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堆 積 学 研 究,51号,55-66,2000 J.Sed.Soc.Japan,No.51,55-66,2000

Sedimentary environments in Lake Pukaki, New Zealand

Augustine K. Chikita*, Ian Halstead** and Glenn Carter***

Sedimentary conditions in glacier-fed Lake Pukaki (20km long, 5km wide, and 98.0m deep at maximum), New Zealand were examined in the glacier-melt season of 1998. Spatial distributions of water temperature and suspended sediment concentration (SSC) obtained by a TTD (temperature-turbidity-depth) profiler indicate that sediment-laden underflow initiated near the river mouths is relatively weak because of their vertical low SSC on the gentle slope, and that the underflow is bifurcated into suspension interflow and other underflow at midlake ca. 1500m distant from the river mouths. The continu- ous measurement of water temperature at some depths revealed that at midlake, the suspension interflow and underflow continuously occur about one day at least.

Key words: glacier-fed lake, sedimentation, suspension flow, sediment-laden underflow, river sediment discharge

Tekapo, New Zealand by analyzing seismic refle- INTRODUCTION ction and sedimentary facies of surface sediment There exist some glacier-fed lakes on the and cores. southeast mountainside in the South Island, New In this study, characteristic behaviors of sus-

Zealand. Of them, Lake Pukaki has the largest pension flow in Lake Pukaki are described by glacier cover (19.0%) in the drainage basin. Sed- obtaining spatial distributions of temperature imentation in the lake could thus be dominated and suspended sediment concentration of lake by glacier-melt sediment discharge of inflowing water and by analyzing grain size of surface

Tasman River in summer (cf. Pickrill and Irwin, sediment. 1983). As representative hydrodynamic agents STUDY AREA AND METHODS of sedimentation in glacier-fed lakes, suspension overflow, suspension interflow and/or sediment- Lake Pukaki,South Island, New Zealand (44•‹4' laden underflow have been observed commonly S, 170•‹10'E ; surface area, 150km2 at mean lake (Gilbert,1975; Sturm and Matter, 1978; Weirich, level of 525m asl) receives large water supply in 1986; Chikita, 1992; Chikita et al., 1996; Chikita summer, due to ice-melt and snowmelt in et al., 1999). Pickrill and Irwin (1983) revealed Tasman Glacier, Hooker Glacier, Mueller Glacier, the sedimentary structure in glacier-fed Lake and other smaller drainage basins (Fig. 1). The

largest inflowing river, Tasman River, then pro-

Received: April 6, 2000, Accepted: April 28, 2000 vides the sediment discharge to the lake, and * Division of Earth and Planetary Sciences , Graduate consequently has developed the outwash plain School of Science, Hokkaido University, Sapporo 060- 0810, Japan: by delta progradation at the uplake end (Fig. 1). ** Environmental Data Division , National Institute of The total glacial cover occupies 19.0% of the Water and Atmospheric Research (NIWA) Ltd., P.O. drainage area (1027km2, excluding the lake sur- Box 29, Tekapo, New Zealand ***National Institute of Water and Atmospheric Re- face). Ice-contact lakes, dammed up by end mo- search (NIWA) Ltd., P.O. Box 8602, Christchurch, raines and/or outwash heads, exist on the termi- New Zealand nals of Tasman, Hooker, Mueller, and Murchison 56 Augustine K.Chikita,Ian Halstead and Glenn Carter 2000

Fig. 1 Location and drainage basin of Lake Pukaki. The terminals of Tasman, Hooker and Mueller Glaciers follow aerial photographs taken in December 1994 (Department of Survey and Land Information, 1996).

Glaciers (Fig. 1; Warren and Kirkbride, 1998). flow of lake water thus possibly have no high These lakes seem to play a role as settling ponds suspended sediment concentration (probably, of suspended sediment produced from the gla- the order of 100mg/L at most), though the ciers (e.g., see Chikita et al., 1999). Tasman fluvial entrainment of deposits could occur on River and Hooker River initiated by the over- the outwash plain where Tasman River is J.Sed.Soc.Japan,No.51 Glacier-fed Lake Pukaki,New Zealand 57

braided. 3 is probably due to the boat's drift during the

Sediment infilling in Lake Pukaki prevails in water sampling.

half the lake basin proximal to the mouths of the Bulk density, ƒÏTC, of lake water at suspended braided river, as shown by the bathymetry in sediment concentration, C (mg/L), and tempera-

Fig. 2. The development of the subaqueous ture, T (•Ž), was calculated by the following delta suggests (1) that the lake level greatly equation. varies throughout the year, (2) that the sediment ƒÏ TC-(1-C•~10-3/ƒÏs)ƒÏT+C•~10-3, (1) input to the lake prevails in the season of the low where ƒÏT is the pure water density in kg/m3 at lake level (possibly, early spring when snowmelt temperature, T (•Ž), and Ps is the sediment densi- begins), and (3) that suspension flows continu- ty (= 2779 kg/m3 for Pukaki). Finally, lake- ously occur far away to the subaqueous delta water density, ƒÏPTC (kg m-3) at water pressure, P, front (near a line of site D2 to site D1 in Fig. 2). was calculated by adopting the compressibility

In order to examine sedimentary conditions in of 0.00045 per equivalent pressure (P=980000 Pa) the lake in the glacier-melt season, vertical pro- of 100m depth at an observed temperature range files of water temperature and suspended sedi- of 10 to 15•Ž. The water pressure, P, at a water ment concentration were obtained every 0.2m in depth, z, is given by the following equation. depth at 13 points in Fig. 2 on 10 and 11 January (2) 1998, using a TTD profiler (Alec Electronics Co.,

Ltd, Japan; model ATU-200; accuracies of •} where g is the gravitational acceleration, ƒ¢zi=zi

0.05•Ž, •}4ppm and •}0.22m for temperature, -zi -1 is the interval between two water depths, zi turbidity and water depth, respectively). The and zi-1, ƒÏTCi-(ƒÏTCi+ƒÏTCi-1)/2, ƒÏTCi is the water profiler was lowered from a boat positioned by a density at zi, and ƒÏTC0 is the water density at the portable GPS. We then spent 3 to 9 mins to get surface, Z0 (=0). vertical profiles at each point. The water Continuous measurements of water tempera- turbidity measured was converted into sus- ture were carried out at 15min intervals for a pended sediment concentration (SSC), using a period of 7 to 11 January 1998 at depths of 10.5m, relation between turbidity and SSC (Fig. 3). 20.5m, 30.5m, 37.5m, and 38.2m of site D by moor-

The SSC was obtained by filtering lake-water ing temperature data loggers (ONSET Computer samples with millipore filters of 0.45ƒÊm opening. Inc., U.S.A.; accuracy of •}0.2•Ž). At sites A, C,

The water sampling was performed with a Van D, F and G, bottom sediment was sampled by the

Dorn water sampler on a boat immediately after Ekman-Birge grab sampler. The sediment was the vertical measurements of water turbidity. analyzed for grain size by the gravitational set-

The relatively low correlation of r=0.6810 in Fig. tling method (grain size, d•…<44ƒÊm) and sieving (d

Fig. 2 Location of observation sites on the bathymetric map of Lake Pukaki, made by NIWA, New Zea- land. The water depths (m) as shown by isoplethic lines correspond to those at a lake level of 524m asl. 58 Augustine K. Chikita, Ian Halstead and Glenn Carter 2000

Fig. 3 Relation between water turbidity (ppm) by the profiler and simultaneous suspended sediment concentration (mg/L) from water sampling.

>44ƒÊm). tion (SSC) of Tasman River and Hooker River

The water discharge of Tasman River to Lake upstream of their confluence was comparable to

Pukaki was estimated by water budget of the each other at the roughly same time (Table 1). lake, because it is difficult to measure in the The SSC and water temperature seem to de- braided river (Figs. 1 and 2). SSC of inflow-river crease with decreasing natural river inflow (see water was obtained from manual water sam- Chikita et al., 1998). SSC and water discharge of pling at four points (black circles in Fig. 1). rivers except the glacial rivers is likely to be

negligibly small as shown by those of Jacks RESULTS AND DISCUSSION Stream in Table 1 (see Fig. 1 for location). SSC

Sediment Discharge of Tasman River of braided Tasman River flowing into Lake

Figure 4 shows water level and natural river Pukaki is possibly comparable to that in up- inflow of Lake Pukaki at 1 h intervals for a stream Tasman and Hooker and at their confl-

period of 0100h, 1 January to 0000 h,1 February uence, because the latter SSC could increase by 1998. The "natural river inflow" here indicates erosion of fluvial deposit, but decrease by the river water inflow from the natural drainage clear water inflow (SSC•`0mg/L) of Jollie River

basin of the lake, mostly that from Tasman (see Fig. 1). Total sediment discharge of

River (Fig. 1). This inflow was estimated by Na- braided Tasman River was thus estimated at 74.7

tional Institute of Water and Atmospheric Re- kg/s and 43.7kg/s, using the values of upstream

search (NIWA), New Zealand, adopting the water Tasman and the confluence in Table 1, respec-

budget of the lake. It is, however, under- tively. During our observation, sediment dis-

estimated because the evaporation from the lake charge of braided Tasman River was relatively

surface is neglected. As a result, the inflow of biased to the left coast of Lake Pukaki (see the

Tasman River to Lake Pukaki for the observa- arrows in Fig. 2). This was also judged by flow

tion period (7 to 11 January 1998) ranged from ca. patterns of sediment plume (suspension over-

100 to 300m3/s. Suspended sediment concentra- flow) on the lake surface starting from the delta J.Sed.Soc.Japan,No.51 Glacier-fed Lake Pukaki,New Zealand 59

Fig. 4 Time series of water level of Lake Pukaki and natural river inflow to the lake, described at 1 h intervals.

Table 1 Water temperature and suspended sediment concentration (SSC) of river water, and simultaneous natural river inflow to Lake Pukaki. The "confluence" means that of Tasman and Hooker Rivers . The "upstream" indicates sampling points upstream of the confluence (see Fig. 1 for location).

* water discharge

front. calculated from Eqs. (1) and (2).

It is seen that river sediment discharge pro-

Behaviors of suspension flow duced relatively cold, turbid and heavy water

Figure 5 shows longitudinal distributions of (temperature of 6 to 11•Ž, SSC of 20 to 85mg/L,

water temperature, suspended sediment concen- and a of -3 to 0.5) in a region proximal to the river

tration (SSC), and water density in situ, ƒÐ, ob- mouths (between site A and site B), and that

tained at 0940h-1444h on 10 January 1998. ƒÐ sediment-laden underflow started near site B

was obtained by ƒÐ=(ƒÏPTC-1000)•~10, using ƒÏPTC after plunging. Meanwhile, relatively cold, 60 Augustine K.Chikita,Ian Halstead and Glenn Carter 2000

Fig. 5 Longitudinal distributions of water temperature, suspended sedi- ment concentration, and density in situ, a, obtained at 0940h to 1444h of 10 January 1998. J.Sed.Soc.Japan,No.51 Glacier-fed Lake Pukaki,New Zealand 61

Fig. 6 Crosssectional distributions of (a) water temperature (•Ž), (b) suspended sediment concentration

(mg/L), and (c) density in situ, a, obtained at sites D2 to Dl at 1240h to 1330 h of 10 January 1998. Interval: (a) 0.25•Ž, (b) 5mg/L, and (c) 0.5.

Fig. 7 Crosssectional distributions of (a) water temperature (•Ž), (b) suspended sediment concentration

(mg/L), and (c) density in situ, ƒÐ, obtained at sites E2 to E1 at 1125h to 1218h of 10 January 1998.

Interval: (a) 0.25•Ž, (b) 5mg/L, and (c) 0.5.

Fig. 8 Longitudinal distributions of (a) water temperature (•Ž), (b) suspended sediment concentration

(mg/L), and (c) density in situ, ƒÐ, obtained at sites D to G at 0912h to 1149 h of 11 January 1998. Interval: (a) 0.5•Ž, (b) 5mg/L, and (c) 0.5. 62 Augustine K.Chikita, Ian Halstead and Glenn Carter 2000

turbid and heavy water (temperature of 10 to 20mg/L and temperature of 11.5 to 12•Ž was

11•Ž, SSC of 20 to 30mg/L, and u of -1 to 1.5) was induced at depths of 15 to 35 m at near site D2, present also near the bottom between site D and moving downstream limited to the left side, and site G. At site D, there is no bottom turbid layer that sediment-laden underflow (SSC of 20 to 25 of more than 20mg/L, but at site D2 (Fig. 6b). mg/L) was simultaneously initiated from near

These indicate that the sediment-laden under- the bottom of sites D2 to D1. The behavior of flow with SSC of more than ca. 20mg/L contin- suspension interflow limited to the left side is

ues up to the deepest region. probably due to the sediment discharge of

River-induced suspension overflow seems to braided Tasman River, biased to the left side

have occurred simultaneously between sites A (Fig. 2). The production of the interflow and and E, as shown by the isopleths of 13•Ž and 40 underflow at near site D2 seems to result from

mg/L. The ƒÐ pattern indicates that density the bifurcation of sediment-laden underflow

stratification in the lake is developed as a whole, dynamically induced by the bottom topography

especially at near the river mouths and the lake of the steep slope (subaqueous foreset slope) (see

surface. the bathymetry in Fig. 2); the sediment-laden

Figures 6 and 7 shows crosssectional distribu- underflow thus continued from near site B to the

tions of (a) water temperature (•Ž), (b) SSC (mg/ deepest region (Fig. 5). The bifurcated sedi-

L), and (c) ƒÐ, obtained at sites D2 to D1 at 1240h- ment-laden underflow could be strengthened on

1330h and at sites E2 to E1 at 1125h-1218 h on 10 the steep slope near site D, since its driving force,

January 1998, respectively (see Fig. 2 for loca- F, is given by F=(ƒÏTC1-ƒÏTC2) gh•Esin ƒÆ under con-

tion). The patterns of SSC and temperature in- dition of uniform flow and same water pressure,

dicate that suspension interflow with SSC of ca. where ƒÏTC1 is the average density of underflow,

Fig. 9 Time series of water temperature at depths of 10.5m, 20.5m, 30.5m, 37.5m, and 38.2m of site D, described at 15min intervals. The two arrows indicate the times of vertical measurement at site D (Fig. 10). J. Sed. Soc. Japan, No. 51 Glacier-fed Lake Pukaki, New Zealand 63

ƒÏ TC2 is the surrounding water density, h is the downlake of the crosssection of site D2 to site D

flow thickness, and Į is the slope angle. The 1 (Fig. 2), as far as sediment-laden underflow is

bottom topography of the steep slope causes an continuously generated by the glacier-melt sedi-

increase in the downslope component of the ment discharge. At site G, however, the bottom

gravitational acceleration. SSC decreased from 30mg/L to less than 20mg/

Figure 8 shows longitudinal distributions of L (see Figs. 5 and 8b). This means that sediment

(a) water temperature (•Ž), (b) SSC (mg/L), and (c) deposition occurred at the deepest point or

between sites D and G, obtained at 0912 h-1149 ƒÐ around, being faster than the rate of advective

h on 11 January 1998. A comparison with the sediment transport from the uplake; this rapid

results of the previous day (Figs. 5, 6 and 7) deposition is probably due to the loss of the

indicates that suspension interflow with SSC of kinetic energy of sediment-laden underflow.

ca. 20mg/L occurs also at near site D, and that Relatively warm suspension overflow seems to

relatively thick sediment-laden underflow with have advanced in the downlake direction, as

SSC of 20 to 25mg/L is produced from near site shown by the isopleths of 40mg/L in Figs. 5 and

D. Hence, the bifurcation of sediment-laden un- 8b, and by the reduction of the<15mg/L area at

derflow into interflow and other underflow is sites F and G.

likely to continuously occur on the steep slope at Figure 9 shows time series of water tempera-

Fig. 10 Vertical profiles of suspended sediment concentration (SSC), water temperature, and density in situ, a, obtained at site D at (a) 1301 h-1304h on 10 January and (b) 1146h-1149h on 11 January. 64 Augustine K.Chikita,Ian Halstead and Glenn Carter 2000

tore at depths of 10.5m, 20.5m, 30.5m, 37.5m, and

38.2m at site D, measured for a period of 7 to 11 Grain size of lake sediment

January 1998. The records at depths of 30.5m, Figure 11 shows grain size distributions of

37.5m and 38.2m indicate consistently similar bottom sediment obtained at sites A, C, D, F and temporal variations. A comparison between G. The bottom sediment consists of clay-sized vertical profiles at site D (Fig. 10) and the simul- grains (ƒÓ>8) more than 80 wt.% except at site A taneous records (two arrows) in Fig. 9 reveals (ca. 70% silt and 30% clay) near the river mouths. that suspension flow passed through near the Hence, suspended sediment from braided bottom of site D while temperature at 30.5m, 37.5 Tasman River is likely composed mostly of silt m and 38.2m rapidly varied with different and clay. The grain size of the bottom sediment values. The temperature and SSC distributions becomes small rapidly from site A to site C. At

at depths of 30m or more in Fig. 10b seem to site D, however, the sediment becomes slightly exhibit the bifurcation into a warm and turbid coarse; compared with site C, the sediment at interflow (water temperature of ca. 11.5•Ž and site D includes wholly coarse (decreasing mean

SSC of 23 to 27mg/L) at depths of 30 to 37 m and phi), excess coarse (increasing skewness), highly a relatively cold and clear, thin underflow at frequent (increasing kurtosis) grains (Table 2). more than 37 m in depth (Fig. 9). This bottom This suggests that the turbulent level of suspen- suspension flow was probably initiated at near sion flow at site D is higher than that at site C the river mouths as sediment-laden underflow (Chikita,1986). Here, this means that the under-

(see Fig. 5). As shown in Fig. 9, water tempera- flow contributing to sediment deposition be- ture greatly varied with different values at comes relatively strong at site D or around (Figs.

depths of 30.5 m or more for a time period of 1200 8b and 10b).

h, 9 January to 0800h, 10 January. This sug- CONCLUSIONS gests that suspension flow then passed continu- ously over the bottom of site D. Our sedimentological observations in Lake

Fig. 11 Grain size distributions of lake bottom sediment sampled at sites A, C, D, F, and G. J.Sed.Soc.Japan,No.51 Glacier-fed Lake Pukaki,New Zealand 65

Table 2 Grain size statistics in phi scale of lake bottom sediments, calculated by the graphic method (Folk and Ward, 1957). Mz: mean,ƒÐI: standard deviation, SkI: skewness, KG: kurtosis, d50: median.

Pukaki revealed that sediment-laden underflow ments. Japanese Jour. Limnology, 47, 53-61. and suspension overflow simultaneously and Chikita, K., 1992 : The role of sediment-laden underfiows in continuously occur near the river mouths by lake sedimentation : glacier-fed Peyto Lake, Canada. Jour. Fac. Sci., Hokkaido Univ., Ser. VII (Geophysics), 9, glacier-melt and snowmelt sediment discharge 211-224. of Tasman River. The sediment-laden under- Chikita, K. A., Smith, N. D., Yonemitsu, N. and Perez-Arlucea, flow is subsequently bifurcated into suspension M., 1996 : Dynamics of sediment-laden underfiows pass- interflow and other underflow at midlake, where ing over a subaqueous sill: glacier-fed Peyto Lake, Al- the subaqueous f oreset slope is developed. The berta, Canada. Sedimentology, 43, 865-875. grain size of lake bottom sediment suggests that Chikita, K., Nakamichi, Y., Smith, N. D. and Perez-Arlucea, M., the turbulent level of the suspension flows at 1998: A comparative study on suspended sediment dis- charge of rivers. Geophysical Bull. Hokkaido Univ., No.61, midlake is higher than that upstream. The hy- 1-10 (in Japanese with English abstract). drodynamics of the suspension flows, however, Chikita, K., Jha, J. and Yamada, T., 1999: Hydrodynamics of could be connected to the sediment discharge of a supraglacial lake and its effect on the basin expan- Tasman River and wind conditions (see Chikita sion: Tsho Rolpa, Rolwaling valley, Nepal Himalaya. et al., 1999). Sedimentation in Lake Pukaki Arctic, Antarctic and Alpine Research, 31, 58-70. should be examined systematically by clarifying Department of Survey and Land Information, 1996: Mount the contribution of the suspension flows quan- Cook. Topographic map H36 of 1: 50000 scale. Folk, R. L. and Ward, W. C., 1957: Brazos River bar: A study titatively. in the significance of grain size parameters. Jour. Sedim. ACKNOWLEDGMENTS Petrol., 27, 3-26. Gilbert, R., 1975: Sedimentation in Lillooet Lake, British Co- We are grateful to Dr. Andrew Harper and Dr. lumbia. Canadian Jour. Earth Sci., 12,1697-1711. Stephen Thompson, NIWA, Wellington for their Pickrill, R. A, and Irwin, J., 1983: Sedimentation in a deep welcome data supply. We are also indebted to glacier-fed lake-Lake Tekapo, New Zealand. Sedimento- logy, 30, 63-75. Dr. Clive Howard-Williams, NIWA, Christchurch Sturm, M. and Matter, A., 1978: Turbidites and varves in for his kind official procedure of our research. Lake Brienz (Switzerland) : deposition of clastic detritus This study is financially supported by Technolo- by density currents. Spec. Publ. of Intern. Assoc. Sedim., gy Research Center, Japan National Oil Corpora- No. 2,147168. tion. Warren, CR. and Kirkbride, M.P., 1998: Temperature and bathymetry of ice-contact lakes in Mount Cook National REFERENCES Park, New Zealand. New Zealand Jour. Geol. and Geophy., Chikita, K., 1986: Sedimentation in an intermountain lake, 41, 133-143. Lake Okotanpe, Hokkaido. I. Sedimentary processes Weirich, F., 1986 : The record of density-induced underfiows derived from the grain size distribution of surficial sedi- in a glacial lake. Sedimentology, 33, 261-277. 66 Augustine K. Chikita,Ian Halstead and Glenn Carter 2000

ニ ュ ー ・ジ ー ラ ン ド,プ カ キ 氷 河 湖 の 堆 積 環 境

知 北 和 久 ・I.ハ ル ス テ ッ ド ・G.カ ー タ ー,堆 積 学 研 究,No.51,55-66 Chikita, A. K., Halstead, I. and Carter, G., 2000: Sedimentary environments in Lake Pukaki, New Zealand. Jour. Sed. Soc. Japan, No. 51, 55-66

1998年1月 の氷河 融 解期 に,ニ ュー ・ジー ラ ン ド南 島 にあ る プカ キ氷 河湖(長 さ20km, 幅5km,最 大水 深98.0m)の 堆積機構 に関す る観 測 を実施 した。 同湖流 域 の19%は タスマ ン 氷河 な どの山岳氷河 で覆 われ,プ カキ湖上流 には夏 期 の氷 河融解 に よる土砂 流 出か ら形成 され た デル タが発 達 してい る。TTDプ ロフ ァイ ラーを用 いた 同湖 で の船上 観測 か ら,プ カキ湖 の 流入河 川 ・タス マ ン河の河 口付近 では,弱 いなが ら懸濁 下層流(sediment-laden underflow) が形 成 され,湖 央付 近 で の懸 濁 中層 流(suspension interflow)と 他 の下 層流 へ の分 岐 を経 て,最 深部 に達 して いる ことが わか った。 また,同 時 に河 口部 か ら懸 濁上層流(suspension overflow)が 発生 し,湖 の下 流端 まで達 して いる ことが わか った。 水温 の係 留観測 か ら,湖 央 で は分岐発生 した中層流 と下 層流 は少 な くと も約一 日間継続 す る ことが判 明 した。