During mid- and late-summer's typically low flow, river walleyes will be found in surprisingly shallow water, often in "flat riffles" that other angler bypass.
Flat riffles are river stretches that feature significant gradient, but lack the protruding rocks and boulders that provide bumps and whitewater. At first glance these spots — which are a foot to 2 feet deep — seem devoid of the character to hold walleyes. But a closer look often reveals scoured divots in the river bed, subtle depressions for feeding walleyes to duck out of the hard current to ambush prey.
On larger warm-water rivers that support walleyes flat riffles may be several hundred yards long, and are often found below major river holes. They won't be mirror smooth; expect to see some short waves as the current flows over the washboard bottom.
Since they are there to feed, catching walleyes from flat riffles is actually a pretty easy deal. Shallow-running minnow baits that retain good action in current excel, as do soft fluke-style jerkbaits. Simply float through the riffle in a shallow-draft boat, making cross-current casts. Maintain a moderate retrieve speed with the occasional twitch. Walleyes are accustomed to chasing riffle-dwelling minnows and will have no trouble tracking down your lure, though some hits will happen at boat-side.
Don't be discouraged if your first attempt at working a flat riffle draws a blank. Walleyes move in and out of these areas in relation to their feeding cycles. Stick with it and you'll hit things right.