Robert, welcome to Indy. I'm from the NE side of town too and have fished Geist off and on through the years. There's very little public access so a boat is just about a necessity. There's A LOT of channel cats in there, some good sized ones too. Average size though is about 5-8lbs but I've seen some 18lb+ come out of there. There's also flatheads in there, but they are few and far between. I've only caught one and that was at the park at the north end of the lake by the bridge (I think its Florida Street?), was about 13lbs, caught it on a live sunfish. Wish I was more familiar with the lake from a boat standpoint, but I've really only fished from the park and from the causeways.
There is some good fishing on about the first 2miles stretch of Fall Creek below the Geist Damn, I've pulled several 10lb+ channels out of that section and saw a guy catch a 19lb flathead on a rubber worm there once. If your targeting flatheads though, White River is the place to go.