Lake Pukaki Research Articles

The glacial sedimentology and geomorphological evolution of an outwash head/moraine-dammed lake, South Island, New Zealand

Extensive exposures through the glacial landforms around southern Lake Pukaki, New Zealand, comprise seven lithofacies (LFs 1–7). LFs 1–3 are grouped together as LFA 1 (Pukaki Member) and record pulsed subaqueous grounding line fan progradation, cohesionless debris flows, underflow activity and rhythmite deposition by suspension settling, iceberg rafting of dropstones, and pulsed traction current activity. Localized disturbance of these deposits by glacitectonic deformation and multi-generational hydrofracture fills records minor readvances by the glacier snout and the emplacement of a glacitectonite (LF 4) derived from cannibalization of glacilacustrine sediments. LFs 4–6 are grouped together as LFA 2 (Twizel Member) and record direct glacigenic deposition of glacitectonite (LF 4), subglacial traction till (LF 5) and supraglacially dumped boulder rubble (LF 6). Stratigraphic relationships between LFA 1 with LFA 2 record the oscillatory behaviour of the former Tasman Glacier snout when it formed a calving margin in a proglacial and locally supraglacial lake dammed by a glacitectonically disturbed outwash head and lateral moraine ridges. This is entirely consistent with the landform–sediment record of its coeval terrestrial margins, where flutings and push moraines are diagnostic of active temperate glacier recession from a glacially overridden outwash head, the latter being recorded by the vertically stacked sub-horizontally bedded and coarse-grained gravels of LF 7 (Waitaki Member). Previous proposals that late Pleistocene lake damming was initiated by an ice-cored moraine arc appear unfounded, because the glacilacustrine deposits only lie above the altitude of the outwash head/lateral moraine arc in locations where they have been glacitectonically compressed. Alternatively, it is proposed that the overdeepened subglacial topography was produced by the construction of an outwash head, leading to a glacilacustrine sediment sink which operates at times when the expanded Tasman Glacier actively retreats from the outwash head apex. The changing landsystem imprint related to the shrinkage of the Tasman Glacier records spatio-temporal landsystem change, involving evolution from a coupled landsystem to a moraine-dammed to an uncoupled landsystem.

Precise mapping and 10Be dating of Last Glaciation glacier landforms in the Lake Pukaki basin, South Island, New Zealand, and paleoclimate implications

Precise mapping and 10Be dating of Last Glaciation glacier landforms in the Lake Pukaki basin, South Island, New Zealand, and paleoclimate implications

Agamospermous seed production of the invasive tussock grass Nardus stricta L. (Poaceae) in New Zealand – evidence from pollination experiments

The breeding system of the European tussock grass Nardus stricta L. (Poaceae) was investigated with pollination experiments. Plants were sampled from two populations at Lake Pukaki, Canterbury, New Zealand, where the species is recognised as an alien invader. Bagging of flowers with king-sized cigarette paper and hand-pollination were used to test for three modes of reproduction in the greenhouse: (1) agamospermy (apomixis), (2) autogamy and self-compatibility, and (3) allogamy (outcrossing). Two control groups without experimental treatments were further tested for seed set under (1) greenhouse and (2) field conditions. The success or failure of all experimental treatments was assessed with seed set and germination trials. All agamospermy treatments showed high seed set and germination proportions arguing for an apomictic mode of reproduction in Nardus stricta. Cross-pollination treatments were also successful making it difficult to estimate the degree of outcrossing, selfing, and agamospermous seed production in Nardus stricta. Fecundity in field populations was considerably reduced, possibly due to environmental factors acting upon seed development during maturation. The reproductive strategy of Nardus stricta might be particularly beneficial during invasion because single tussocks can form reproducing colonies and high reproductive output is ensured even in the absence of pollination. Genetic studies in combination with pollination experiments would be necessary to gain deeper insights into the breeding strategy of Nardus stricta.

Invasion ecology of the alien tussock grass Nardus stricta (Poaceae) at Lake Pukaki, Canterbury, New Zealand

The European matgrass Nardus stricta has naturalised in New Zealand, often on damp soils within wetlands and grasslands. In this paper, we present for the first time field data on the ecology of this alien invader in New Zealand, from eight kettle‐hole wetlands on lateral moraine along the western side of Lake Pukaki, South Canterbury. The invaded wetland sites were all acidic but varied in other soil characteristics. Nardus stricta was the most dominant species within these wetland communities with 40% of all plots showing more than 50% coverage, and 21% having more than 90% cover. Species richness (including vascular plants and mosses) at some sites was relatively high (c. 40 species), but species richness and abundance were significantly reduced in quadrats with high Nardus stricta density. Seedling densities of Nardus stricta were high but variable (overall mean of 38.6 ± 116 seedlings per m2), and establishment mainly occurred on the cushion‐forming sedge Oreobolus pectinatus. Most reproductive tussocks (90%) produced up to 1000 florets, but single tussocks were able to produce up to 10000 and more florets. Our results suggest that Nardus stricta is still expanding its local range and, therefore, is likely to increase in abundance and dominance, resulting in further negative effects on native species diversity.

Pollen analysis of Pliocene‐Pleistocene Kowai Formation (Kurow Group), Mackenzie Basin, South Canterbury, New Zealand

Kowai Formation in the Lake Pukaki area of the Mackenzie Basin, is a tectonically deformed gravel sequence containing rare, fossiliferous, fine‐grained horizons with pollen assemblages that are unusual in their composition. The sequence of vegetation types from oldest to youngest is: (1) assemblages with high percentages of Brassospora‐type beech and Casuarina; (2) assemblages with high percentages of araucarian pollen (position uncertain); (3) Fuscospora‐type beech assemblages; and (4) grassland/ scrubland assemblages preceded by a major angular unconformity. The Fuscospora‐type beech assemblages are further divided into sequences where, from oldest to youngest, the paleovegetation is: (a) Fuscospora‐type beech with Brassospora‐type beech and Casuarina; (b) Fuscospora‐type beech with Phyllocladus (position uncertain); (c) Fuscospora‐type beech with a variety of pollen types; (d) Fuscospora‐type beech with podocarps; and (e) Fuscospora‐type beech totally dominates. The age of the beds is Pliocene, as determined by the presence of a number of pollen species that last appeared in the Pliocene, along with a few taxa that first appear at the same time. The youngest grassland/ scrubland assemblages, lacking extinct taxa, may be from fluvioglacial sediments of Pleistocene age. During the early phases of deposition, the climate was warmer and more humid than the present day, as evidenced by the presence of abundant araucarian, Nothofagus (Brassospora) beech and Casuarina pollen. Most of the Kowai Formation was deposited at a time when the area was covered in a Nothofagus (Fuscospora) beech forest, indicative of an interglacial climate not dissimilar from that experienced in present‐day beech forests. From the top of the sequence, evidence of glacial conditions starts appearing as the forest vegetation disappears to be replaced by grassland/scrubland vegetation.

A focused look at the Alpine fault, New Zealand: Seismicity, focal mechanisms, and stress observations

The Alpine fault is the Pacific‐Australian plate boundary in the South Island of New Zealand. This study analyzes 195 earthquakes recorded during the 6 month duration of the Southern Alps Passive Seismic Experiment (SAPSE) in 1995/1996 and two ML 5.0 earthquakes and aftershocks in 1997, which occurred close to the central part of the Alpine fault. Precise earthquake locations are derived by simultaneous inversion for hypocenter parameters, a one‐dimensional velocity model, and station corrections. Together with focal mechanisms calculated using a first motion and amplitude ratio method, these results provide a picture of the seismotectonics in the central South Island over a 6 month period. Moment tensor inversions of three earthquakes provide an independent means of comparison to the focal mechanisms derived using the amplitude/first motion method. To validate our observations over time, we compare the SAPSE seismicity with the seismicity recorded by the New Zealand National Seismic Network (NZNSN) and a local network at Lake Pukaki east of the Southern Alps (6 months versus 8 years). Our study indicates that the Alpine fault releases elastic strain seismically from the surface down to 10–12 km depth between Milford Sound in the south and the Hope fault in the north. The seismicity rate of the Alpine fault is low but comparable to locked sections of the San Andreas fault, with large earthquakes expected. Seismicity decreases north of Bruce Bay at the Alpine fault and within a triangular region along the Alpine fault located between the Hope and Porters Pass fault zones. We interpret this as the result of deformation distributed on the Alpine fault and the Hope and Porters Pass fault zones. The base of the seismogenic zone is fairly uniform at 12 km ± 2 km over large parts of the South Island. The high Alps region has a shallower base of the seismogenic zone, indicating localized elevated temperatures east of the Alpine fault. Most of the study region deforms under a uniform stress field with a maximum principal horizontal shortening direction of 110°–120°, similar to geodetic observations and plate motions. This confirms that the region is not undergoing strain partitioning. The earthquake data show that the deformation away from the Alpine fault is distributed on mainly NNE trending thrust faults and strike‐slip transfer faults with a maximum seismogenic depth of 12 km.

ニュー・ジーランド, プカキ氷河湖の堆積環境

ニュー・ジーランド, プカキ氷河湖の堆積環境

Processes and rates of ice loss at the terminus of Tasman Glacier, New Zealand

The Tasman Glacier is the largest glacier in the Southern Alps of New Zealand. Despite a century of climate warming, the glacier has until recently remained at its “Little Ice Age” terminus position, although there has been substantial downwasting. The lower glacier is covered by an extensive debris layer, which has redistributed ice losses in both space and time compared to the conventional glacier response to climate change. Ice is lost at the terminus through melt of bare ice slopes, melt under the debris layer, and calving of ice into a newly established pro-glacial lake. Over the past ten years, the glacier has evolved from an ablation regime of melt under the debris and around sinkholes, to one where calving into the lake is of greater importance. This study investigates the processes and rates of ice loss at the terminus during the summer of 1995. Melt rates of ice are greatest on bare ice slopes, averaging 96 mm day −1 of ice depth over the summer period. Melt under the debris averaged 7 mm day −1, and calving accounted for a specific ice loss of 125 mm day −1. Overall, calving is the dominant form of ice loss at the terminus, accounting for 73% of the total. Melt under the debris layer accounted for 26% of terminus ablation, and melt on bare ice slopes just 1%, due to the relatively small area covered by these. Ice loss at the terminus is therefore largely de-coupled from climatic influences, and due mainly to the effect of the pro-glacial lake. However, when water loss from the whole ablation zone of the glacier is considered, calving accounts for just 4% of ice loss, with the largest portion of ice loss (80%) coming from melt on bare ice upstream of the debris cover. A total amount of 21 million m 3 a −1 of water is estimated to be supplied to Tasman River due to ice loss from the lowest 4 km 2 of glacier. 135 million m 3 a −1 of water is estimated to be lost below Ball Hut (approximately 10 km up-glacier), with 20% of this being calved from the terminus. This proportion will increase as the glacier terminus retreats up the valley, but at present rates, it will take over a century for the glacier to retreat to Ball Hut. Water stored as ice and then released from Tasman Glacier is resulting in a mean annual flow into Lake Pukaki of 4.3 m 3 s −1. The water released from all glaciers in the region due to glacier ice loss is 6% of the annual inflow to Lake Pukaki.

Application of natural and artificial fallout radionuclides to determining sedimentation rates in New Zealand lakes

Sedimentation rates have been estimated for New Zealand Lakes Rotoiti, Taupo, Ngahewa, Tikitapu, Okareka, Pukaki, Rotorua, Waikaremoana, Wakatipu, Tekapo, and Ohau, and also the Falls Dam, Central Otago, the Pauatahanui inlet (Wellington), and Lake Vanda (Antarctica). Sedimentation rates were able to be calculated using the 210Pb and 210Po methods in 26 out of 28 cores. For the 137Cs method (shown equivalent to the previous methods) calculable rates were obtained in 13 out of 19 cores. An age estimation was always possible by one of the methods. Values of 147Pm and 144Ce showed that most surface layers were undisturbed, hence the hypothetical mixing often invoked to explain 137Cs profiles lacking the expected 1965 peak is incorrect for most of these cores. Deficits in surface layer 210Pb caused by 222Rn loss did not affect the estimates of sedimentation rate. One core contained a record dated back to 1800 A.D. Deposition rates of 210Pb depended on rainfall, not river input. Some local geothermal activity emits 222Rn and deposits more 210Pb than usual for New Zealand lakes, but otherwise absolute 210Pb levels are generally very low compared to other studies. Deposition rates varied from a low of 0.11 kg m‐2 yr‐1 (Lake Tikitapu) to a high of 28 kg m‐2 yr‐1 (one Lake Pukaki core).

Crustal structure in the central South Island, New Zealand, from the Lake Pukaki seismic experiment

The crustal structure of the central South Island, New Zealand, has been investigated using explosion seismology measurements. A series of 114 shots were fired in Lake Pukaki and recorded in three modes: on a 120 channel, 6 km long, seismic reflection array rolled along a 27 km profile on the eastern margin of Lake Pukaki; on a Reftek seismograph array consisting of 40 units spread over a 52 km long line, partially coincident with the reflection profile; and wide angle reflections from the lower crust recorded on permanent stations of the New Zealand Seismograph Network that were located between 80 and 120 km from the shots. The data show that the greywacke‐schist forming the mid‐upper crust of the Pacific plate is c. 25 km thick near the east coast and thickens as a crustal root to >35 km beneath the Southern Alps. A strong reflection was recorded from a depth of 25 km and is interpreted to be from the base of the greywacke‐schist crust overlying an old oceanic crust. Below 6 km depth the compressional seismic velocity is relatively uniform and increases only slightly from 6.2 km/ s to 6.25 km/s at the lower crustal interface. A relatively large velocity gradient of 0.22 (km/s)/km is derived for the upper 6 km of the greywacke basement. Whether this velocity gradient, derived from a relatively short profile, is regional in extent needs to be verified. A Poisson's ratio of 0.21 ± 0.03 was determined from apparent velocities of Pg and Sg for the greywacke/schist and is interpreted to indicate fracturing in this layer. Reflections, inferred to be from segments of the downdip extension of a broad Alpine Fault Zone, occur at depths of c. 22 and 28 km and indicate a dip of 33 ± 5°. A width of 7.5 km is estimated for the fault zone at this depth. The dipping reflector segments would intercept the surface west of the Alpine Fault, and suggest that the fault zone is steeper at shallower depths. The total amount of material, which has been subjected to uplift, has been estimated from the amount of crustal shortening and the shape of the crustal root. These estimates indicate that 50% of this material has not been accounted for in previous studies, and suggests either a non‐exponential distribution, or larger rates of uplift southeast of the Alpine Fault than thought previously.

Dryland Holocene vegetation history, Central Otago and the Mackenzie Basin, South Island, New Zealand

Pollen and charcoal analyses are presented from three south‐central South Island Holocene age deposits. A spring bog site in the Idaburn Valley records small‐leaved Olearia scrub and grassland in its immediate vicinity from c. 7100 yr B.P., when peat growth began, to 5000 yr B.P. After that date the valley bottom vegetation became increasingly short and open, and the bog ceased growing, probably as a result of increasingly droughty summers. The adjacent hilly country supported a forest/scrub cover of Podocarpus hallii, Phyllocladus alpinus, Halocarpus bidwillii, and small‐leaved shrubs. A site in the Mackenzie Basin near Lake Pukaki recorded near total dominance by Phyllocladus alpinus scrub from c. 8000 yr B.P. until 5000 yr B.P. after which time Halocarpus bidwillii, Aciphylla, and grassland became increasingly important in response to drought and local fires. At a third site, again in the Mackenzie Basin, Halocarpus bidwillii formed a complete scrub cover at the time of Maori settlement at about 800–600 yr B.P. but fire then rapidly reduced the scrub to grassland. On the evidence of these and other southern South Island vegetation history records, the early Holocene appears to have had a relatively stable climate with moist summers. From 5000 yrB.P. on, evidence for drought and fire point to drier summers and unstable, ENSO‐affected climates.

Proglacial glaciotectonic deformation associated with glaciolacustrine sedimentation, Lake Pukaki, New Zealand

Proglacial glaciotectonic deformation associated with glaciolacustrine sedimentation, Lake Pukaki, New Zealand

Proglacial glaciotectonic deformation associated with glaciolacustrine sedimentation, Lake Pukaki, New Zealand

The Quaternary glaciogenic sediments exposed on the southwest shore of Lake Pukaki were investigated. The sections consisted of the Pukaki Diamicton, which is composed of four lithofacies: (i) homogeneous facies (PDH)—a grey matrix-supported homogeneous subglacial diamicton; (ii) coarse facies (PDC)—a very coarse matrix-supported diamicton, which was interpreted as a proximal glaciolacustrine sediment; (iii) laminated facies (PDL)—a cream coloured, fine-grained, matrix-supported diamicton, with grade laminations of silt, sand and gravel, interpreted as a more distal glaciolacustrine facies; and (iv) fine facies (PDF)—a cream coloured fine-grained, silt-rich matrix-supported diamicton, with lenses of sand and gravels, which was interpreted as the most distal glaciolacustrine facies. It is suggested that these sediments were produced by two small ice advances during a period of general retreat. Furthermore, the sections showed a combination of three types of glaciotectonic deformation; gravity tectonics, proglacial glaciotectonics and subglacial glaciotectonics. Two of the moraines showed an unusual style of glaciotectonic deformation, i.e. proglacial deformation on the proximal face and gravitational slumping on the distal face. It is suggested that this style of deformation is diagnostic of proglacial deformation into a waterbody associated with a retreating margin.

An estimation of temperature changes near Lake Pukaki, South Canterbury, since the enlargement of the lake

Abstract Evidence of a change in the thermal climate about Lake Pukaki (South Canterbury, New Zealand) since the lake was enlarged in 1979 is determined by several different means. Estimation of lapse rate structure enables height-corrected temperature estimates, before and after the enlargement, from a single station above the lake. The lake has cooled its immediate thermal environment in summer and warmed it in winter. There has been a decrease in mean daily range of approximately 1°C in most months, except July and August, when small increases in range occurred.

886012 Reservoir-induced seismicity at Lake Pukaki, New Zealand : Reyners, M Geophs J R Astr Soc V93, N1, April 1988, P127–135

886012 Reservoir-induced seismicity at Lake Pukaki, New Zealand : Reyners, M Geophs J R Astr Soc V93, N1, April 1988, P127–135

Reservoir-induced seismicity at Lake Pukaki, New Zealand

SUMMARY The Pukaki microearthquake network was installed in 1975 to monitor possible changes in seismicity accompanying the raising of the water level of Lake Pukaki for hydroelectric power generation. During the period 1976-79, the depth of the lake was increased by 37 m, thereby adding 4.9 × 109 m3 to its volume. An analysis of 8 1/2 years of data from the microearthquake network has revealed widespread changes in seismicity which correlate with groundwater changes. During the period when groundwater level was higher than any previous maximum, there was both an increase in seismicity rate within 15 km of the lake and a decrease in the background (i.e. 15-50 km from the lake). The largest earthquake of the sample, the ML= 4.6 event of 1978 December located 10 km northwest of the Pukaki High Dam, occurred during this period. When groundwater level stabilized, the seismicity rate near the lake returned to normal, while there was a significant increase in both seismicity rate and moment release in the background. Strike-slip failure currently predominates in the Pukaki region, and the water load added to the lake will tend to stabilize such a stress regime. The increased seismicity near the lake during the period when groundwater level was rising clusters around the periphery of the lake, rather than directly beneath it. This suggests that earthquakes are induced only when an increase in pore pressure has diffused beyond the region where loading stresses predominate. The close correspondence in time of the period of rising groundwater and the period of increased seismicity near the lake, and the fact that they both lag the period of rising lake level by approximately one year, indicate that pore-pressure diffusion plays an important role in inducing seismicity near the lake. However, it is difficult to explain seismicity changes in the background in terms of pore-pressure changes, and these may be a result of stress redistribution following induced seismicity near the lake.

Effect of rainfall on pedogenesis in a climosequence of soils near Lake Pukaki, New Zealand

Abstract Four soils were sampled along a 42 km transect near Lake Pukaki, South Island, New Zealand, to study the physical, chemical, and mineralogical changes that have occurred under mean annual rainfalls of 640 mm at profile 1 to 2000 mm at profile 4. Sampling sites were chosen to minimise difference in soil-forming factors other than rainfall. All soils were formed in loess, ranging in thickness from 40 to 76 cm, over till. The marked differences found in soil properties of the four profiles illustrated the major effect rainfall has in determining the rates of leaching, weathering, and podzolisation. Leaching of bases, acidity, and exchangeable aluminium were found to increase markedly with rainfall. The main differences occurred between profiles 1 and 2, which reflected the marked soil moisture deficit and low soil water surplus characteristic of profile 1. A progressive increase in weathering was shown by increasing profile development and clay content with rainfall, by increasing conversion of total aluminium and iron to readily extractable forms, and by changes in clay mineralogy. Soil profiles showed progressive development of podzol morphology with increasing rainfall. Horizons identified in profile 1 mainly reflected accumulation of organic matter and weak weathering whereas profile 4 had a distinct eluvial horizon and evidence of accumulation of sesquioxides and humus in B horizons. This trend of increasing podzolisation was substantiated by chemical evidence.

Phytoplankton periodicity of the Waitaki lakes, New Zealand

The Waitaki River system in the South Island of New Zealand includes three large glacially-formed headwater lakes, Tekapo, Pukaki and Ohau, which drain into the manmade Lake Benmore. Phytoplankton periodicity was followed from December 1975 to January 1980 as part of a study investigating possible changes in these lakes as a consequence of hydroelectric development. The phytoplankton was highly dominated by diatoms, e.g., Diatoma elongatum, Cyclotella stelligera, Asterionella formosa, and Synedra acus, but in lakes Ohau and Benmore populations of green algae occasionally developed. In all four lakes seasonal phytoplankton periodicity was observed with maximum biomass in spring and summer. In Lake Tekapo, the first lake in the chain, maximum biomass did not exceed 300 mg m-3, but in the very turbid Lake Pukaki the maximum summer biomass ranged between 300 and 800 mg m-3. In Lake Ohau, the least turbid lake, maximum biomass was around 1 000 mg m-3. In the newly created Lake Benmore periodicity was less evident and summer maxima reached over 1 500 mg m-3. The phytoplankton periodicity in these lakes is greatly influenced by seasonal patterns of turbidity from inflowing glacial silt.

Microseismicity of the upper Clutha valley, South Island, New Zealand

Abstract The spatial distribution of microseismicity observed during a 16-day survey of the upper Clutha valley is broadly similar to that of the longterm seismicity, All but one of the microearthquakes were located within the crust, with the proportion of lower crustal events being greater than in the Lake Pukaki region to the northeast. The single subcrustal event, located at 73 km depth, exhibited a definite phase between the P- and S-phases, similar to the S to P conversion at the base of the crust recently identified in records of 2 subcrustal earthquakes west of Lake Pukaki. A composite focal mechanism for all the crustal microearthquakes indicates strike-slip faulting with a P-axis acting southeast-northwest. This mechanism is very similar to that found previously for crustal microearthquakes to the west and northeast of the study area.

Pukaki earthquake of 17 December 1978

Lake Pukaki is a natural lake but has been raised twice to provide water storage for power stations constructed or planned for the Upper and Middle Waitaki Hydro Schemes. The lake was first raised 9 metres in 1955 to give a depth of 71.5 metres and a volume of about 5 x 109 m3. A second dam, the Pukaki High Dam, was completed in 1978 to raise the lake a further 37 metres and add 5.5 x 109 m3 to the volume. This will create a lake with a maximum depth of 108 metres and a volume of more than 10 x 109 m3 .
 In accordance with the recommendation of the 'UNESCO Working Group on Seismic Phenomena Associated with Large Reservoirs' (Adams, et al, 1973), the New Zealand Electricity Division of the Ministry of Energy installed a seismic network around Lake Pukaki comprising 8 vertical and 1 three-component seismometers. This has been operating continuously since June 1975.