Tread is the part of a trail that is actually walked on. Usually, and especially in the mountains, tread is constructed by cutting away varying amounts of the slope. The upslope must be cut into to form a flat area, which is the tread. Different ratios are used to measure the cut into the upslope, often depending on the soil type. A rocky upslope can have a steeper cut (backslope) into the slope because a harder material can hold together better than a loamier soil.
Other than water, two enemies of tread are slough and berms. On hillside trails, slough is soil, rock, and debris that has moved downhill, narrowing the tread. If slough is not removed from the tread, a trail will tend to "creep" downhill. Sloughing can be caused by hikers walking closer to the upslope side of the trail than the downslope or by too steep of a backslope.
Where as slough builds up on the inside of trails, berms build up on the outside. Berms will keep water from sheeting across a trail, keeping water on the trail. When the water can’t sheet, it flows down the length of the trail, carries away soil and, and as a result, builds up an even bigger berm.
Next: Stream Crossings