I lived on Bob Sandlin from 89~97 and only saw 2 the entire time (that includes many many hours spend on Cypress and Monticello). Occasionally we would see something we thought was a slide on the bank, and we found some egg shells that my father collected and took to the local vet who collects exotic animals and she said they were alligator eggs.
Of the two we did see, one was sunning on the bank in late April. Red clay with some tall grass on the back side of the bank in a narrow cove. We lived on the hill across from it and watched him for 10 min before we called parks and wildlife. They said thanks, and that they'd send some one out. Never heard anything back, might have been 4 foot at best.
The second time was crappie fishing along the damn between Cypress and Sandlin and one came swimming out of the lily pads on the south side. Came north for a bit, and then headed back south east.
A few weeks ago there was a 10' gator spotted on Sandlin as well.
http://www.texashuntingrepublic.com/ubbthreads.php/topics/160809/Re_Alligator_seen_at_Lake_Bob_If you want to talk about alligators in lakes, you need to look at Millwood Lake northeast of Texarkana. According to the guys at the 'Maud TX Baitshop' (so take that for it's worth) Arkansas game officials have been relocating alligator to that lake for years, specifically the north west section where the Little River flows into the lake.
In fact at the Wilton Landing Public Use area on Highway 59/71 there's a sign that reads "Caution: Alligators - Please do not feed the alligators"
http://g.co/maps/jxbc7On a side note, that public use area was remodeled over last winter and looks BEAUTIFUL. Bathrooms, ramp, dock, picnic area and a VERY slow moving river with lots of cypress tress and well.. alligators.
Not sure how long of a paddle it is to the White Cliffs campground on the lake but looks like fun.