During the summer, my son, Jeff, likes to fish for smallmouth bass at dawn. I like to fish at dusk. However, we both agree that the middle of the day when the sun is shining brightly on the river is our least productive time to catch large smallmouths.
Lets compare what you expect at both dawn and dusk.
At dawn during the hot part of the summer, the water temperature may be several degrees cooler than later in the day. This can definitely improve the fishing. At dawn, you definitely have fewer fishermen on the river to compete with. At dawn, the low light level prompts many of the large bass to feed heavily.
At dusk, many large bass move onto the tails of the pools to feed heavily on the large populations of minnows that live there. At dusk, I catch many bass that crash into the shallow grass beds along the river banks to feed on the shiner minnows who live there. As darkness approaches, many crayfish come out from under their cobblestones streambottom homes to feed and are thus an easy meal for the bass. The madtoms also come out from beneath their streambottom stones to forage through the river in search of their dinners and are exposed to the bass.
As you can see, we get our best fishing at dawn and dust so adjust your schedule accordingly.