-- Last Updated: Dec-13-12 8:24 PM EST --
It is a shallow lake, and considering its size it's extremely shallow, with very gradual slopes, so I'd say you are correct. However, at a glance, I'd estimate the average depth to be more like 20 feet, rather than just six. I'd like to see if the surrounding topography matches the character of the lake bottom. I suspect that "getting rough very fast" is more due to the tremendous size (very long fetch) than the bottom, since shallow water tends to restrict wave size, except at locations where the waves first enter the shallower water, but maybe there are localized areas where the bottom shape makes things rougher. Since the OP's lake is extremely deep, I'm still expecting the overall topography to be irregular, even if not steep on average, but I guess we can't know unless we learn its identity.