Erosion thresholds and suspended sediment yields, Waipaoa River Basin, New Zealand

Abstract

Characteristics of the suspended sediment load in the Waipaoa River, New Zealand, and in two of its tributaries (Mangatu and Te Arai Rivers) are examined for evidence of different dominant erosion processes in the basins upstream. Different shapes of the suspended sediment concentration‐water discharge relations lead to differences in long‐term average yield, event‐yield magnitude‐frequency relations, and relative importance of large flows and rare 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 long‐term average load is transported by events with return periods less than ∼1 year, and there is little evidence of an erosion threshold limitation 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 and moderate flows, frequent runoff events transport less of the long‐term yield (4600 t km−2 yr−1) than do rarer, large‐magnitude flood events, event sediment yields are an order of magnitude lower during events with subannual return periods, and half of the long‐term average load is transported during events with a return period of >2 years. Some of these characteristics appear to result in part from two populations of runoff events in the Te Arai basin; nonetheless, they are consistent with field evidence that most of the sediment supplied to the Te Arai stream network is generated by shallow landslides which are activated once a rainfall threshold is exceeded.