I've been wearing the Simms fingerless gloves with the fold over mitten. They have a pocket on the inside of the wrist for a hand warmer packet. It's important to keep your cuffs out of the way and make sure you can feel the heat so it is warming the blood flowing to your hands. I also keep another pair if hand warmers in my jacket pocket to add extra warmth to my hands when they start to get cold. Make sure you open the hand warmers about 30 minutes before you get to the river.
This has worked for us when it was -15f.
The biggest downside is they are expensive.
I agree with the others that you want to take action when your fingers start to get cold, rather then waiting until they are cold.