It's possible to cross a stream while barely wetting your feet. All you have to do is high-step across as quickly as possible, allowing your feet to touch down only for a fraction of a second. And don't be afraid to run right through a stream. Too many competitors lose time by halting at the edge of a stream midrace. Every type of trail offers unique challenges.
Sand is never easy to run on, especially when the weather's been hot and dry and the sand is loose.
For all-around annoyance, nothing beats running down a muddy trail. It's dirty, there's a greater chance of slipping, and mud tends to stick to your shoes, making them heavy. Shiny mud is invariably wet and liable to suck you in. Mud with a dull appearance is usually firmer and faster to run on. Step lively here, using a higher leg lift to avoid tripping.