They justify it as removal of a hazard to the life and limb of other paddlers.
Aside from the safety aspect of risking your neck to cut the sweeper, is it ethical to alter the river for this reason? Assume a competent and alert canoeist can see the sweeper soon enough to skirt it "with ease."
To be very picky, if it was not hazardous I would say it is "unethical" to remove the trees simply to make it an easier paddle. Sweepers like that create ripples, deep eddy pools, aerate the water, create woody debris, slowly alter the river shape, etc... - in other words they provide very healthy ecological benefits.
Check out this doc if you want (it's from Australia but explains it all very well):
However, compare a couple of sweepers removed a year by concientous canoeists versus putting in a golf course, laying down a new ATV trail, crossing rivers in pickups & ATVs etc... Overall - very ethical compared to the other changes made by humans in the name of recreation.
Is it ethical To make portage trails around unrunable rapids? I know of one I would like to see spruced up a little with out going overboard. I guess one could always talk to the land owner as well but trying to find out who to talk to would be quite the chore.
There is only one place on the upper Nashwaak that could use a official portage trail on the right bank while running down river . I Assume that it its crown land or maybe Irving owned. Most of what needs moved is dead fall. Last trip down the Croix I had a chainsaw in my kayak which I did not even use. Now that I know it fits I think I would take it down the upper Nashwaak next time. Last thing I would want to see would be a clear cut operation, Just some gentle sculpting of a narrow path.