I've been fishing the same stretch of the Brazos for many years. Here are some observations on where to find different species in the river. These are not hard and fast rules, but fairly general in nature, yet they hold true an awful lot of the time.
Bluegills are typically close to the bank, within about 6' of the edge. Bluegills love cover, so a good place to hunt for them is where there are tree branches in the water, or tree roots. I've seen a lot of bass tearing up the water right at the edge, going for the Bluegills, they are under constant attack, probably why they like cover so much. The further you get away from the bank the larger the Bluegill is likely to be. I've caught them in the middle of the deep zones, but not all that often unless the deep zone doesn't extend to the bank. Typically the further from the bank the larger the Bluegill will be.
Bass generally stay in a zone near the bank, starting at about six feet away from the edge. But not often in open water, they too love cover. My most productive bass spot is a submerged log that is at an angle to the bank, closest point is about 6' and furthest point is about 18'. The water depth there is about chest deep. I cast over the log and let the fly or bait settle a bit then strip it back. I've caught Bluegills close to the bank there and had Bass attack the BG as I brought it in over that Bass zone. It's rare for me to catch a LM bass in open deep water, although I pick up a few hybrids out there. Hybrids are relatively few where I fish and I haven't figured a pattern for them, they just seem to pop up in random places.
Carp and Buffalo tend to be out in the middle of the channel where the water has depth. They aren't interested in structure/cover. They roam around quite a bit and if you can't see them, as I generally can not, then it's a matter of the presentation intersecting with one of them moving around, a game of numbers and chance primarily. It can be a long time between bites, especially with carp, but it's always worth it when you get one of those big guys on a hook.
Catfish are also open water roamers. They'll often be in schools that circle around and around the deep area. They seem to be able to see further as I'll get more hook ups with them than I do with the Carp or Buffalo. A good bet for the Catfish, Carp and Buffalo is to dead drift your presentation from as far up stream to as far down stream as you can, using a strike indicator. Sometimes catfish will bite with aggression but mostly they bite softly. Even with an indicator you have to pay very close attention to detect a bite. Same with Carp and Buffalo, usually soft biters. Without the indicator it can be much more difficult. Mostly cats will pick up off the bottom so have the leader at a length that lets the presentation bounce along the bottom and let it bounce at the same speed as the current. Windy days where the water is rippling heavily are better for cats. The "wave action" imparts more up and down bounce via the indicator and floating line, I always get more bites on days like that.
Drum also tend to be in deeper water and not so interested in structure. The technique for drum is identical to that for cats. I catch more cats than drum so either they are more short sighted or just more finicky, but they put up great battles. Drum mostly hit aggressively, but not always.
Gar go everywhere but they mostly feed in the top three feet of the water. They also appear to be mostly sight feeders. One myth that I have discovered is not true is that you need a heavy or metal leader because of their teeth. I've caught a lot of gar on 8 lb monofilament leader without the leader being cut. If you look closely at a gar's teeth they are conical coming down to a needle point, and they overlap, they do not come together. So a leader "slides" between their teeth. You can get some fraying damage now and then, but it's surprisingly uncommon to get the line "cut". Gar are a bugger to get a hook into though because they have a tendency to pick up the presentation in the very tip point of their mouth, swim around with it for a while tasting it, before stopping to swallow it. Bony mouths will deflect even the sharpest hook, but sometimes you can get a hook in.
Fish usually don't like shallow water, they will make the occasional foray into it, typically when chasing another fish as it tries to escape that way. There are so many birds that eat fish that the fish are averse to being in shallow water. Calm water makes them go deeper, if the water is rippled sufficiently that birds can't see the fish, the fish will move up more and go into shallower areas, but still not "shallow" water.
The best "holes" in my experience are the ones that have long sections of shallow water up and down stream. The best part of the hole depends on the species, as described above. Each hole can be separated into species zones. So, when you get a good hole you can fish it several different ways and get several different species. If you find a hole surrounded by shallow water that has some structure near the bank in still fairly deep water and an open bottom in the middle, you've got yourself a gold mine.
Work it all over with different presentations and get ready to get your rod bent.