Find the bait (with Humminbird Down Imaging), and you'll find the fish!
Dr. Jason Halfen
Professional Walleye Angler
Navigation:
One of the oldest adages in the angler's manual is that you must find the bait in order to find the fish. Aside from their annual spawning activity and the predictable movements associated with them, fish spend much of their time looking for food, or trying to avoid becoming another fish's meal! An important forage species in lakes and rivers across the continent is the shad. On the Mississippi River, abundant, high-fat shad are the primary forage  for species from crappies to walleyes, catfish to pike, and bass to sturgeon. Just a couple of test runs of Humminbird's Down Imaging software package have deomonstrated the power of this technololgy for identifying schools of shad, both large and small, and the sportfish associated with them.
Navigation:
The first few cold fall nights in the upper Mississippi River valley trigger a predictable movement of shad from shallow bays and backwaters towards the deeper, more thermally stable waters of the main channel. This bait movement pulls many shallow water sportfish, particularly walleyes, into these deeper areas. One area that holds bait, and walleyes, all winter long are the outside bends of main river channels, where current action in spring and fall scours the bottom to a greater extent than occurs in straight sections of the channel. Here's a Down Imaging/Side Imaging split screen capture that shows one of these deep outside bends, and a large school of shad (marked with red stars) within it. The shad are so numerous that their sonar shadow is clearly observed in the right hand Side Image (noted with blue arrows).

I collected this next screen capture just down the channel from the one above (note the times: 1:13 and 1:14). As the channel became just a bit shallower, the depressions in the washboard bottom became absolutely full of fish. These rows of fish, which appear as white returns in the otherwise darker depressions in the left hand side image, include a variety of species (walleye, sauger, crappie, sheephead) that find relief from the current by tucking themselves into the depressions; many of these fish are inactive midday, but feed intensely on the shad found in the same region during low light periods. Most of the noise in this screen capture is present because I was following a barge heding downstream, which was mixing a fair amount of air and small organic debris into the water column
Here's another deep outside bend of the main navigation channel. In this area, the sand bottom features the characteristic "washboard" appearance caused by long-term current flow across the sandy substrate. Another high-riding school of shad, this one packed a bit more tightly than the one illustrated above, is noted with the red stars in both the Down Imaging and Side Imaging views.
This is a close-up view of all of those fish tucked into the depressions in the washboard bottom. If you fish a river system, washboard bottoms like this should be easy to find (particularly with the help of your Humminbird electronics), and are excellent places to find and catch fish year 'round.
Here's a sampling of the fish we found by concentrating on this particular section of washboard bottom. Not giants by any means, but 3 nice fish destined for a hot grease release. Humminbird's Down Imaging and Side Imaging helped us locate the bait, and then the fish...the rest was easy!