EROSION in the Black Hills After the Burning of the Forest Cover
Many surveys have been made by foresters and engineers to size up the extent and import of the erosion problem, and detailed studies have been initiated to determine the effect of the removal of the land’s natural cover—forest, brush, grass—upon erosion and run-off. It has become increasingly apparent in the United States that a detriment to forest cover, particularly on steep slopes, means a detriment to soil and water supply.
A notable example of severe erosion immediately following the destruction of the forest cover by fire, in contrast with the very satisfactory protection afforded by forest cover on an adjacent area, is found near Rochford in the Black Hills National Forest, S. Dak. The destruction of the protective cover was the only change that occurred prior to the time the erosion took place—all other factors remaining unchanged. Here the direct relationship between the removal of forest cover and subsequent erosion is clearly demonstrated.
Conditions throughout the timbered portion of the Black Hills region, which includes between 1 and 2 million acres, are generally ideal with regard to ground cover and its effect upon the prevention of erosion (fig. 19). Forage is not abundant on the more densely timbered areas. The grasses are of unpalatable species and grazing is relatively light. Consequently, there is seldom heavy tramping by livestock with resultant compacting of the soil, favoring rapid runoff. The watersheds are generally well timbered and a thick mat of humus and litter covers the ground. This thick layer of vegetable matter is a very important factor in delaying run-off and in preventing erosion.
Reproduction of ponderosa pine comes in abundantly on sites suitable for tree growth, especially where the soil is coarse and light. As a result young forests are becoming established on many bottom lands and slopes formerly covered only with grass or farmed. In fact,there are few burned-over areas in the Black Hills that have not become stocked with ponderosa pine trees within a period of 10 years after fire (fig. 20).
On some areas within this section, however, there has been considerable active erosion during past years. But the old gullies have generally become well sodded, indicating that the former surface run-off and the accompanying active erosion have been effectively checked. Frequent examples of such “healing” of former erosion may be found.
In contrast to these conditions the situation that exists on an area in the northern portion of the Black Hills where the forest was destroyed by the disastrous fire near Rochford in the fall of 1931 is significant. Incendiaries set a number of fires which burned over an area of 22,000 acres and were extinguished only after a 10-day battle by 3,800 fire fighters. On many slopes all of the trees, as well as the cover of grasses and weeds, were killed; duff and humus were completely burned.
The effect of this destruction soon became evident. During the following year (1932) rains washed down the bare hillsides carrying quantities of rock and earth to the valleys below. Deep gullies were washed in the bottoms, and homesteads were covered with silt, rocks, and debris (fig. 21).
This destructive erosion was very pronounced along the road paralleling South Rapid Creek. A culvert in the road was washed out three times and the bridge which was finally installed had to be replaced (fig. 22). No such damage had occurred before the adjacent slopes were burned over. The stream bed was deeply gullied and large fan-shaped deposits of detritus varying from a few inches to 4 feet in depth were washed onto the homestead meadowlands (fig. 23).
It is significant to note that no gullying, depositing of soil and rocks, or washing away of culverts, bridges, and roadbeds occurred in other comparable situations where the cover on the nearby slopes had not been destroyed or damaged by fire. The contrasting areas provide a clear demonstration of the importance of keeping watersheds green if serious erosion is to be avoided.