Paddling and skiing have much less impact - but it's still better than motorized recreations.
Please stop spreading this b.s. Paddling rivers usually has about the same driving with it's impacts as shuttled mountain bike rides (which you don't have to do to ride). Skiing usually doesn't have shuttles, but the commute is almost always longer because mountain passes and summits are the farthest from civilization, not to mention the diesel fumes spewing in the air in lift-serve skiing.
Sure, you burn some gas when you ride a moto or quad (and for the commute, too) but since these activities have limited accesses and set aside locations, the "worseness" is hardly measurable, but really just repugnant bias.
If you are simply referring to the "leave no trace" aspect of sport, again refer to the previous statement about locations.
Bob: "I'm a so much better citizen than you because I engage in low-impact sport, unlike your motorsport!"
Bill: "Then why did I burn less fuel than you doing so?"
Furthermore about trail erosion, Eastern Mich Univ. did a study quite some time ago to see if any one of three groups (equestrian, hiker, mountain biker) actually cause erosion faster than the others. They found that there is no measurable difference, unless it was really wet and muddy, in which case the horses did far more damage. It is often incorrectly touted that the bike has to be worse, probably because it maintains more contact with the ground in the process, but this is patently untrue.