Was out hiking today and was reminded of this thread when I encountered this today.
This picture doesn't really do it justice or adequately describe whats going on here. There is a small creek that runs continually beneath this patch of liverworts. When it rains very heavily the creek apparently runs both beneath the earth and then overflows into this liverwort covered layer of substrate that is made up almost entirely of fine grains of creek sand and chunks of quartz. I guess when it rains only normally(?) this area is totally "dry" and the watercourse continues only beneath it. It has been raining somewhat consistently these last few weeks/months; so it would seem that this secondary watercourse only comes into being during the most intense periods of precipitation, which for around here means something like 3+ inches of rain per day in succession.