Walleye can become incredibly hesitant to bite during the summer months when the water warms up and the flow drops.
There are a couple of things that are more "general rule of thumb" for fish in the summer that are not necessarily specific to walleye:
1) Fish will bite after dark. Bass, bluegills, crappie, catfish, and walleye (along with an occasional gar which is more of a nuisance as they are expert bait stealers when targeting catfish). In fact, fish will generally bite significantly better after the sun goes down. I routinely pull fish out of holes that have been hammered during the day by folks reporting that fish are not biting.
2) Live bait will outperform everything else.
That being said, here is how I target walleye in the river when the flow is low, the temp is up, and the mosquitoes are thick:
Rivers in Michigan typically follow the "rapids, hole, gravel, flat, rapids" pattern. Rapids will be characterized by faster current constricted to a narrower run of the river and characterized by bottom structure that is primarily rock and (in the more urbanized settings of lower Michigan) concrete chunks. Often, these rapids will be located on the upstream side of a bend. As the water picks up velocity and then makes the turn on such a bend, a few interesting things happen. First, the water gets oxygenated. Second, the water scours the river bottom on the outside of the bend when it is high in the spring. Also, baitfish, crayfish, worms - fish food in general - will be getting carried downstream by the swifter current.
Directly after the big rocks that make up the rapids will be the hole - the deeper spots you will find in the river. Often, the hole is combined with an outside bend to form an eddy where the water circles back and flows opposite to the main current.
There is one spot in particular in each of these holes that will almost always hold a decent fish - the first 3 feet of water at the very head of the hole, directly below the rapids. To have a shot at catching this fish, you will want to cast slightly upstream and work your bait or lure with the current. Let it wash down through the rocks and drift through the area directly under the rapids. Many times you will get bit on the first or second cast, and more often than not this will be one of the better fish available in the particular run. If you get bit and either don't hook the fish or the fish gets off, you will want to stop fishing that pocket for 15-30 minutes in my experience. Give the fish a chance to re-position itself and get comfortable again.
I use this very particular location to figure out what bait or lure I am going to focus on. I always start with suspending cranks, then transition to a jig and soft plastic, and then try either a crawler, half crawler, leech, or (if I have them) a big creek chub. If I cannot get bit in 10 casts, I switch. If you catch that first fish, keep fishing that same spot. Other fish will move up and occupy that prime spot that is now vacant.
Now, continuing to focus on the same rapids / pool area, my next location will be downstream, but not necessarily right along the seam. You will pick up some little guys right there, and tons of smallmouth and catfish, but the bigger walleye will feel too exposed.
During the day the bigger fish will position themselves right at the point of any timber that lays in the pool / eddy, just out of the current. Be prepared to horse them out. They will also lay on the UPSTREAM side of boulders or humps in the channel. And the final spot they will hide is the secondary pool directly behind a big lay-down. I am talking about the one made by a tree, not a stick or rock.
At night, these fish will move out from their cover and up onto the flatter area on the side of the river channel opposite the hole. There is normally a bit more current and the water they are in is typically three feet deep or less. These are actively feeding fish and are the ones you can catch just by casting and retrieving pretty much anything artificial that they can see in the dark. If you are catching small ones, don't be afraid to try a bigger bait. You want to hit the bottom, but not drag on it. Start by tossing upstream and working with an up and down pattern with the current, then work against the current with a slow steady retrieve. Work it as slow as you can. Finally, try to dead stick - throw the crank out, reel it down, and let it float for 10 seconds or more, then barely twitch it. You may only get two or three twitches in a cast if you are doing it correctly.
In the dark, I do best with 3 1/2" to 6" jerkbaits that are green. Particular pattern doesn't much matter, as long as it is green. I have a couple of go to lures: HJ12 in firetiger, Matzuo Zahnder Shad in perch or (on occasion) brown and silver, Shad Rap 07 or 09, X-Rap 10 in silver or perch, Shadow Rap in olive or carbon. I also have an assortment of other brands, and they all seem to work, depending on the size I want.
For the artificial, I don't worry about leader and such. I tie cranks directly to my braided line. I like Power Pro in either 8 pound or 10 pound test. I have experimented around with leaders and fluoro, and snaps, and swivels, and I truly believe that a fish is either going to hit a crankbait or not, regardless of how it is attached.
Now, the reality is that in the middle of summer during the day I am most often going to be resorting to a half of a night crawler. I use a very simple set up:
1/8 to 1/2 ounce bell or dipsy sinker. I use the least amount of weight that will let the sinker tick along the bottom and drift, but not get washed up to shore. Under 1000 cubic feet per second (CFS) of flow it will be 1/4. Under 750 CFS it will be 1/8 or no weight at all. Over 1000 CFS 3/8. Over 1500 1/2. Obviously, I vary this as the particular location in the river will also dictate the correct weight but these general rules of thumb are a good place to start.
I use a white bead and then a small black swivel. I use 8 or 10 pound fluor for a leader. I tie on a size 4 baitholder hook. I use a leader that is between 6 inches and a foot long.
The single most important thing to do is to hook the crawler correctly. Feed the point of the hook into the mouth. Run the crawler up the shank. Pop the hook point out JUST AHEAD OF THE SECOND COLLAR (orange band). Then spin the crawler half a turn and run the hook through the crawler just behind that second orange piece. I pinch the crawler off and leave 3/4 to 1" hanging off. You want it to be completely straight on the hook, with just a small piece of the tip of the hook exposed. Cast it out and let it drift.
You can use the current to get a drift downstream to one of those trees or humps or boulders I talked about.
I don't typically park the bait. When it stops bouncing along because it washes into the seam, I let it sit for maybe a minute before reeling in and casting again. Letting it sits allows bass, suckers, and catfish to find it, which is fine if you just want to catch fish, but which won't typically get you walleye for a stringer.