Residence time of alluvium in an aggrading fluvial system

Relationships between the surface area and age of alluvial deposits were used to estimate the residence time of alluvium in the 2205 km2 Waipaoa River basin, New Zealand. The contemporary Waipaoa River is an efficient transporter of sediment to the continental shelf, but the basin has been characterized by rapid channel and valley aggradation in the historic period, and by extensive mid‐ to late Holocene alluvial storage in the lower reaches. The area‐weighted mean age of alluvial deposits in the lower part of the river basin is ∼4400 yr. These deposits comprise terrace remnants isolated by downcutting, and Holocene to Recent sediments that are potentially remobilizable by the modern river. Even though the amount of storage is small relative to downstream transport, the majority of the potentially remobilizable alluvium is likely to remain in storage for >100 yr, and its half‐life (time for 50 per cent removal) is >2000 yr. Within the confines of the flfloodplain, the apparent ‘loss’ of older deposits is due primarily to burial, but losses of the most recent deposits are due almost entirely to remobilization (30–40 per cent), with the remainder preserved in the alluvial record for at least 104 yr. Most of this sediment is likely to remain in storage until there is a shift to a degradational state. Copyright © 2006 John Wiley & Sons, Ltd.