Gotta be inside the waterway, watercourse, marsh, creek... Etc... If it is above the banks I can guarantee it is illegal. If it is inside the banks then it could be questionable if it is up off the ground and water.
I can guarantee you this, if you catch something in it, do not post pics on here. Whether on a log or in a sluice pipe and legal, you will be ridiculed by people for it. Trust me on that one. The comments might not be horrible, but the pm's you will get will be a joke. I know first hand on an incidental bobcat catch in a 160 (100% legal) mink set in a sluice.
A fallen tree is considered the watercourse. I have a ton of 110's set on log crossings for mink all 100% legal. One of the streams I trap comes up and down about 6-8 feet. Within the high water mark is considered the water course, so a bridge with a pipe and a "water mark" is fair game for a 110 as far as im concerned. A friend of mine got grilled by a WCO for using 160's in coon boxes, but everything checked out 100% legal. The only gripe I have about using the 160 as a lethal trap is that it becomes less species specific. Everything fits through a 160 (look at the bobcat example), very few critters go through a 110 except the water targets.
The bobcat only weighed in at 18lbs, and it's head barely fit into it. Barely bigger and it wouldn't have fit its head there. It was dumb luck to have it out its head down and go through it instead of jumping over it.