Sediment yield response to large storm events and forest harvesting, Motueka River, New Zealand

Abstract

Suspended sediment yields (SY) were measured at seven sites in the Motueka catchment to characterise major influences on sediment yield variation and estimate sediment delivery to Tasman Bay. Relationships between storm event SY and peak flow were used with long-term flow records to calculate SY. At large catchment scale, there were clear differences in SY related to geology and rainfall, but the major influence on SY was the impact of a >50-year return period storm that caused an increase in event SY ranging from 10–20 times in the headwaters to 2–3 times at the coast. Forest harvesting was an important influence at small catchment scale, producing a five-fold increase in event SY. The magnitude of the SY increase, the time to recover and the total area affected were all smaller than the impact of large storms. Annual SY at the coast was extremely variable, ranging from 49,000 t to 1.7 Mt.