Lake Powell's kind of a special case, because depending on the precipitation for the last few years the water level can vary 100 feet or more. So the position of the shoreline and rocks near the surface that you need to worry about varies tremendously.
Pretty simple paper charts such as the Fish-n-Maps (2 cover the whole lake) are handy to help you know roughly where you are on the lake, but you have to do hazard avoidance by fishfinder and eyeball. Charts make it fairly easy to see where the main river channel is, and it's always way deep there, but sometimes there's a chunk of shallow rock right at the edge of that main channel, out a good distance from the shoreline where you might not expect it. Depth can go from 200 feet to 2 feet in no time flat.
Fortunately, if there's sunlight, rock shallow enough to hurt you will generally show in a light tan or almost white color - except in the upper reaches of the side canyons, where the water is murky enough you can't see through it. There you have to go slow and watch your fishfinder like a hawk.
Most of the time (not always, however) when you get a ways up a side canyon the bottom will become very flat, getting shallower very slowly as you go further up. This is silt deposit, and pretty soft, but sometimes there's a rock sticking up. The silt bottom goes up and down each year too - but not necessarily in concert with the water level. The position of the silt bottom depends on how much has been washed down by rainstorms, or how much has been scoured out by really major water flow.
