FLOODPLAIN CONSTRUCTION BY RECENT, RAPID VERTICAL ACCRETION - WAIPAOARIVER, NEW-ZEALAND

The rate of vertical accretion (typically 14-18mmh(-1)) during eight f loods in the Waipaoa River basin, with recurrence intervals of 5 to 60 years, was determined by relating the floodplain stratigraphy at McPh ail'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 mmh(-1). By contrast, because the flow velocity across the floodplain was too high to permit deposition from suspension, dur ing the record flood of March 1988 the rate of vertical accretion was only 6 mmh(-1). The sequence of deposition is highly discontinuous, an d the rapid vertical accretion is a response to a late 19th to early 2 0th century phase of deforestation in the headwaters that probably ini tiated a far greater change in suspended sediment yield than in discha rge. 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 channe l width although, due to the overbank deposition, channel capacity rem ained constant. (C) 1998 John Wiley & Sons, Ltd.