J. Irwin

Predominant headwater inflow and its control of lake‐river interactions in Lake Wakatipu

Abstract Temperature and suspended particulate matter concentrations are used to identify lake‐river interactions in Lake Wakatipu. Results from sampling at seasonal maxima and minima of water temperature and inflow, in conjunction with density estimates, suggest that both annual and diurnal cycles from inflow from the predominant headwaters are generated. From mid autumn to mid spring, underflows predominate; in summer, river water may warm sufficiently to allow inflowing water to interflow or overflow. In summer the large diurnal temperature range in the rivers sets up a diurnal cycle of inflows, with underflow in the early morning, progressing through to interflows and finally overflows in the middle of the day, before returning to underflows again at night. In winter, when river temperatures remain low throughout the day, inflowing water underflows continuously. Floods normally enter as turbid underflows, disrupting the diurnal summer cycle and strengthening the underflows of winter. Flood underflows ...

Observations of temperatures in some Rotorua district lakes

Summary A preliminary study has been made of temperatures in some of the lakes of the Rotorua district, in the North Island of New Zealand. Seasonal and diurnal variations are discussed. Two main thermal structure types are present. One, in which stratification is irregular or absent, is present in lakes that are relatively shallow and large in area, and the other, in which there is thermal Stratification from early summer to early winter, is present in lakes that are comparatively deep and large in area or else of medium depth and size.

Sediments of lake Pukaki, South Island, New Zealand

Abstract Lake Pukaki, fed by glacial melt water, is turbid all year round and is subject to an average of four major floods each year. Lake level is controlled by a dam at the south end of the lake. Bottom surface sediments are fine silt and clay, and these become finer down lake; the coarsest sediments are from the delta area and from close to the outlet. Analyses show the samples to contain rougly equal amounts of quartz and albite (43%), and clay minerals. Short cores (47 cm and 15 cm) show alternating light (silt) and dark (clay) layers. It was not determined whether these rhythmic layers are annual deposits, or controlled by flood conditions.

Bottom sediments of Lake Tekapo compared with adjacent Lakes Pukaki and Ohau, South Island, New Zealand

Abstract Bottom sediments in Lake Tekapo are mainly silty clays which become progressively finer grained away from the main sediment supply, the glacially‐fed Godley River. 57% of the sediment samples have over 50% of clay size material. A comparison of morphometric data, sediment source areas, turbidity and bottom sediments is made with nearby Lakes Pukaki and Ohau which are also fed by glacial melt water.

Morphology and sedimentation in lake Wakatipu, New Zealand

Summary Lake Wakatipu occupies an overdeepened glacial trough dammed by former terminal moraine. The steep subaerial slopes above the lake continue steeply below the surface of the water to a plane floor that is horizontal in the central and most of the southern sections and slopes longitudinally in the northern section. Below the general level of the sediment surface, a system of current channels has been developed, extending from the delta of the Dart and Rees Rivers to the flat‐floored sedimentary basin in the central and southern section. Active deposition of sediment from density currents is taking place; the action of such currents is discussed.

Water temperature and turbidity in glacially fed Lake Tekapo

Abstract Inflow into Lake Tekapo is dominated by the glacially‐fed headwater Godley River. Measurements of water temperature and transmissivity at periods of seasonal maxima and minima provide data on dispersion of river water entering the lake. During spring, lake waters warming from isothermal winter conditions receive turbid cold meltwater which interflows or underflows down‐slope to the deepest basin to pond against the rising lake floor. Waters stratify weakly in summer, and turbid inflowing water interflows. In winter, near isothermal lake water receives cold clear water underflowing to the deepest basin. In all seasons inflowing water is deflected towards the eastern side of the lake by Coriolis force. Diurnal changes in inflow across the Godley delta in spring are complex, with interflow and overflow influenced by heating of waters flowing over wide, braided river channels. In winter, underflows are strongest in early morning when inflows are coldest, and they weaken through the day as river water...

Sublacustrine channels in Lake Wanaka

Sublacustrine channels off the two major inflowing rivers in Lake Wanaka are described. They are compared with, and found to be similar to, channels in nearby Lake Wakatipu. The channels are probably caused and maintained by density flows from the turbid, inflowing river waters.

Water clarity records from twenty‐two New Zealand lakes

Abstract Water clarity values from 22 New Zealand lakes show that the largest and deepest have the highest values, except those affected by glacial silt or humic coloration. Smaller and shallower lakes have lower water clarity values.

Morphological stability of pauatahanui inlet, porirua harbour

Abstract Vertical aerial photographs made over a 31‐y period (1942–73) show only a few small changes in channel and bank morphology of the inlet; the changes apparently occur more slowly than in the Avon‐Heathcote Estuary.

Shallow‐water sand bars on the Ruamahanga River delta Lake Wairarapa

Abstract In the shallow waters of Lake Wairarapa, a multiple series of sublimnic sand bars has developed around the outer edge of the Ruamahanga River delta. Shore‐normal movements in bar positions occur in response to changing lake level and wave conditions. Unlike many bar systems, most of these movements are lakeward of the breaker zone. Sediments on the bar crests are well‐sorted fine sands; the troughs are a mixture of fine sand and mud settling out of the turbid lake waters during calm conditions.