by Thurston Griggs

Flashback: How Can Drought Cause Erosion?

Original article published in the November 1983 issue of the The Register.

“For some parts of the A.T., 1983 has been a year of drought. Springs that used to flow continuously have gone dry, and vegetation known to flourish during July and August failed to appear on the Trail.

It might seem that, with rainfall being so sparse, the treadway would be free from erosion. Not so. Though infrequent, the rain that has descended has poured down heavily and quickly in thunderstorms. When that has happened, some sections of sloping trail have become like little streams.

For one thing, the ground has been baked by the drought, caked hard, with little ability to absorb moisture. In some places it has become almost like pavement. Clearing of brush for the trailway is another factor. Where fewer leaves and branches intercept the pelting rain, the impact is immediate along the course of the trailway. Yet another factor is the dry leaves, dirt, and dust that lie loose, ready to be swept along in the stream of water. When the ground is damp or moist, these things tend to remain better attached where they lie. During the flood, they are dislodged and swept along, to be deposited in waterbars or wherever the water has to slow down.

In Shenandoah National Park we cleaned out some waterbars back in June, along a mile of Trail that rises about 800 feet. By Labor Day weekend, those waterbars had completely filled up– the result of only two or three thunderstorms. Last year, by contrast, despite continual intermittent rains and occasional thunderstorms, the same waterbars remained clear throughout the summer.

No one set of guidelines will suit all kinds of soils, of course, but on this particular stretch of Trail, which was built originally by the CCC, we noticed that the waterbars seemed adequately spaced to halt erosion each year–including this particularly bad one. They are set at an average of 20 yards apart where the grade is about a steady 10 percent. For that kind of soil condition, this appears to be just about right.

With a steeper slope, and with drainage from adjacent areas, the waterbars might be closer than 20 yards– perhaps as little as 10 yards. One thing, however, is clear. Lots of water and lots of soil get moved down the course of the Trail even in a year of drought.”