Bismarck area, but Audubon, Darling, Antelope are all in the 5-7" range.
I heard Alice and Irving (probably Dry as well) are in the 8"+ range.
No such thing as "Safe Ice". However, most smaller lakes seem to have between 4 and 8 inches. Be Safe and check the whole darn way out and back. Unexpected ice baths......SUCK!!!!!!