Agreed. Around here the rivers get good once the ponds freeze up. And then they get even better when the big lakes start freezing up (or at least the shallow water on the lakes). Then the action gets fast and furious when the slow parts of the river freeze up too. River hunting is absolutely my favorite. I like to set up just above or below a good riffle with open water. The current keeps the decoys moving too. In my experience, the ducks will come at the normal peak times you would expect on a pond or lake. Here, they usually come upriver from the main lake in the morning, and opposite in the evening. I wouldn't necessarily say that its an all day flight, but more so than on a farm pond. We've sat many times all day. Its usually good early for a couple hours, slows down, then picks up around 10-11, slows down, then again around 2-3. We don't get a lot of flight in those in between times, but don't be surprised if you get a big flock of mallards while you're munching on a sandwich. It also helps to have other hunters out on the river so the birds don't stack up in one or two holes. Keeping them moving is the key. Sometimes our group will split up and hunt 2 or 3 riffles just to keep the birds moving a little better.
One other thing. I remembered you saying you were going to hunt a bend in the river. I've noticed that a lot of the flocks will fly right up the river channel between the trees that line each of the banks. But, I've also noticed a good chunk of the ducks will sometimes cut corners across a field. For example... I live about 200 yards off the river. Hunting behind the house can be good, but sometimes frustrating. There are times when not much flies right on the river channel past the decoys, but you'll see several groups cutting across the field and dumping back into the river on the next straight away. It took me a while to realize what was happening, but now I almost always walk the extra quarter or half mile up the river to that long straight away. More times than not, it holds more ducks than on the bend where I live. I don't know how big of a bend you're hunting or if it will be the same there, but it might be something to watch for. I know there's times that it can make all the difference in the world here. I remember once we split up and my group of 3 walked up the river and shot a 3 man limit. After coming back down the river to the other guys, we found out they only fired a hand full of shots. All I can say is river hunting is the best. They can come up that channel and never be seen until they're landing in the decoys. Or they can drop right over the top of the trees on the bank and dive bomb your spread. But then there's the days nothing seems to work out quite right, but that's hunting. Just get out there and see what they do. And keep a note book. You might not notice their pattern tomorrow. Or this year. But you might find the golden key when you look back over your notes all at once. Good luck!