This is another one of those colorful terms from trail building. The feature that goes by this name is used to control runoff. It's a ditch. (Whoopee,etc.)
Such a ditch runs parallel to the trail on the uphill side. The ditch collects water and redirects that water so it doesn't slosh across the trail (bad) or run downhill along the trail and chase backpackers away (even worse). After it has done its duty, a ditch like this usually dumps whatever it has collected into a culvert. After that, it's the culvert's problem.
As for the water itself, we don't care where that ultimately ends up. Really, we don't. We don't care.
But then again, other sidehill ditches are ruts worn by the passage of any of several small annoying animals we can lump under the general name of "sidehill gougers". It's them what do that (the gouging).
These gougers are smallish mammals who root on hillsides, tearing up earth in their search for food (small leafy plants, roots, insects, and grubs). There are gouger species on several continents and they are known by a variety of other names such as "sidehill dodge hodag", "rackabore", "dahu", and "wampahoofus".
The wild haggis is a related species found exclusively in Scotland. Although rare now, it was once abundant enough to become the national baked dish of that country. In case you ever wondered about that.
Anyhow, these various sideways gouging animals sort themselves out early in life into left-handed and right-handed variants, and after that they consistently turn one flank uphill. Gradually, as these critters mature, the downhill pair of legs lengthens while the uphill pair shortens until an individual animal loses the ability to turn around, or even to walk on level ground.
Difficulty in finding suitable mates, not to mention the mechanics of the mating process itself, may be a key reason for their declining numbers and bad temperament.
Yes, they are desperate. If you encounter one, keep your distance — you know what the neighbor's dog tries to do with your leg every time it gets close enough to make a grab for it. And these critters are much, much worse.
So very much worse.
And when they do get going, they drool a lot. So. Think about that then.
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See if that helps.