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.