SOIL AVALANCHING, EROSION, AND THE EFFECTS OF DEFORESTATION IN THE PANAMA CANAL BASIN

Soil avalanching (landslides in soil that has developed in situ from bedrock) is ultimately caused by chemical weathering, which supplies the loose soil material that eventually slides. The slide is initiated by events such as major rainstorms or earthquakes. Human activities, such as road building and agriculture, enhance the physical interactions. Because of the erosional significance of landslides and their hazard potential, global assessment of landslide-affected landscapes is important. Accordingly, we investigated the roles of surficial physical erosion and soil avalanching in supplying suspended sediment to rivers across a rainfall gradient in the Panama Canal Basin.

Daily sediment data have been collected by the Panama Canal Commission for six rivers: three since 1981, and three since 1987. The time series indicate a possible decrease in sediment yields since 1981 despite continued development and urbanization. Two alternative hypotheses are proposed for this apparent decrease. One is that revegetation following deforestation has caused a decrease in yield. This hypothesis is confounded by a major jump in measured sediment yield in 1996, an especially wet year. Yields did not rise to levels seen in earlier wet years, however. The second hypothesis is that sediment yields are controlled by both surficial erosion and deeper soil avalanching. Landslides require that rainfall (and by inference runoff) exceed a threshold. With a suitable correction factor to account for evapotranspiration, infiltration, and rainfall patchiness, runoff can be used to derive a parameter, "landslide days." This model describes spatial and temporal patterns of annual yields with a high degree of efficacy, far better than the reforestation model. Based on this model, 1996 had lower sediment yields despite high runoff because there were fewer days that exceeded the landslide threshold. The smaller number of landslide days in 1996 could be strictly meteorological, or perhaps growth of secondary forest modified the runoff peaks, dropping them below the landslide threshold.