It was my first visit at Eufaula after recently moving to this area. I think that it is seasonal and predictable from searches on the web, and past observations. The area I moved from, the upper Black Warrior River, NW of Birmingham, had depth, but also had current at times. With current the crappie moved into the creek mouth. When generation stopped at the dam, they disappeared. The water in the mouth of the creek was not as deep, but current was minimal compared to the river. With more cooling I predict that they will move closer to the river channel as the deeper water will likely be warmer there. Bream fishing on the Mobile River Delta where I grew up revealed the same behavior. Hot and cold months the fish were deep, to a point, but 30 feet was not unusual if there was that much water, and shallow spring and fall. In some smaller creeks we fished out of the other side of the boat as the fish moved to the center or shore depending on the season.
Granted, I have not been much over 10 feet under the surface, and that swimming during the summer, but it was noticeably cooler than the quite warm surface water, which was more like bath water during the hot summer months. I am not sure how much difference there is in temp at that depth, and it seems that the shad are close by. Correlation, causation, or both? A guess is that the fish, crappie and shad, are searching for the warmest temp and most oxygen concentrated water for metabolism and energy management.
Having no experience with long lining I can offer no comment