THE INITIATION AND GROWTH OF GULLIES IN MADAGASCAR - ARE HUMANS TO BLAME

Lavaka (Madagascar's unusual gullies) require thick lateritic regolith on smoothly convex hills, and they are initiated locally by a variety of cultural and natural proximate causes, such as paths, roads, defensive trenches around hillforts, steep fields, unchanneled to slightly concentrated storm run-off, springs, mass movements, etc. Growth continues by rain attack, incision by run-off, earthfalls caused by wetting and drying cycles, and especially basal sapping by diffuse groundwater flow above bedrock, somewhat in that order. Lavaka are less directly permitted and promoted by (1) hardening of exposed laterite surfaces, which creates a hard-soft-hard profile from bedrock to laterite, which favors incision relative to lateral erosion, (2) the superimposition of concave run-off profiles onto convex hills, and, locally, (3) re-equilibration of watersheds after stream piracy and faulting. Run-off and hardening are interrelated with devegetation, because hardening results from exposure following devegetation, because both hardening and devegetation increase run-off relative to infiltration, and because hardening, rain attack, and run-off impede vegetation. Devegetation is certainly worsened by harmful human activities (such as range burning, forest cutting, and/or overgrazing), but tectonism and natural climatic aridification are probably ultimately more important, given that many lavaka are simply a natural part of the landscape's evolution and that some lavaka clearly predate primary (uncut) rain forest.