In the 1950s, increased erosion, flooding and sedimentation was widely observed in New Zealand. The ruling opinion then was that the forests prevented erosion and floods, and browsing mammals were primarily responsible for the increased erosion of mountain lands. It followed that effective control of browsing mammal populations was necessary to prevent erosion and alleviate lowland flooding and alluviation. In the 19608 evidence was found for much severe erosion on the Ruahines in the 1840s -long before browsing mammals were there.
During the last 1800 years there have been eight periods of increased erosion and alluvial sedimentation in New Zealand, which have generally decreased in magnitude towards the present. Throughout New Zealand, alluvium of all erosion periods contains abundant remains of plants as evidence of widespread destruction of vegetation during erosion periods. Indices of the relative magnitude of alluviation, and estimates of the damage to vegetation in the current Waipawa Period (since 1950), are applied to estimate the impact of earlier erosion periods.