Dee, you and T.P.H. are both right on the money; what's being described at Twin Lakes smacks suspiciously of our local autumnal lake turnover. Can't be certain without time series recordings of air and surface water temps over the last 3-4 months, and historical year-round temperatures for Twin Lakes. I am a BIT puzzled, as air temps in College Station haven't gone down much lately, and we're more climatologically variable than the coastal plain. To get true fall turnover the surface waters must cool appreciably... that takes time in large, deep lakes. Twin Lakes goes down to 50 feet, right?
There can also be wind-induced (polymictic) mixing stirring things up, but you'd need a great deal of recent wind activity to make a dent in a lake that deep. Maybe those hurricane fringes did something down there. Friggin' hurricanes...
Most Texas lakes don't get pronounced algal blooms this time of year, at least enough to make much dent in underwater visibility. Zooplankton populations start crashing in the summertime, which'll facilitate a slight upwards blip in the phytoplankters. This doesn't last long, as you get a secondary zooplankton pulse to compensate.
Twin Lakes is susceptible to massive nutrient inputs via runoff. If there was the usual late summer dry spell, and then some episodic rainfall events later, all those pent-up agricultural, municipal, and industrial nutrients are gonna do some serious hurt to T.L.. That'll cause the mother of all algal blooms, and deliver piles of fine sediments to boot.
Hmmm... I like this last scenario the best now. If you Twin Lakes groupies want to run a little survey for me, record the air and surface water temps. the next few times you go down there, and post 'em to this thread. There isn't an online record is there, like at Lake Travis?
