Interesting catch ranman. Sauger are great eating just like walleyes. It's cool how the springs concentrate multiple species. I caught a sauger once in the middle of the summer too but it was on Guntersville. Most of those are caught in holes out on the main river channels in the dead of winter from what I've read and heard from folks who fish for them. Maybe next time you can get out there with your electronics and see something else interesting. Do you use bouys very much? If I were you I would start at the main Hiwassee channel and go back and forth accross the ditch from east to west and drop a bouy on either the up current or down current side of the ditch. I would drop a bouy every 50 yards or so as to map out the course of the ditch and then I'd go back and bring my boat right down the middle of the ditch initially throwing a deep running crankbait on either side of the ditch and sometimes in the middle of the ditch. I would continue this until I caught a fish, then I'd either throw out another bouy at that spot to make a note as to where I was and the trajectory of the cast I made to catch the fish. I'd work the spot until I stopped catching fish then I'd pick that bouy up and keep moving down the ditch. Any spots where I caught multiple fish or where I felt something really good on the bottom I'd mark on my GPS for future reference. I would expect, just like on most structures that there will be sweet spots that hold lots of fish on a consistent basis and areas that are less productive. It's hard from the surface to tell why the fish bunch up in certain places but when and if you would get out there and snorkel you could see why. A bunch of stumps together, a little ditch running from the main ditch, a spring putting out lots of cool clear water, something different that concentrates the fish. It's interesting, I read years ago and saw pictures of bass in a tank that had nothing in it other than say one piece of wood or even just a quarter sitting on the bottom of the tank and the fish consistently would school over whatever was "different" on the bottom of the tank. Finding those different spots quickly and efficiently is what I think makes the pros able to catch fish consistently and find bigger fish.
How wide did the ditch seem? If you couldn't cast from one side to the other you may have to check out one side and then the other. You have to be sneeky about all of this too as if there are good fishermen out there at the same time they will later come over with their electronics and check it out. I know that I do, not to say that I'm good but to say that I'm curious when someone else is too. If you can do this at night first and then just use your GPS to go along the channel that is the ultimate stealthy way to keep people guessing. I don't have a WAAS enabled GPS but I still can find things again if I mark them. Let me know if you find anything else interesting.