Erosion thresholds and suspended sediment yields, Waipaoa River Basin, NewZealand

Characteristics of the suspended sediment load in the Waipaoa River, New Ze aland, and in two of its tributaries (Mangatu and Te Arai Rivers) are exami ned for evidence of different dominant erosion processes in the basins upst ream. Different shapes of the suspended sediment concentration-water discha rge relations lead to differences in long-term average yield, event-yield m agnitude-frequency relations, and relative importance of large flows and ra re events. In the Mangatu River, frequent runoff events are relatively more important to the long-term yield (11,540 t km(-2) yr(-1)), half of the lon gterm average load is transported by events with return periods less than s imilar to 1 year, and there is little evidence of an erosion threshold limi tation on sediment supply. This is consistent with the predominance in the Mangatu basin of hillslope erosion processes that involve scour by surface and channelized runoff, particularly gully erosion. This contrasts with the Te Arai River where sediment concentration tends to be much lower at low a nd moderate flows, frequent runoff events transport less of the long-term y ield (4600 t km(-2) yr(-1)) than do rarer, large-magnitude flood events, ev ent sediment yields are an order of magnitude lower during events with suba nnual return periods, and half of the long-term average load is transported during events with a return period of >2 years. Some of these characterist ics appear to result in part from two populations of runoff events in the T e Arai basin; nonetheless, they are consistent with field evidence that mos t of the sediment supplied to the Te Arai stream network is generated by sh allow landslides which are activated once a rainfall threshold is exceeded.