if the tree is important to you, i would not cut the roots. i would build a retaining wall to support the tree and add soil back over the roots. the erosion will eventually bring the tree down. if you must, i would take less than one third of the roots per season. remember that the roots are helping to stabalize the bank. without them, the bank will erode more bringing the tree down sooner.

I would want to know what kind of a tree it is. Some trees have shallow roots while others have a very deep main tap root. If it is the latter, you might get away with cutting some.

It is a very mature Tulip Poplar tree. Probably forty or fifty years old. The two roots in question stick out over the stream bank for about twelve feet, they are only three or four inches thick.

Found a great article on root pruning from Iowa State University. Specifically it says..."Trees intolerant of root pruning include beech (Fagus grandifolia), birch (Betula spp.), conifers, Callery pear (Pyrus calleryana 'Bradford'), sugar maple, and tuliptree (Liriodendron tulipifera)" Check it out at http://www.ipm.iastate.edu/ipm/hortnews/1995/3-31-1995/sidetree.html What about Plan B?

I don't have a plan B at the moment. Possibly a small retaining wall with some back fill.