There are a lot of articles on the internet on summer walleye fishing. They seem to contradict each other. The general consensus is that walleye go deep in the summer. While this is true on many lakes it is not the case on all lakes. Most articles will also state that you're wasting your time fishing below the thermocline in the summer months. Here is an excerpt from an article.
Fish need dissolved oxygen to survive, so the layer of the water below the thermocline becomes a dead zone as the air temperatures rise. Lures or bait presented there is the same as casting into a desert.
Here's an excerpt from another article.
There’s no arguing that there’s less oxygen below the thermocline, but there’s less need for oxygen because the water’s cold and the fish aren’t moving around very much. In my opinion the really big ones drop down that deep because of the safety of deep water and because the water’s a lot colder.
Here's the catch.(From the same article).
Now, I’ll be the first to admit that my experiences with this are all on Dale Hollow Lake, a deep, clear highland reservoir. But I can’t see why the same thing isn’t taking place all around the country. It’s common sense to me.
It's true that fish need dissolved oxygen to survive but many clear deep lakes have plenty of oxygen below the thermocline. The author of the second article fishes a clear reservoir. In many deep clear lakes photosynthesis can occur below the thermocline. When you have phytoplankton growth and oxygen levels high because of photosynthesis, fish will seek out deeper water.
Walleye need DO (dissolved oxygen) levels of > 5.0 ppm to survive. They can tolerate levels down to 2.0 ppm for a short time. Less than 1.0 ppm is lethal. Walleye also have a range of temperatures in which they are comfortable. The optimum water temperature for walleye is 67 degrees with a low limit of 50 degrees and an upper limit of 76 degrees. Walleye also prefer low levels of light so in clear lakes they have a tendency to go deeper during the day and come in shallow at night to feed.
Get to know the lakes you fish. What works on one lake may not work on the next. The most important thing you need to know is the oxygen levels in the lake. If there is no dissolved oxygen at a given depth there will be no fish there. Walleye can tolerate higher than comfortable temperatures but not low DO levels. In the hot summer months on many mid western stained lakes the DO levels are lethal to walleye at depths as low as 23'. On these lakes walleye can be found in extremely shallow water in the middle of the day. Horizontal jigging a 1/8 or 1/4 oz Bink Spoon is deadly on these shallow fish. On clearer lakes the walleye will typically be deeper. Horizontal jigging a 1/2 oz to 3/4 oz Bink Spoon is effective to about 30'. You can also slow troll the Bink Spoon or drift with it anywhere you would drag a worm or other live bait. Unless you are slow trolling or drifting, vertical jigging the Bink Spoon is the preferred method in water deeper than 30'.
DO levels can also vary from year to year. Lake Norfork in Arkansas is normally an extremely clear lake with dissolved oxygen available in the hot summer months well in excess of 90'. The following is an article I posted about Lake Norfork for striper fishermen.
Norfork is in a unique situation this year because of the high water. The lake has color to it because of this and the oxygen levels are messed up. Currently there is good oxygen from 0 to 15' but this area is too warm for stripers. This is where most of the other fish are including the bait fish. They are mostly in the brush making spooning difficult. From 15' to 50' there is no oxygen so there won't be fish there. From 50' to 90' there is some oxygen. About 3.5 ppm. Stripers like about 6.5 ppm but they can survive at 3.5. Because the oxygen level is low they are very lethargic. There is no bait fish at these depths so they are not feeding, they are just trying to survive. To catch them you need to work the spoon like you would in the winter. Gently move the spoon up and down about 12". Do not snap the spoon. They will fight hard to stay below 50' but once you get them above 50' where there's no oxygen they will give up. There is no catch and release because you will kill them bringing them up. The lake is starting to come down and hopefully conditions will change. There is worry about a striper kill this year. Bink is catching them on the 1 oz white Many Shad and Red Throat. Because of the color in the lake the silver Many Shad is working good too.
So, as you can see, conditions can change from year to year also. To find the walleye first find the depths that have DO levels of 5 ppm or higher. 9 ppm is optimal. In the areas that contain these levels of oxygen look for the bait fish. That is likely where you will find walleye.