Water is so important to the Buffalo River.  As obvious as that statement sounds, I haven't seen anywhere else where it is more true.  The river changes constantly with rain or without.  It is one place to visit with a full river, and a completely different one when the water runs dry.  Here are some of the best guides to use to see what Buffalo River will greet you if go to visit.
Water Level (Air Space) at Ponca Bridge
NPS River and Rain Reports
USGS River Gage near Boxley  
USGS River Gage at Ponca Bridge
USGS Gage at Pruitt
USGS Gage on Richland Creek
Here is some other information that can come in handy when planning a trip to the area.  There isn't much on the web about the Buffalo that's really useful, but these few sites can help tremendously.
National Park Service Page
Cloudland - Tim Ernst's Website
Buffalo Outdoor Center
Ozark Mountain Region Tourism