George,
Had the start of the same problem on my inflow. I also used 80lb bags and it has worked out great. The only problem I have is the paper is starting to disintegrate and floating in the pond. I have been sending the kids around to pick it up every evening. Looked at getting limestone instead but the cost was higher and getting the tractor down there to spread it would have made my problems worse. Quikrete actually makes a riprap bag of concrete where the paper is biodegradable but I could not find a source. Very nicely done and I think you will be quite pleased with the results.