Gully erosion and sediment load: Waipaoa, Waiapu and Uawa rivers, eastern North Island, New Zealand

The onset of gully erosion following deforestation was mapped for the three largest and heaviest sediment-laden rivers within the East Coast Region, North Island, New Zealand. Gullies were then re- mapped after a ~40-year reforestation period (~1957-1997) and sediment production from gullies was calculated from these data bases using a degradation rate based on DEMs of gullies at differing stages of development in each of two different geologic/tectonic terrains. At the end of the measurement period the total composite gully area for the Waipaoa, Waiapu and Uawa catchments was 5%, 33% and 39% greater than before reforestation, and for the study period, gullies in both terrains collectively contributed the equivalent of 43%, 49% and 54% of the average annual suspended sediment yield from just 0.8%, 2.4% and 1.7% of hill country areas in these respective river systems. A potentially significant reduction in sediment production and yield at catchment-scale could be achieved through a more targeted approach to reforesta- tion, particularly of gullies in the most highly erodible and unstable pastoral hill country areas of Waiapu catchment.