Riding when the trails are wet and mushy (rain, freeze-thaw) creates a lot of trail damage, some of which is difficult or impossible to repair. When your tires or trail shoes or horse shoes are caked in mud you are physically removing part of the singletrack tread. Over time the trail will deepen into a channel and funnel water down the trail to the next lowest point further accelerating the process of dirt removal. Once the trail is below the natural dirt level on a hill or flat area there's is no way to fix the trail "cupping". That's why building trails on low gradient to flat ground is troublesome as there's no way to "raise" the trail again without rerouting it. Think of the grassy flats of Chubb or Castlewood where the tread is several inches deep and is always looking to take you down with tire or pedal strikes.
Anyway, most of you that read this email understand the importance of good trail responsibility. We ask you pass the word along to newer or occasional riders that may not understand the negative long-term effect of muddy trail rides.
This sign, while not found in our own region, conveys a universal message from all midwestern trail builders.