Phillips, Nelson: Sedimentation in an artificial . . .

New Zealand Journal of Marine and Freshwater Research abstracts

Sedimentation in an artificial lake -Lake Matahina, Bay of Plenty

CHRISTOPHER J. PHILLIPS*

CAMPBELL S. NELSON

Department of Earth Sciences University of Waikato Private Bag Hamilton, New Zealand

Abstract Lake Matahina, an 8 km long hydroelectric storage reservoir, is a small (2.5 km²), 50 m deep, warm monomictic, gorge-type lake whose internal circulation is controlled by the inflowing Rangitaiki River which drains a greywacke and acid volcanic catchment. Three major proximal to distal subenvironments are defined for the lake on the basis of surficial sediment character and dominant depositional process: (a) fluvial-glassy, quartzofeld-spathic, and lithic gravel-sand mixtures deposited from contact and saltation loads in less than 3 m depth; (b) (pro-)deltaic-quartzofeldspathic and glassy sand-silt mixtures deposited from graded and uniform suspension loads in 3-20 m depth; and (c) basinal-diatomaceous, argillaceous, and glassy silt-clay mixtures deposited from uniform and pelagic suspension loads in 20-50 m depth. The delta face has been prograding into the lake at a rate of 35-40 m/year and vertical accretion rates in pro-delta areas are 15-20 cm/year. Basinal deposits are fed mainly from river plume dispersion involving overflows, interflows, and underflows, and by pelagic settling, and sedimentation rates behind the dam have averaged about 2 cm/year. Occasional fine sand layers in muds of basinal cores attest to density currents or underflows generated during river flooding flowing the length of the lake along a sublacustrine channel marking the position of the now submerged channel of the Rangitaiki River.

Keywords Lake Matahina; lacustrine environments; lacustrine sediments; lacustrine sedimentation; sedimentation rates; hydro-electric storage lakes.

New Zealand Journal of Marine and Freshwater Research, 1981, Vol. 15:459-473 Received 29 May 1981

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