Koshkenong. I've been by it on I-90 but never actually to it. Any idea why it was so green? (There's a lot of dairy around there -- I might be able to guess.)
From what little I know/remember of that area, Lake Koshkenog was formely farmland. The native americans farmed it previously and then I believe white settlers also farmed it. Somehow we altered the Rock River (most likely dammed upstream) and the farmland flooded. I remember hearing the Lake has only a maximum depth of 7 feet, so that would support that theory-- pretty shallow for a lake that size. So I would assume the lake bed soil was already pretty nutrient rich to begin with and I'm sure the nearby dairy industry doesn't help.