Floodplain construction by recent, rapid vertical accretion: Waipaoa River, New Zealand

The rate of vertical accretion (typically 14–18 mm h−1) during eight floods in the Waipaoa River basin, with recurrence intervals of 5 to 60 years, was determined by relating the floodplain stratigraphy at McPhail's bend to the 1948–1995 flood history. Overbank deposits remaining after a flood that occurred in March 1996 suggest a rate of vertical accretion of 15 mm h−1. By contrast, because the flow velocity across the floodplain was too high to permit deposition from suspension, during the record flood of March 1988 the rate of vertical accretion was only 6 mm h−1. The sequence of deposition is highly discontinuous, and the rapid vertical accretion is a response to a late 19th to early 20th century phase of deforestation in the headwaters that probably initiated a far greater change in suspended sediment yield than in discharge. Cross‐section surveys conducted since 1948 indicate that the high suspended sediment load of the Waipaoa River also promoted in‐channel deposition, which effected a progressive reduction in bankfull channel width although, due to the overbank deposition, channel capacity remained constant. © 1998 John Wiley & Sons, Ltd.