Here are two pics roughly the same spot. My buddy Chuck up on the rock. Notice the large rock in the background and the flat water to the right.
My son Andrew is standing behind where Chuck is, rocks in background, flat water on the right.
Here is the rock (labeled)to the left that Chuck is standing on and Andrew was in the grassy area by it. The flat water and big rock are under the other label in the pic. This picture was taken from the middle of the river channel and all of the rocks in the foreground and up to the top are generally under water when we fish for winter Steelhead. The fish will often hold out of the current behind these different rocks and hit your offering as it goes through the different channels. There is also a shelf along the rock to the left which consitently holds fish. The water will be 4-6' deep directly below the rock during winter months.
Ths is just an example of what you can find out during low flow summer months to help you find the hiding spots of your targeted fish in winter. Each hole reveals something you can use at a later time.
I spend a lot of time in both summer and winter on several of my favorite rivers. One of the keys to successful fishing is learning several bodies of water very well and applying what you learn when everthing is exposed to when you can't see it. You know it's there.