I hear you on the drive. I too live up close to the dam area and have driven to go fish other area's, only to find that the fish like to bite just about as well there as they do anywhere else. I do think there is a time and place for driving out of a persons way, but I think they are few and far between.
I like to hear someone elses take on marking fish and bait with the sonar. I am a newbee when it comes to sonars. Got my first one this year. But I have been noticing the same thing as you. I was seeing a LOT of bait around the 15' mark Monday night(correct that one post about it being Sunday night when I went out) and it almost seemed that I was seeing the thermocline around the 15' mark also. I could be wrong on that though, would love to hear what your take is on it.
I too have noticed a lot of "stuff" in the deeper water by the dam. I have just thought it was old tree's and trash down there, never thought it was bait down that far. I know a lot of times they can be found down there, but just figured it was later in the year. I guess reading and correctly inturpreting a sonar is something that comes with time?
Oh, and don't kick yourself too hard. I went out last night to test out my new downriggers and managed to reel in one little ole walleye(which got off right at the boat) but did not see much at all in the way of fish on the screen like I did the other day. Makes me wonder, but I just would say it was an off day all around.