This is the point at which either fartleks, pickups or intervals can be used to deter a deadly confrontation. Wearing a mask backwards is also an excellent idea. Get one with a picture of a timid-looking gazelle or bunny rabbit. Then get ready to bolt. Honing these skills harkens back to the days of the truly tuff distance runners and will help establish your legacy on multi-day runs.
Nothing is more worrisome to an ultra runner than a babbling brook.
But take heart, flowing water can be your friend. Before you take to the trails, visit your local scuba shop. Purchase a wet suit, flippers, goggles, and a bathing cap. In your next ultra, think how envious your competitors will be when you whip out your water gear at the appropriate moment. If packed correctly this will all fit in a survival pack.