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Contemporary Floodplain Process |
1 Geomorphology Laboratory, Indiana State University, Terre Haute, IN 47809, USA bgomez{at}indstate.edu
2 Landcare Research Ltd, PO Box 11052, Palmerston North, New Zealand
3 National Institute of Water and Atmospheric Research, PO Box 8602, Christchurch, New Zealand
4 Gisborne District Council, PO Box 747, Gisborne, New Zealand
5 Landcare Research Ltd, PO Box 69, Lincoln, New Zealand
Rapid vertical accretion on the Waipaoa River floodplain is conditioned by the river’s high suspended sediment load (30 000–40 000 mg l–1 at flood stage). Cumulative sediment accumulation curves derived from three cores suggest an average (post-1850) rate of vertical accretion of c. 60 mm a–1, though a 15 year lacuna in flood activity has depressed the post-1948 rate to c. 40 mm a–1. Rates of aggradation during floods are several orders of magnitude larger than the time-averaged rate. Within a 44 km long reach, cross-section surveys indicate that 0.2–0.8 m of sediment was deposited between 1979 and 1990. Over this period floodplain storage accounted for 5% of the total suspended sediment load, and 16% of the suspended sediment load transported during events that exceeded bankfull stage. The Waipaoa River floodplain may be representative of floodplains bordering rivers with high suspended sediment loads, produced by rapid, episodic vertical accretion, on which overbank deposition occurs across the entire floodplain, and is complemented by channel aggradation. Such rivers are able to construct high banks. Thus channel capacities are greater and the incidence of overbank flows is less than in rivers where overbank deposition is slow relative to the rate of floodplain destruction by lateral migration. The difference between our time-averaged estimate for sequestration on the Waipaoa River floodplain and comparable estimates for actively meandering rivers, and meandering rivers with low sediment loads, reinforces the notion that there is a link between the sediment transport regime of a river and its sedimentary record. To elucidate this link it is necessary to view vertical accretion in the context of the flood events that generated it, rather than in the context of a time-averaged sediment budget.
