I spoke with someone and they said they were staying off certain trails because snow melt and mud would ruin the trails. I had never thought of this and really want to be considerate.
Is this a valid concern? Should I avoid a trail because of this? How do I know when it is ok to use?
It's common etiquette, at least everywhere I've been, to stay away from wet and muddy trails (unless they're supposed to be or always are a mudfest). Riding on a muddy trail makes ruts which make the trail conditions worse for everyone once the mud dries and the ground hardens.
In addition to ruts, if the trail is muddy and you slide out a front wheel going around a turn, for example, your wheel can scrape several inches of trail off. This can cause erosion problems later on.
However, as alluded to earlier, if the trail is a known mudbog and stays muddy year-round, one more rut probably won't hurt anything.
Another common counterexample is when the trail is melting in the afternoons but can be ridden early in the day when the ground is frozen. If you're crazy enough to be out riding then, more power to you.
But generally speaking, it is in good taste to stay off melting, muddy trails. Any time your presence can alter the trail physically (ruts, slide-offs, feature damage), everyone else who shares the trail with you will be glad you stayed off.
As Alesplin puts it, it is common practice to stay off of trails that are muddy.
I'd like to expand on his answer a little. I think there are a few factors that come into play with a question like this.
One of the biggest contributors of damage to a muddy trail is large amounts of traffic. The more people that use the trail, the more damage will be done. If you are one of a few people to use a trail, you most likely won't do any permanent damage. The worst thing you will do is create ruts, that when dry are not the nicest things to ride on/in.
Ruts can also lead to other forms of damage. A rut will collect water, which will not run off the trail. Quite often, these puddles of water will get bigger and bigger until they consume the width of the trail. This, I think, is when the real damage occurs. Trail users (both rides and hikers) will most often avoid going through the standing water and simply go around (at the cost of the surrounding vegetation). This can lead to parts of the trail being expanded unnecessarily, and there will still be the problem of the expanding rutted water hole in the middle of the trail.
In the end, if the trail is used very lightly, riding in the mud will not do much damage at all. For most trails though, it is best to avoid the muddy ones. Of course, there are always sections of trail that are muddy, no matter what. Those are a whole other matter of how to properly build and route a trail. Get dirty and have fun.
The issue is not just muddiness and ruts. Activities like biking and hiking promote soil erosion (and soil is much more susceptible where erosion already has taken place). We try to strike a balance between preserving the condition of the trail and enjoying our activities. If the ground is very wet, the damage caused by our use is much greater, since we move more material around including whatever plants are helping to reinforce the soil (roots act like the rebar in concrete). The damage caused in one trailride could be greater than several years of use in decent conditions.
Even under normal use, trails need to be reconstructed and/or moved from time to time to allow the land to "heal". I'm grateful for the volunteers that work to maintain the trails where I ride (Thank you, Waterloo Cycling Club!)
In short, thank you for your desire to be courteous and thoughtful, and please save your riding until the trail has dried up.
IMBA wrote a great article summarizing the various studies of mountain biking's impact on trails. Their conclusion was as follows:
Furthermore, while the impact mechanics and forces may be different from foot traffic, mountain biking impacts are little different from hiking, the most common and traditional form of trail-based recreational activity.
[...]
So what does this mean for mountain biking? The existing body of research does not support the prohibition or restriction of mountain biking from a resource or environmental protection perspective. Existing impacts, which may be in evidence on many trails used by mountain bikers, are likely associated for the most part with poor trail designs or insufficient maintenance.
Managers should look first to correcting design-related deficiencies before considering restrictions on low-impact users. By enlisting the aid of all trail users through permanent volunteer trail maintenance efforts, they can improve trail conditions and allow for sustainable recreation.
In other words, current studies show that mountain bikers do no more harm than hikers and that muddy trails are a product of bad trail building and management, and not from mountain bikers riding them.
The problem here is more with perception I believe, as some small groups of extremely reactionary hikers usually wants to ban everything that is not hikers from venturing into the great outdoors. If they can keep mountain bikers off the trails by saying that they damage them, they suddenly have a great argument for doing so. And if we as mountain bikers accept this as a fact and the way that it should be, then we are suddenly at the mercy of the hikers, and this would be very unfortunate for mountain biking as a recreational activity.
So instead of staying of the trail you should help doing maintenance on them so that problems such as mud and erosion are lessened and the trails become more pleasant for everyone who uses them.
