Floodplain stages in the braided Ngaruroro River, New Zealand

This study investigates floodplain deposits in the braided Ngaruroro River, New Zealand, to characterize and quantify the spatial extent of deposits at different stages of development. Using a sequence of aerial photography, depositional surfaces were mapped and then assigned a floodplain development stage based on age, the discharge causing inundation, and field observations of sediment and vegetation characteristics. The ensemble of floodplain deposits makes up 40% of the river valley, the remainder being the active river bed. Stabilizing bars and mature floodplains dominate the floodplain ensemble. While the existence of stabilizing bars points to a recent shift or fluctuation in controlling conditions, the extent and location of mature units suggest an earlier change in hydrologic and sediment supply conditions that limited braidtrain mobility. Mosaics of different aged deposits range from simple two-stage mosaics to more complex arrangements of three and four stages. In the rapid evolution from stabilized bars to mature floodplains, a large amount of fine-grained sediment is stored in the Ngaruroro River valley, highlighting the possibility of finding extensive fine-grained facies in the ancient sedimentary record of braided systems. Understanding floodplain ensembles and their controls will contribute to effective restoration of braided rivers.