The Buffalo River is my favorite place in Arkansas. Unfortunatley you will not find any crappie there. However, you will have a chance to catch first class smallmouth bass. I recommend using a tube jig called a "getzit" or another crayfish pattern. Dark brown and greens with orage or red fleck work the best for me. This time of year concentrate on the riffles and the transition areas right beneath the riffles.
Here is the biggest one I have ever seen caught in the river. My wife caught it several years ago on our first anniversary. It was 21.5 inches long. I estimated it to be 4.5 - 5 pounds. We let it go. Have fun! I wish I were going to be there soon.