The Physical Limnology and Sedirnentology of Montane Meziadin Lake, Northem British Columbia, Canada

by

RICHARD D. BUTLER

A thesis submitted to the Department of Geography

in conformity with the requirernents for

the degree of Master of Science

Queen's University

Kingston, Ontario, Canada

March, 2001

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The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be printed or otherwise de ceile-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. ABSTRACT

This work is an assessment of the landfoms and processes that influence the hydrology, limnology, and sedirnentology of Meziadin Lake and its basin.

The impetus for this study is to provide a better understanding of the nature of distal glacilacustrine deposition in fiord lakes. Meziadin Lake, British Columbia,

Canada (56.5 ON; 129. 2 9N) demonstrates physical processes and sedimentary fil1 comparable to other fiord lakes of the Canadian Cordillera.

Fieldwork was carried out on site frorn May 20 to August 10, 1999 to examine lake and basin responses to meteorological forcing, limnologic circulation. turbidity cunent fkquency and duration, and sedirnent distribution patterns and rates of accumulation. The discharge hydrograph for the gauged portion of inflow to Meziadin Lake peaked on June 16 in response to a signifiant rise in air temperature. Delayed rnelting of seasonal snowpack augmented glacial melt and created a significant freshet peak in fluvial suspended sediment

(max. 1425.5 mg/!-). Temperature-Conductivrty-Turbidity profiles illustrate dominant çontrol of lacustrine circulation by inflowing water masses, development of weak thermal stratification, and Coriolis deflection of inteflowing plumes. A subrnerged temperature logger in the delta proximal region recorded the passage of 2 types of undeflow events as positive temperature anomalies: a) quasiantinuous underflows across the delta front wrresponding to highenergy nival melt; and b) lower energy, laterally shifting turbidity currents wrresponding to diurnal variation of discharge and suspended sediment. Eleven sediment-trap rnoon'ngs were deployed, covering the geographical extent of the lake, recording significant proximalldistal, and northlsouth trends in Mass Accumulation Rate

(MAR). Proximal trap mootings and Ekman grab sarnples recordeci laminae with bimodal particle size distribution that were determined to be varves. Lateral and inflow distal regions are separated into irregutariy laminated and massive depositional environments relating to a reduction in deposition from turbidity currents and low MAR. In the infiow proximal region, the present annual accumulation rate of 17 mm a-', determined from Ekman sample examination and corroborated by sediment trap data. Taking this value for annual accumulation, and a value of 185 rn for total sedimentary thickness. a reasonable date of 10.6 x 1o3 years since the lake became free of glacial ice is obtained. ACKNOWLEDGMENTS

For the last two and half yearç I have been focused on trying to produce a large piece of independent work. The tnith is, however. that so much of this work is the result of many helping hands I am in debt to the large number of people who have supported my work and person during this time.

First. I would like to express my thanks to my supervisor, Or. Robert

Gilbert for his support. Sinœ taking his GPHY 304 course as an undergraduate. his interest and cornmitment to science have been contagious. I thank him for his many suggestions and ideas, as well as his attention to detail. both of which have raised the level of this work significantly.

Second, 1 would like to thank Dr. Scott Lamoureux for his advice and suggestions, but particularly his enthusiasm. Your interest and support of my work and has made me a better scientist.

As well. I would like to thank the many memben of the Geography

Department with whorn I have interacted, at work and at play. over the past two and half years. The caliber of this department as a whole has instilled in me much respect for this discipline and its advocates. Everyone has contributed to this work in their own way, with novel ideas and discussion. stimulating lunchtime conversation, or Grad Club Fridays. Thanks to Jackie, a great office-mate. for a fun-filled year. To Brandon, thanks for so many things including the ranting and raving, cornputer support, and least of all, the snowballs. To Scott. for his help in

BC and in Kingston, as well as his abilities with prime rib. Thanks to Nicole, for

her friendship, and her assistance, in the field. Thanks also to Ted, for king an in-house source of ideas, and for rny addiction to coffee. Finally, to everyone in the basement, it's dark and smelly. but it's home.

I would also like to thank al1 of my fnends near and far. without whorn I am sure I would have gone postal, or worçe. become an engineer. In no particular order: Kristen, Simon. TC, Jay. Juwan, Matt, Beynon. the Rector. Mo, Schachs,

Annie. Matty. the Caledon Gangsters. Keltie and Katherine. the Kingshott family. al1 the Central boys, camp people. soccer magic and Grads United. the QP. la belle provence. thehun. and Napster.

I would like to thank rny parents for al1 of their unconditional love and support throughout my 7 (oh my) years here at Queen's. To Caroline and

Antonia. who keep me going with lots of love and quality chat. 1 could not have corne close without rny family.

7.3 Implications for Future Research ...... 156 7.3.1 Post-Glacial Depositional History of Canadian Cordillera ...... 156 7.3.2 High-Reçolution Reconstructions of Environmental Events ...... 156

References ...... 158 Appendix 1 .Press Releases ...... 164 Vita ...... 169 LIST OF TABLES

Table 4.1 Mean and maximum suspended sediment concentrations for the Meziadin drainage fluvial systems ...... 77 LIST OF FIGURES

Figure 1.1 Meziadin Lake and surrounding drainage basin ...... 3

Figure 2.1 The major physiographic unes of Canadian Cordillera ...... 9

Figure 2.2 Location of Canadian Cordillera fiord-lake sedirnentological investigations ...... 1 1

Figure 2.3 A depiction of idealized thenally driven inflow variation, demonstrating cabelling. From Camack et al.. (1979) ...... 22

Figure 2.4 Density determined infiow variation. From Ashley et al.. (1985) ...... 22

Figure 2.5 A. A reconstruction of Kalamalka Lake acoustic profiles. From Eyles et al, (1990). B. An acoustic profile of Hamson Lake demonstrating two different acoustic facies. From Desloges and Gilbert, (1991) ...... 30

Figure 2.6 A Two parts of a single lacustrine cure demonstrating significant variation in sedimentation rate. From Smith (1981) B. A map of the Bow Lake and drainage basin. From Smith (1981) ...... 35

Figure 3.1 Meziadin Lake and drainage basin ...... 38

Figure 3.2 Locations of Ternperature/ConductivrtyTTurbidity profiling, Tidbit temperature recorder. sediment trap mootings, and Ekman Grab sampling sites ...... 46

Figure 3.3 A. A schematic of the paired sediment trap moonngs used in Meziadin lake. B. A photo of the funnel-shaped traps attached tu aluminum rod supports ...... 49

Figure 3.4 A. A photo of water velocity measurements, at 1 m intervals, for Strohn Creek. B. A photo of the Strohn Creek stilling well ...... 49

Figure 4.1 A. A photo of glacial recession in the Meziadin area, exposing Little Ice Age moraines. B. A photo of retreat at the toe of Meziadin basin glaciers exposing unstable driftmaterial ...... 60 Figure 4.2 A. A photo of Surprise Creek on June 7, demonstrating the braided morphology of the channel and quantity of debris stranded on channel bars. B. A photo of Surprise Creek on June 14 during a pend of increased stage associated with nival melt ......

Figure 4.3 Climate nomals for Stewart, Srnithers, and Dease Lake, British Columbia. (sources: Environment Canada. Canadian Meteorological Centre) ......

Figure 4.4 A. A comparison of standard mean daily temperatures for Meziadin and Smithers. BC. B. A scatter plot demonstrating the strength of the relationship between the temperature records ......

Figure 4.5 A. Continuous records of precipitation, Meziadin lake level. Strohn Creek discharge. and Smithers mean daily air temperature. B. A scatter plot indicating the strength of the relationship between discharge and air temperature ......

Figure 4.6 Rating Curve for Lesser Strohn Creek discharge calculation ......

Figure 4.7 A cornparison of Lesser Strohn Creek discharge with air temperature at lake level in the Meziadin Basin ......

Figure 4.8 A cornparison of the continuous discharge record from Lesser Strohn Creek to the suspended sediment concentrations of Lesser Strohn, Surprise. Hannah, and Tintina Creeks ......

Figure 5.1 Bathymetry of Meziadin Lake, constnicted from the interpolation of acoustic survey data. Map includes the locations of transects followed in subbottom survey ......

Figure 5.2 Subbottom acoustic record from several transects in Meziadin Lake Delta region B. Proximal am C. Distal am ......

Figure 5.3 Thickness of sedirnentary fiIl in Meziadin Lake constwcted Rom the acoustic subbottorn record ......

Figure 5.4 Subbottom acoustic record fiom transect 22, in the distal arrn of Meziadin Lake, illustrating three discrete ost tala ci al sedimentarv facies ...... Figure 5.5 Temperature profiles from several CTD stations illustrating temporal and spatial variations in the thermal structure of the water column ...... 93

Figure 5.6 Conductivity profiles from several CTD stations illustrating temporal and spatial variations in the conductivity characteristics of the water culumn ...... 97

Figure 5.7 Turbidity profiles from several CTD stations illustrating temporal and spatial variations in the suspended sediment concentration within the water column ...... 100

Figure 5.8 Longitudinal profiles of suspended sediment concentration constnicted from CTD profile data on June 10 and June 17 ...... 101

Figure 5.9 Transect of suspended sediment concentration constnicted from CTD profile data on June 17 across the width of Meziadin Lake ...... 103

Figure 5.1 0 Longitudinal profiles of suspended sediment concentration constructed from CTD profile data on July 5 and August 6 ...... 105

Figure 5.1 1 A. A cornparison of continuous discharge data from Lesser Strohn Creek with averaged hourly undeflow temperature data from the lÏdbit submersible logger and suspended sediment concentration from Surprise Creek. B. a scatter plot illustrating the strength of the relationship between underfiow temperature and discharge...... 107

Figure 6.1 Meziadin Lake and drainage basin ...... 114

Figure 6.2 Mass Accumulation Rates (MAR) for sediment trap moorings ...... 11 5

Figure 6.3.1 Sediment trap mooring 1. A. Photo of sediment stratigraphy captured by trap bottles B. Particle Size Distribution plot C. Organic and lnorganic Carbon D. Map of Meziadin Lake and location of trap mooring ...... 120 Figure 6.3.2 Statistical data (mean, standard deviation, kurtosis. skewness) dernonstrating changes to the distribution symmetry of individual grain size samples from sediment trap mooring 1. All data shown in this figure correlates to the stratigraphy examined in Figure 6.3.1 ......

Figure 6.4.1 Sediment trap mooring 2. A. Photo of sediment stratigraphy captured by trap bottles B. Particle Size Distribution plot C. Organic and lnorganic Carbon 0. Map of Meziadin Lake and location of trap rnooring ......

Figure 6.4.2 Statistiml data (rnean, standard deviation, kurtosis. skewness) dernonstrating changes to the distribution symmetry of individual grain size çamples from sediment trap rnoaring 2. Al data shown in this figure correlates to the stratigraphy examined in Figure 6.4.1 ......

Figure 6.5.1 For Ekman sample 3. A. Photo of sediment stratigraphy B. Particle Size Distribution plot C. Organic and lnorganic Carbon D. Map of Meziadin Lake and location of Ekman recovery ......

Figure 6.5.2 Statistical data (mean, standard deviation, kurtosis, skewness) demonstrating changes to the distribution syrnmetry of individual grain size samples from Ekman sample 3. Ail data shown in this figure correlates to the stratigraphy examined in Figure 6.5.1 ......

Figure 6.6.1 For Ekman sample 5. A. Photo of sediment stratigraphy B. Thin section of a portion of the Ekman sample C. Particle Size Distribution plot 0. Organic and lnorganic Carbon E. Map of Meziadin Lake and location of Ekman recovery ......

Figure 6.6.2 Statistical data (mean, standard deviation, kurtosis. skewness) demonstrating changes to the distribution symmetry of individual grain site samples from Ekman sample 5. All data show in this figure correlates to the stratigraphy exarnined in Figure 6.6.1 ......

Figure 6.7.1 For Ekman sample 7. A. Photo of sediment stratigraphy B. Thin section of a portion of the Ekman sample C. Partide Size Distribution plot D. Organic and lnorganic Carbon E. Map of MeAadin Lake and location of Ekman recovery ...... Figure 6.7.2 Statistical data (mean, standard deviation. kurtosis, skewness) demonstrating changes to the dimibution symmetry of individual grain size samples from Ekman sample 7. Ail data shown in this figure correlates to the stratigraphy examined in Figure 6.7.1 ...... 136

Figure 6.8.1 For Ekman sample 16. A. Photo of sediment stratigraphy B. Thin section of a portion of the Ekman sample C. Particle Size Distribution plot D. Organic and lnorganic Carhn E. Map of Meziadin Lake and location of Ekman recovery ...... 138

Figure 6.8.2 Statistical data (mean, standard deviation. kurtosis, skewness) demonstrating changes to the distribution symmetry of individual grain size samples from Ekman sample 16. Ail data shown in this figure correlates to the stratigraphy examined in Figure 6.8.1 ...... 139

Figure 6.9 A description of the sedimentary environments within Meziadin lake ...... 142 CHAPTER 1 - INTRODUCTION

1.'l MONTANE LACUSTRINE INVESTIGATIONS

Large, deep lakes, located in a postglacial montane setting, potentially contain a record of Quatemary environmental change preserved in the stratigraphy of their sediments. The fiord lakes of British Columbia, Canada. represent important depositional environments as they rnay record valuable long- terni information on glacial retreat, possible manne incursion, drainage basin evolution. and lacustrine sedimentation since the final retreat of the Cordilleran

Ice Sheet (Eyles et al.. 1990; Gilbert & Desloges, 1992; Desloges & Gilbert.

1998). Glacially overdeepened lakes contain a continuous. or quasi continuous. record of sediment delivery from their watersheds, integrated by the processes and landforrns within their adjacent drainage basin (Gilbert et al., 1997) and further wnditioned by unique intemal limnologie characteristics (Smith. 1978;

Sturm, 1979).

The seasonal nature of drainage from Canadian Cordillera montane basins, in conjunction with sediment trapping efkiency of the lakes. affords the potential for accurate construction of lake sediment chronology (Smith. 1981), thereby establishing a control for sediment flux (O'Sullivan. 1983). Specific drainage basin characteristics such as the glacial covefage, sediment availability. proximity of the lake to glaciers, and the lack of sediment sinks en route to the tacustrine basin provide the potential for an annually or sub-annually resolved sedimentary records. Lacustrine sedirnentary records may contain proxy evidence of environmental variability, such as changes to basin clirnatic (Leeman

& Neissen, 1994; Desloges 1994). hydrologie (Desloges 8 Gilbert, 1994). and glacial regimes (Leonard. 1986 a & b).

In addition to long- and short-term sedimentary records, fiord lakes with one prirnary inflow provide a dynamic environment for the examination of physical interaction between lacustrine and fluvial systems. Due to significant differences in physical properties. the processes associated with interaction of lake and river water masses have continued influence on water column structure and stability (Gilbert, 1975; Canack et aL, 1979). inflow induced circulation

(Hamblin 8 Canack, 1978; Wright & Nydegger. 1980). and the relative importance of interflowing and undemowing density currents as depositional processes (Sturm 8 Matter, 1978).

1.2 MEZlADlN LAKE AND DRAINAGE BASlN

Meziadin Lake (56.5 ON; 129. 2 OW). British Columbia, Canada, is a deep, postglacial lake on the eastem edge of the Boundary Range, northem Coastal

Mountains (Figure 1.1). Meziadin Lake occupies 34 km2 in the southeastern portion of its 530 km2 drainage basin (16.5:l basin to lake ratio). a small northern portion of Nass Valley Drainage System. The total range of elevation in the basin is 1800 m (5900 R) with Meziadin Lake situated at 244 rn (800 ft) above sea level (ad). The elevation of treeline is generally between 91 5 and 1O67 rn

(3000-3500 ft), depending on the orientation of the slope face. while some valley glaciers extend as low as 915 m. There is one primary fluvial input into Meziadin ft. as1

Figure 1 1 : Meziadin Lake (244 m - 800 ft. asl) and its surrounding drainage

basin. Strohn-- - Creek enters Meziadin from the northwest, draining the centres of glaciation. Meziadin River drains the lake towards the southeast. as part of the Nass River drainage system. Elevations from NTS map 104 A13 Lake. Strohn Creek. entering frorn the west and one output. the Meziadin River, which drains the lake in the southeast. The lake is 18.5 km long and a near unifom 2 km wide (thinning slightly at the elbow and widening at the point of outfiow). with a maximum depth of 135 m in the inflow proximal an,which nses to approximately 20 m in the most distal regions near the mouth of the Meziadin

River.

Climatic forcing of basin hydrology is highly seasonal, controlled prirnanly by nival and glacial melt, thereby providing temporal variation in the transportation and deposition of sediment. Strohn Creek drains 45% of the total catchment, including the IO0' covered by glaciers, with two srnaller fluvial systems, Hannah and Tintina Creeks. draining most of the remaining non- glaciated low-lying regions of the north and northeast. The Strohn Creek delta is a stable feature with one dominant fluvial channel, several extinct laterai channels, and significant forest and other vegetation growth. Sediment available to basin fluvial systems is considered a function of strearn and delta channel re- organization related to discharge variation. and the production of weathered matenal by glaciers.

1.3 RESEARCH QUESTIONS

The focus of the Meùadin Lake study is to explore the genesis of a lacustrine sedimentary record in a northem Cordilleran montane environment, in relation to drainage basin landfoms and lacustrine processes. A review of relevant lacustrine and postglacial lacustrine literature (Chapter 2) highlights fiord lakes as key depositional environments in the examination of Quaternary environrnental change. The field methods and equipment used in the monitoring of environrnental variables over the summer of 1999, as well as the laboratory analysis of sediment samples, are reviewed in Chapter 3. An examination of basin hydrociimatology as a forcing mechanism of sediment routing (Chapter 4) assists in explaining the timing, quantity. and character of mobile sediment entering Meziadin Lake. By combining morphological and physical limnological data (Chapter 5) with sedimentological observations. a conceptual mode1 of sediment transportation and deposition will be constructed, based primarily on the interaction between lake and river water, to ascertain the distribution and properties of sediments in Meziadin Lake (Chapter 6).

Relevant to this thesis are several research questions that formed the basis of field-work camed out primarily during the spring and summer of 1999.

1. Hydrorneteorology 1-1 What are the general dimate patterns for the eastem side of the northern Coast Mountains and how does this clirnate signal manifest itself in the Meziadin basin? 1-2 What is the difference in magnitude and character of nival and glacial discharge pulses for Strohn Creek? 1-3 What are the dominant forcing mechanisms of discharge variation in Meziadin basin fluvial systems? 1-4 What is the strength of the relationship between fluvial suspended sediment concentration and discharge?

2. Physical Lirnnology 2-1 What factors influence the development and stability of thermal stratification and the production of the intemal circulatory system? 2-1.1 How do these physical systems change over the course of the summer? 2-2 In response to river-lake interaction at point of infiow. what are the resuiting circulatory systerns at work within the lake? 2-2.1 What weather variables inf uence lake circulation? 2-3 Do underflows exist in Meziadin? 2-3.1 If so, do events demonstrate any association with discharge or fluvial suspend4 sediment mncentration?

3. Sedimentology 3-1 How does the quantity and charader of sedimentation Vary spatially throughout Meziadin Lake? 3-1.1 Are there recognizable proximal to distal trends? 3-1.2 1s there lateral variation related to Coriolis deflection? 3-2 How does sedimentation Vary ternporally throughout the summer field season? 3-2.1 What is the relative importance of nival and glacial runoff on sedimentation? 3-3 What are the sedirnentological characteristics associated with their deposits? 3-4 Having examined the sediment record, what is the temporal resolution at which variations in sediment quantity or charader can be disthguished? 3-4.1 What is the spatial differentiation of deposits?

In summary. this work represents an assessrnent of the processes that affect the hydrology. limnology. and sedimentology of Meziadin Lake and its basin to provide a basis for cornparison to other large. postglacial lacustrine systems in the southem and northem Canadian Cordillera. CHAPTER 2 - LITERATURE REVIEW

2.1 INTRODUCTION

Meziadin Lake. northem British Columbia, is a large. deep, post-glacial lake containing a thick sedimentary fill. Because of its ability to efficiently trap sediment. Meziadin Lake is believed to contain a record of changes to its drainage basin sedirnentary regime, possibiy since deglaciation. An examination of lacustrine sediments. therefore, may provide insight into drainage basin evolution including changes to basin processes and landfons.

This chapter reviews lacustrine sedimentology in montane environments by first examining the physiography of northem British Columbia and its

Quatemary history. Following this. a review of montane basin hydrology focuses on glaciated basins and their sedimentary dynamics. The physical limnology of fiord-type lakes follows. providing insight into the interaction of lake and river water masses, lacustrine circulation patterns, and the distribution of coarse and fine sediments. The initiation and development of turbidity currents is also exarnined in this section. Finally, lacustrine sedimentation is examined using examples frorn basins in the Rocky Mountains and Coast Mountains of the

Canadian Cordillera as well as other montane basins in Norway, Switzerland. and New Zealand. 2.2 PHYSIOGRAPHY AND QUATERNARY DEGLACIATION

2.2.1 Physiography and Geology

The bedrock of the western margin of the North Amencan Craton is a collage of cnistal fragments (belts), defomed in response to convergence with the Pacific Ocean plate (Clague, 1989). Geologically. this region represents five northwest-trending geological belts (from east to west): The Foreland Belt, The

Omineca Belt, the Intermontane Belt, the Coastal Belt, and the Insular Beit. The

Meziadin drainage basin lies on the eastem margin of the southem Coastal Belt.

This region, created by collisions in the Cretaceous and eady Tertiary periods. is characterized by the meeting of plutonic bodies with Paleozoic sedimentary and volcanic rocks.

The physiography of the Canadian Cordillera can be divided into three major systems: Western, Interior. and Eastern (Figure 2.1). This classification can be further broken down into four main landscape elements: mountains, plateaus, lowlands. and valleys (Clague. 2989). The Meziadin drainage basin is located near the boundary behveen the Western and Interior systems. More specifically, the catchment lies on the eastem margin of the Boundary Ranges. part of the northem Coast Mountains. The Coast Mountains are a northwest trending belt that was a centre of ice accumulation during Quatemary glaciations

(Ryder & Thompson, 1986). The southem portion of the drainage basin. including the oufflowing Meziadin River, includes part of the Nass River drainage

basin. The Nass depression stretches several hundred kilometres southeast. bounded by the Coast Mountains to the west, and the Skeena Mountains to the INTERIOR PLAINS

EASTERN SYSTEM

I , INTERIOR SYSTEM

. ... WESTERN SYSTEM

Figure 2.1 : The major physiographic units of the Canadian Cordillera east (Ryder & Maynard, 1991).

Meziadin Lake itself occupies a long, deep, glacially scoured valley and can be characterized as a fiord-type lake; a tenthat refers to lake morphotogy as well as relevant physical processes. The origin of such lakes has been linked to the overdeepening of structural basins by ice scoufing, during the extensive

Pleistocene glaciation (Muilins et al.. 1990). Research in the Southem Cordillera.

British Columbia, represents an atternpt to document the trends in sedirnentation from deglaciation to present, taking into consideration glacial influence and drainage system development. Much of the work done on lakes sudi as Lillooet.

Kalamalka. Hamson, Stave, and Oakanagan has been undertaken in an effort to understand better distal glacilacustrine sedirnentation in montane basins (Gilbert.

1975; Eyles et al.. 1990, 1991; Mullins et al. 1990: Desloges and Gilbert. 1991;

Gilbert and Desloges, 1992). More recent work has also focused on fiord lakes in the northem Coast Mountains and Rocky Mountains (Desloges and Gilbert.

1995. 1998; Gilbert et al., 1997) (Figure 2.2). An attempt is made to determine the distribution and properties of the postglacial sediments; as well as '. ..to infer from this record processes of sediment delivery and postglacial accumulation history." (Desloges & Gilbert. 1991 ; P. 800).

2.2.2 Quaternary History of Canadian Cordillera

The Quatemary stratigraphy of the Cordillera provides insight into British

Columbian lacustrine records discusçed in this work. The stratigraphic record described below represents deposition and reorganization during the episodic growth and decay of the Cordilleran Ice Sheet. Quaternary units generally fall Meziadin Lake A

Figure 2.2: Location of Canadian Cordillera fiord-lake sedimentologica investigations: 8-Bowser Lake; C-Chiiko Lake; H-Hamson Lake: L-Lillooet Lake; K-Kalamalka;O-Okanagan; M-Moose Lake; S-Stave Lake; Ua-Upper Arrow; Ko-Kootenay into two categories of deposit based on their ongin (Clague. 1986): 1. till deposited subglacially and supraglacially and 2. stratified sediments deposited in pro-glacial and ice-contact fluvial. marine and lacustrine environrnents. Clague

(1986) relates the stratigraphie record in the Cordillera to bnef, depositional events separated by long periods of erosion; describing the record as ". ..a complex response of geornorphic systerns to threshold events of varying magnitude." (p. 892)

Till is the most extensive surficial material in the Cordillera (Clague, 1989). having kendeposited at the base of the ice sheet and at the base of outlying glacien in regions of low to rnoderate relief. Although glacien were effective in scou ring valleys parailel to flow direction, some sediment, comprised of several units of till. glaciofluvial, and glaciolacustrine sedirnents, has remained in some coastal lowlands and valleys (Ryder & Clague, 1989). The existence of large. icedarnmed lakeç in the BC interior valley dunng the Quatemary and most recently dunng the Fraser Glaciation is evident in the volumes of ice-contact and glaciolacustrine sediments, as well as high-level terraces along valley walls

(Eyles et al.. 1991 ).

Particulariy important to this review is the Fraser Glacation, the final

Cordilleran glaciation, during which glacial maxima in the Coast Mountains occurred approximately 13 ka BP (Ryder & Thornpson. 1986). The Fraser

Glaciation has bendescribed, by several authors, as a series of complex advances and retreats associated with regional climate modifications and topographical restrictions, which resulted in glacial responses at varying time scales (Ryder & Clague, 1989; Fulton, 1991). Glacial retreat at the end of the

Fraser is hypothesized to have begun with the reduction of alpine domes, isolating individual tongues in lowiand intefior valleys (Fulton, 1991; Ryder 8

Maynard, 1991). During deglaciation, exposed sediments were easily eroded from poorly vegetated. unstable drift deposits found in the regions of upland slopes and valley walls (Eyles et a/., 1987). Large amounts of sediment were deposited at the close of the last glaciation on proglacial flood plains. deltas, and in subaqueous fans in valleys and coastal lowlands. At this time, the yield of sediment into fluvial systems was many times greater than the production of weathered debris (Church and Ryder. 1972). As such. streams rapidly aggraded their valleys because they were unable to cope with the large amounts of sediment made available. These streams became choked with glaciofluvial and fluvial sediments, causing extensive drainage changes. In addition to these sediment-dammed drainage alterations. remnant ice played a major role in directing and redirecting sediment distribution and deposition. For example, shifting ice sheet drainage patterns (Gilbert and Desloges. 1992) and in situ disintegration (Eyles et al., 1987) are believed to have played an important role in changing directions of sediment movement and, therefore, changes to

Quatemary sediment deposition in respective basins. Late Pleistocene valley trains. outwash plains, and deltas were incised by streams during the Holocene, and now occur as terraces above presentday floodplains (Clague. 1989). In the

Fraser River Valley, glaciofluviai and glacilacustrine sediments were excavated by the downcutting of the Fraser River foming several. distinct terraces (Church and Ryder, 1986). Sequences of starvation and aggradation were also documented by Eyles et al.. (1987). Downcutting seen in many other British

Columbia rivers is generally atttibuted to a decline in sedirnent supplies throughout the Holocene.

Church and Slaymaker (1989) further investigate the impact of this depositional regime throughout the Holocene. Due to secondary remobilization of sediments, specific sediment yields in British Columbia strearns do not decline sttictly according to an increase in area drained. Therefore. spatial scale cannot be the sole determinant in examining specific yield; a temporal component must be considered. Church and Slaymaker (1989) hypothesize that major river systems are in fact redistributing sediment that was originally delivered by glacial events at the end of the last glacial period. This 'paraglacial cycle of sedimentationn(also in Church and Ryder. 1972). proposes that the larger basins within the Cordillera are actually still responding to Wsconsinan aged sediment inputs and are still finding a point of equilibrium.

2.3 GLACIATED BASIN HYOROLOGY

A large proportion of allochthonous sediment entering montane lakes originates from fluvial sources; therefore, a study of drainage baçin hydrological regimes is important to any review of lacustrine sedirnentology. Glaciers represent a major reservoir of water and sediment that dominates the hydrologie systems within their respective basins. This, in tum, influences sedirnent transport to the lake system. Seasonal variation creates the potential for researchers to differentiate sedimentation temporally in the lake record and. in some circumstances, allows the establishment of chronologies within lacustrine sedirnents. A combination of glaciohydraulic and glaciohydrological process influences the fluvial geomorphology of glacieiized basins (Lawson, 1993).

2.3.1 Glacial Hydraulics

Although an in depth examination of glacial hydraulics is beyond the sape of this work, a bnef review of glacial thermal conditions and englaciallsubglacial drainage systems is relevant, as they are important controls of water and sediment discharge from the glacial system. The thermal regime of a glacier is a diagnostic characteristic of the basal erosion. entrainment. and transportation proceçses at work. thereby serving to classify glaciers as temperate or non-temperate. Temperate glaciers are generally defined as those of which the ice is almost ubiquitously (except for a transient surface layer which

is cooler in winter) at the pressure-metting point (Lawon, 1993; Alley et al..

1998). Non-temperate glaciers have been generally assurned to be more

restrictive in their drainage systems, forming a seasonal aquidude that releases

surface storage rapidly at the start of the melt season; this perception has

recently been questioned and is currently under review (Hodgkins, 1998). Since

temperate glaciers dominate the Canadian Cordillera, they represent the majority

of the review.

The entrainment and transport of basal debris is a cornplex process

involving glacier bed deformation and regelation of subglacial material (Alley et

al., 1997). The ability of the glacier to concentrate ninoff from precipitation and ice melt provides the cornpetence to move large amounts of sediment. and control local hydrology. Daily and seasonal trends in runoff are linked to the energy available to produce meltwater from snow and ice, generally in the form of net radiation, and both latent and sensible heat. Longer scale trends in mnoff are a fundion of climatic cuntrols on mass balance (Lawson. 1993).

Runoff is focused beneath a glacier in one of three ways: 1. as a thin. basal film; 2. through a porous basal debris layer, or 3. through a network of conduits and cavïties (Culter, 1998). Flow may exist as a combination of these foms. but if an extensive channelized system exists, this flow dominates al1 otherç (Alley et al.. 1998). The size and shape of subglacial channels are signrficant in the detenination of pressure and speed of meltwater flow through a glacier (Hubbard and Nienow. 1997). During the ablation season. these conduit systems expand as the rate of tunnel melting due to the presence of viscous energy exceeds the rate of tunnel closure due to glacial creep (Culter.

1998). This 'snowballeffecf corresponds to the increase of availa ble energy for ice melt: more energy (net radiation, sensiblenatent heat) gives rise to more meR water, thus more kinetic energy and increased channel growth with the result of an increased ability to transport sediment. The seasonal evolution and stability of the englacial-subglacial drainage system greatly influences the quality and quantity of sediment transported within and beneath a glacier. This. in tum. seasonally influences glacial stream dixharge.

2.3.2 Glacial Hydrology

In the above review of glacial hydraulics. the ice mass is understood to represent major resewoir of water and sediment. Therefore. acting as dynamic source for both, the glacier partially controls the magnitude of runoff within the catchment. Glaciers effectively control mnoff in montane hydrologie systems by modifying peak discharges. variability. and daily and seasonal characteristics

(Ward and Robinson, 1990). Therefore, runoff in a glaciated basin can be wnsidered to be influenced by a combination of the themal conditions of the glacier, recent flow charactenstics, and other meteorological charactetisticç

(Church and Gilbert. 1975)

As previously mentioned. seasonal characteristics of sediment flux within

a glacier are related to the energy balance within the ice rnass. variations in

extent and development of an englacial-subglacial drainage system, and

variations in sediment production and availabilty. This glaciofluvial system does

not necessarily remain stable over an entire melt season; the majority of melt

occurs within several months, dominating stream flow in glaciated basins,

corresponding to a period of increased energy for melt (Gilbert. 1975; Lawson,

1993). Since streamflow is the major source for clastic input in proglacial lakes,

changes to the suspended sediment tlux will be dominated by glacial dynamics.

If a relationship between suspended sediment discharge and glacial melt

is assumed. the increases in glacial ablation should, in theory, create an increase

in mnoff that is correlated with increased suspended sediment flux (Leemann,

and Niessen, 1994). This relationship has been hypothesized at annual (Leonard.

1997). interseasonal (0strem and Olsen, 1987). and diumal temporal scales

(Weirich 1986). This fundamental assumption for high-resolution glaciated basin paleoenvironmental reconstruction is that sediment discharge relates directly to glacial meltwater discharge. Seasonal characteristiffi of sediment flux within a glacier are related to the energy balance within the ice mas, variations in the extent and development of the drainage system, and variations in sediment

production and availability (Lawson. 1993). 0strem (1975) found that in

rneltwater streams there was no simple relationship between seasonal variations in discharge and suspended sediment concentrations.

Diurnal variation in air temperature and runoff can be characterized by a

peak and return cycle that is associated with daily cycles of net radiation and.

therefore. energy for melting. Recent examinations of glaciated basins suggest

that even moderate diumal alterations in the hydrology of the inflowing stream

cm greatly affect lacustrine processes. The sensitivity of density to the thermal

and sedimentary charactefistics of the water masses is such that small changes

to either variable can generate recognizable events in lake circulatory systems

and their sedimentary records (Gilbert, 1975; Weirich, 1986 ).

In summary. the interaction between variations in energy supplies, as well

as variations in precipitation dictate glacial variation in storage versus meltwater

production. Glacial inputs into montane hydrologie system can be characterized.

generally, as a periodic, thermally driven regime with distinctive diumal and

seasonal variations (Ward and Robinson, 1990). 2.4 PHYSICAL LIMNOLOGY OF MONTANE LAKES

Within this section, the relationship between glacierized basins and the post-glacial lake system is examined. According to the previous sections, the presence of a glacier heavily influences basin hydrology, increasing the seasonal nature of basin physical processes. At this time it is important to understand the physical processes within a montane lacustrine systern and the resulting sedirnent deposition.

2.4.1 Inflow, Circulation, and Fine Sediment Distribution

The major source for allocthonous sedirnent in montane lacustrine systems is, generally, inflow from rivers and streams. Mass wasting events cm, in some circurnstances, influence lake sediment budgets (Werîch, 1985). but generally these inputs are sporadic and unpredidable. In many circumstances. river-induced currents and resuitant fiow influence and even dominate circulation and sedimentation in fiord lakes (Gilbert, 1975; Hamblin and Carmack, 1978;

Pharo and Carmack, 1979; Pickrill and Irwin, 1982).

When a stream with enters a lake with different physical characteristics

(e-g. temperature, concentration of dissolved or suspended sediment), the

interaction of these two masses is important to the entire lacustrine system and

not restricted to the point of entry. According to Pharo et aL(1979), fluctuations

in the sediment content and temperature of an inflowing strearn can greatly

influence the structure and strength of physical stratification within a lake. This

being so, an understanding of the general circulatory patterns of a lake, including

the role of inflowing currents on water wlumn stability. is important in reconstructions of lake-wide sedimentation (Wright and Nydegger. 1980). Gilbert

(1975) distinguished five factors that were important to the movement of fine- grained sediment within Lillooet Lake and. therefore, influential in generating the lacustrine sedimentary record: (p. 1701 )

1. density characteristics of the lake and inflowing water based on

temperature and, particularly, suspended sediment content.

2. currents induœd by infiow and wind.

3. the thermal structure of the lake body, which may detemine the nature

of circulation patterns.

4. diumal and seasonal fluctuations in the inflow parameters.

5. large annual inflow.

Of particular importance to several of these factors is the differing densities of the inwming river water versus the ambient lake water. The density variation. partly related to the differing temperatures of the water masses. has a profound effect on the interaction of inflowing water. Based on the variable temperatures of the river. the lake surface, and the lake bottom. the circulation of incoming water can be predicted based on seasonality (Figure 2.3) (Canack et al.. 1979). Hamblin and Camack (1978) outline three stages for the inflow of a stream into a lake, where the physical characteristics of the two bodies differ.

Initially. water entering the lake moves to a depth where it's density matches that of the lake (Interflow) (Hamblin and Carmack, 1978; Pharo et al., 1979; Carmack et al.. 1979; Pickrill and Invin, 1982). Therefore, annual variations of stream temperature and lake stratification ensure that their relationship at the point of inflow is dynamic. Secondly, mixing and entrainment occur as the plunging fluvial inflow feels a frictional drag, at its boundaries, from the lake where it then draws lacustrine water into turbulent motion. The Richardson number defines the relationship across this interface: a ratio of the density gradient to the shear force creating said instability, thereby providing a model for plume entrainment.

The process of cabelling (Figure 2.3, D and G) represents the mixing of waters slightly above and below 4OC. which combine to reach the temperature of maximum density of water. This effect has been related to thermal bars on the

Great Lakes in that circulation is restricted for a penod of time based on temperature induced horizontal stratification (Cannack et al.. 1979). Finally. when the plunging water has reached a depth at which its density matches the surrounding lake, the plume will fiatten and begin to spread along this surface of constant density creating an interflow (Hamblin and Camack, 1978; Carmack et al., 1979; Pharo and Camack, 1979).

The above description of infiow and lake interaction represents an idealized situation in which differing temperatures solely affect densrty; this is not the case in the majonty of montane lacustrine systems. The suspended

sediment concentration of inflowing water has a much more profound effect on its density (Gilbert, 1975; Smith, 1981). Figure 2.4 shows the rnovement of

inflowing water at a given density (pi) and its interaction the epilimnion and

hypolimnion at densities (pe) and (fi)respectively. Therefore, based on the temperature and suspended sediment concentration. inflow moves through the

lake as overftow, interfiow, or underflow. Several forces within lake influence the Figure 2.3: A depiction of idealized themally driven infiow variation. Boxes D and G represent the periods when cabbeling occun. T represents temperature with the subçcn'pts R-River; L-Lake. S- Surface, B-Bottom. Source: Canack et al. (1979)

Figure 2.4: Density determined inflow. Depending on the suspended sediment content of the inflowing river, a turbid plume will disperse along a plane of equivalent density: pi is the densrty of inflowing water, pc is the density of the epilimnion, and ph is the densrty of the hypolimnion. Source: modified from Ashley et al., (1985). movement of this plume: frictional forces associated with flow dong the boundary of equal density, cross stream pressure fields determined by the geometry of the plume's gradient, and the impingement of flow on one side of the plume by a boundary (Coanda Effect) (Hamblin and Camack. 1978; Camack et ai., 1979;

Pharo et al., 1979).

ln addition to the above-rnentioned forces within the water wtumn. the entire lake is subjected to geostrophic forces associated with the rotation of the

Earth. The Coriolis Effect greatly influences the distribution of turbid plumes within lakes and. therefore. influences the deposition of sediment from suspension. The deflection of mass to the right (Northem Hemisphere) has been documented in other lakes (Smith. 1978; Leonard. 1986). Smith (1978). documents the movement of suspend4 sediment within Hector Lake and demonstrated the plume favoured the right-hand side of the lake. Evidence of this rnovement exists as mean sedimentation rates (from suspension), grain size. and carbonate measurements Vary according to this right-hand deflection. The

magnitude this effect on a plume varies directly with the latitude, and inversely with the speed of the plume.

The role of Coriolis deflection was augmented in Hector Lake by the

influence of katabatic winds generating downlake currents. Smith (1978) showed

that katabatic winds were responsible for a turbulent epilimnion. particulady in

regions distal to the point of inflow. The sediment transporthg power of the

upper layer of the lake was increased by this turbulence, generating a distinctive

southward (following And direction) trend in transportation and. therefore. deposition of diment. As such, proximal-âistal trends were important to Hector

Lake analysis. in addition to right to left distributions (Smith, 1978. Smith et al.,

1982). This preferential transport on one side of the lake and wind influence holds implications for the distribution of grain size throughout the lake, which, in tum. introduces important considerations for sediment sampling and analysis.

2.4.2 Turbidity Currents and Coarse Gain Distribution

2.4.2.1 Physical Parameters of Turbidity Currents

The deposition of sediment from suspension foms an important part of a lacustrine sedimentary record. In Hector Lake, for example. the role of sedirnentation from suspension and variations in circulation were determined to be the controllhg forces in sedimentary record generation (Smith. 1978).

However. just as important to sedimentary record production are the distribution and reorganization capabilities of turbidtty currents. An understanding of the turbulent structure of such currents is required if predictions are to made regarding their behavior or the nature and distribution of their deposits. As such, the mechanics and ongins of turbidity currents are reviewed, followed by an examination of their impact on lacustrine sedimentation.

Turbidity currents represent an underflow of water with a densrty greater than that of ambient lake water. These difier fmm underflows in that their density is linked diredly to the concentration of suspended sediment and does not necessanly relate to a temperature near that of maximum density (approximately

4OC), or dissolved load. The structure and mechanics of turbidity currents are generally cornparecl to that of a stream, forced by gravity, to flow beneath water rather than air (Le. a particularly dense fluid flowing beneath a less dense Ruid)

(Chikita. 1989). According to this analogy, the fnctional forces at the upper fiuid interface must be greater than that of a regutar stream systern (Weirich, 1986), while at the same time. the buoyancy of the current is increased (reduced acceleration due to gravity). Flow is examined by several mathematical parameters including the densometric Froude number (ratio between inertial and gravitational forces), the Reynolds number (ratio between inertial forces expressed as velocity and the viscous forces expressed as Ruid viscosity); and the rnodified Chezy Equation (examines velocity of flow). Although this type of study represents a discipline w'thin itself, the character of flow (i.e. the erosional and depositional potential) of a turbidity current can be predicted based on these parameters and their effect on velocity distribution (Lambert et. al. 1976; Chikita.

1989; IWO). The Shield's diagram represents a relationship between grain

Reynolds number and shear stress that attempts to predict whether a given current will cause motion in basai sediments (Boggs, 1982). As the grain

Reynolds nurnber increases, there is a decrease in grain size. increase in frictional velocity. or a decrease in kinematic velocity; each is an important factor in turbidity current behaviour (Chikita, 1989).

Mixing of turbidity currents with the ambient lake is comparable to the physical entrainment descnbed previously for infiowing water masses and is. therefore, modeled by the Richardson number (Chikita. 1989). This mixing occurs in the form of large eddies; these plumes dominate the turbulent energy spectrum within the generally heterogeneous flow of the wrrent (Lambert et al.,

1976).

2.4.2.2 Origin and Control of Turbidity Currents

The origin of turbidity cumnts has ken linked to 1. the plunging of river water as a dense underfiow and 2. dumping of the delta forefront (Lambert and

Giovanoli. 1988; Pickrill and Irwin, 1983). Stun and Matter (1978) further classify turbidity currents as either seasonal (mused by annual river Roods), or catastrophic (larger events once or twice per century related to rare geomorphic events). However. this classification system may only be pertinent for a limited number of lacustrine systems, specifically. those systems wherein large-scale geomorphic events have produced distinguishable sediment facies and are likely to record future phenomena in a similar manner. Weirich (1986). who delineates these currents into surge type undedlows and continuous feeding underfiowç. has advanced this type of categorization. While these ternis broaden the sape of lacustrine systerns that may be influenced by dense underfîows, this classification does not adequately differentiate between a turbid plume moving within the water wlumn and a turbidtty current. A plume represents flow along a boundary of qua1 density (temperature or suspended sediment rnay be the infiuential variable), while turbidity currents are underffows related almost entirely to suspended sediment concentration and, as such, shape lacustrine records differently. Due to the specific nature of some lake-river environments. incongruities rnay arise when generalizations are made regarding the frequency or the timing of these events. Aithough currents generally occur throughout the year, they are particularfy important dunng periods of increased discharge (Le. glacial melt season andlor periods of increased precipitation)(Lambert and Hsü,

1979; Lambert and Giovanoli. 1988). Seasonal and diumal variations may be apparent in these currents (Gilbert, 1975; Wiench, 1986), although such characteristics are not definitive. Lambert and Hsü, (1979) examined Lake

Walensee. Switzedand. to correlate flood stage to turbidity curent strength.

Their conclusions linked the frequency of turbidity current deposition to the magnitude and frequency of high-water stage and not, necessarily, to annual cycles.

Sediment deposition from turbidity currents has been documented as an important phenornenon in lacustrine sedimentation (Gilbert. 1975; Lambert et ai..

1976; Sturm and Matter. 1978; Lambert and Hsü, 1979. Weirich, 1986). The influence of gravity dictates that, for the most part, turbidity currents follow the thalweg of the lake (Chikita. 1989) or may be restricted to channels moving outward from the deltaic environment (Lambert and Giovanoli, 1988). Smith et al., (1982) documented bottom topography as an important factor in sedimentation patterns in a Rocky Mountain lake that was. at the time, dominated by underflows. As much as the lake bottorn may influence turbidity currents and underflow, erosional and depositional abilities are able to significantly alter basal sediments. Eyles et al., (1987) found channels in awustic profiles that appeared ta have been ait and subsequentiy filied by turbidity currents. Differential deposition has also been documented resulting from turbidity currents, induding the gradua1 slowing and eventual temination in the distal regions of lake as bottom slopes upward. It is not uncornmon, therefore, tu find thicker distal sedimentary deposits (higher mean sedimentation rates) in lakes headwater inflows and established turbid underRows (Gilbert.

1975. Pickrill and IMn, 1983). A description of the sedimentary facies produced by these currents is addressed in the next section.

2.4.2.3 Deltaic Sedimentation and Turbidity Currents

As previously mentioned, the production of turbidity currents has been linked to the failure of delta fronts, creating a pulse of sediment which flows under the influence of gravity. Since the delta region represents a unique combination of fluvial and lacustrine processes, deltaic deposition holds particular importance for lacustrine sedimentation. In glacially fed Lake Tekapo.

New Zeaiand. deltaic sedimentation accaunts for approximately 55 */O of al1 annual sediment deposition (Pickfl and Invin, 1983). The frequency or precise rnechanism by which episodic-type turûidity currents are produced by delta failures is not campletely understwd. In addition to oversteepening of the delta front. earthquakes, low lake-levels (potential for reduced hydraulic support of delta), and wave action are possible episode tnggerç (Gilbert. 1975; Pickrill and

Irwin, 1983).

2.5 LACUSTRINE SEDIMENTOLOGY OF MONTANE LAKES

Once an understanding of the physical limnology of montane systems exists. the sedimentary stratigraphy containeci within the lakes of British

Columbia can be useâ to create a frarnework for examining paleosedimentary conditions. Studies focusing on the sediments contained within fiord-type lakes have provided a strong background on Southern Canadian Cordillera montane lacustrine sedimentation since deglaciation (Gilbert, 1975; Mullins et ai., 1990;

Eyles et al.. 1991; Desloges and Gilbert, 1991; Gilbert and Desloges. 1992;

Desloges, 1994; Desloges and Gilbert, 1995). In addition to the southem regions of the Cordillera, investigations at Boswer Lake and Chilko Lake represent a more recent shift in study sites to the northem reaches of the Cordillera (Gilbert et al., 1997; Gilbert and Desloges, 1998). Other studies (not all from Canadian lakes) have focused on laminated sediments, carrelating these with hydrometeorological conditions within the basin. changes to basin glacial wverage, and long-terni climate change (Pickrill and InMn, 1983; 0strem and

Olsen, 1987; Desloges, 1994; Leonard. 1985; 1986; 1997).

2.5.1 Canadian Cordilleran Investigations

Shifts in the depositional environment within a single lake basin over tirne have been docurnented in Cordillera lakes. thereby assisting in the

reconstruction of Pleistocene glacial ice retreat. These lakes contain a record of the paleosedimentary conditions in the region since the initiation of ice sheet

retreat and eventual disintegration. As such, the stratigraphie record contains an

integrated record of catchment evolution and lacustrine condlions.

Figures 2.5 a and b represent acoustic profiles hmtwo Southem

Cordilleran Lakes. According to Eyles et al. (1990), Unit II may represent a high-

energy depositional environment, possibly associated with retreating ice margins.

Unit III was hypothesized be vawed sediment. which were related to a lower kW- -.

'l.. * UH C

Figure 2.5: A. A reconstruction of a Lake Kalamalka acoustic profile highlighting two depositional facies (II and III). Source: Eyles et al. (1990). B. A profile from Hamson Lake dernonstrating the different acoustic signatures of transparent facies T and stratfied facies S. The respective sedimentary units (II and III; S and T) from A and B represent demonstrate different depositional environments possibly due to changes in sediment source or changes to depositional energy since deglaciation. Source: Desloges and Gilbert. (1991 ). energy environment with decreased overall sediment inputs. Note: the classification of Unit III as varved was made without the recovery of sediment samples and is. therefore, speculation. This summary is similar to Gilbert and

Desloges (1991 ), who demark the acoustically stratified lower unitS as a Late

Glacial sedimentary environment; also a highenergy depositional system. The acoustically transparent nature of the unit-T sediments indicated a smaller grain sire and, therefore. deposition was likely in a lowerenergy environment. This unit is designated as post-glacial. This large-scale shift in depositional energy has been associated with the retreat of the source glacier northward, exposing another lake (Lillooet Lake) upstrearn. thereby causing a drastic alteration to the sedirnent budget of Harrison lake.

Similar studies are Eyles et al. (1991) and Gilbert and Desloges (1992). both of whom identified other lakes (Okanagan and Stave. respectively) whose sediments demonstrate significant changes to their depositional regime (in situ ice disintegration, possible manne incursion, etc.). Shifk from proximal to distal glaciolacustrine conditions (Desloges and Gilbert. 1985; Desloges and Gilbert,

1998) or shifts in direction of sediment input (Gilbert and Desloges, 1992) have aiso been researched,

Several lawstrine studies have addressed the reduction of sediment inputs to a given lawstrine system over tirne. Ponding of glacial sediment behind terminal moraines (Smith, 1981; Gilbert and Desloges, 1987: Leonard,

1985; 1986), sediment exhaustion (Desloges. 1994). and the exposure of sedirnent reservoirs en route to the lacustrine system (Gilbert et al., 1997) indude some of the systems that have been examined.

2.5.2 Laminated Sedirnents

Although a wide variety of a sediment facies can be found in montane lacustrine systems, for the purpose of this review only laminated sediments are addressed. The presence of laminated sediments is diagnostic of a lacustrine system with cyciic sediment input over tirne. According to Smith (1978) the presence of laminations within a lacustrine record can be credited to five possible variations in sediment input: 1. diurnal 2. subseasonal 3. seasonal 4. annual and

5. exceptional. with the first contributing the greatest overall input of sediment, and the iast producing the least. The nature of these sediments depends greatly on the individual characteristics of the lake. even the relative positioning within the lake itself, thereby making generalizations about their appearance and mechanism of formation difficult.

2.5-2.1 Vawe Formation

Laminations that are annual are refemd to as varves. Recognition of varves stems from variations in sediment structure, rooted in changes to the energy associated w'th deposition. Generally, each varve consists of a aMne sumrner fraction laid down dunng the higher depositional energy environment during the mek season. The siltfsand dominated layer is deposited during periods of higher energy, and can indude deposition from suspension (interfiows and overfiows) and turbidity currents (high and lowdensity underfiows)

(O'SullNan, 1983). During the winter, when glaciers are less active and ice may cover the lake, pelagic sedirnentation dominates. creating a cap of finer clay- sized material. Therefore. the clay-dominated layer represents deposition during a pend of time when depositional energy is at a minimum and sedimentation occurs from suspension (Ashley et al.. 1985; Stum. 1979). The lamina grades

normally upward with. generally. a gradua1 transition between silt and clay

portions of couplets for the same depositional year and a sharp boundary between top of the clay and sik deposited at the start of the next season. The

boundaries of these layers and the consistency of the depositional regime depends on environmental factors such ice cover or the presenœ of winter

cydonic stoms (Desloges. 1994).

Dense, turbid undertlows have been reqnized for their importance to the

formation of varved sediment, due to their potential for transporting the coaner

fraction of inflow throughout the lake basin (Smith. 1981.Gilbert and Desloges,

1987). Turbidites are generally recognized within sedimentary facies as a sharp

basal surface (possibly erosional). sand or coarser silt grading nomally into finer

materials, overiain by silty clay at the top (Sturm and Matter. 1978; Desloges.

1994).

In detennining the necessary cornponents for varve production, Stum and

Matter (1978) discount the earliest models of vawe formation produced by

researchers such as de Greer, Kuenen, and Antevs. Stum and Matter's mode1

introduces turbidity currents as the sole mechanism of varve formation, while

Antevs links varves only overflows. Sturm and Matter (1978) research suggests

that in addition to both overfiow and turbidity currents, the seasonal stratification of the water column is essential for the formation of annual couplets. However. according to other work. underflows and stratification may not be wholly compatible. Although stratification does not restrict underfiow events, Gilbert

(1975). and Gilbert and Desloges (1987) documented a breakdown in thermal stratification linked to the plunging of waner turbidity currents. According to

Ashley et al. (1985). with the breakdown of stratification, turbid currents are disrupted and inflow results in homopycnal mixing, rather than discrete underflow currents. The notion that a combination of over and undeflow foms vawed sediments is reinforced by a number of other researchers (Lambert and HSU.

1979; Smith et al.. 1982). although these studies do not suggest that this combination of flow type is necessarily required.

Figure 2.6a shows two portions of a core taken from Bow Lake. Alberta. that shows a change in the character of sediment deposition (Smith. 1981). The younger lamination (ca. 1960) on the left are thinner than the older lamination

(ca. 1940) on the right due to a reduction in the input of sediment into Bow Lake.

Figure 2.6b shows a map of the region. indicating the position of the lake and the ice-contact pond that has fomed behind a recessional moraine. decoupling the glacier and it's production of sediment fmm the lake system. This type of

Holocene basin evolution has also been reported in the Arctic by Lemmen et al..

(1988). who documented changes in sedimentation related to direct dimatic and indirect geornorphic controls.

In sumrnary, the high sediment production and seasonality of glaciated basins seerns to be the only consistent requirement for the production of varves. Figure 2.6: A. Two parts of a single lacustrine sediment corn demonstrating a signifiant decrease in sedimentation rate in the reduced thickness of the annual larninations. B. A map of the Bow Lake region, Alberta. Partiailady important to this map is the presenœ of the pond near the Bow Glacier which has becorne a sediment sink between the glaciers and Bow Lake. reducing the sedirnent supply. Source: Smith. (1981).

CHAPTER 3 - METHODS

3.1 INTRODUCTION

This chapter describes al1 the methods used in the acquisition and analysis of data. The field season for this work was from May 20 to August 10,

1999. Descriptions are separated into sections based on monitoring and sample recovery carried out in the field and laboratory analysis camed out at Queen's

University.

3.2 FIELD MONITORING

3.2.1 Meteorological Observations

3.2.1.1 Temperature

Air temperature at lake level was recorded using a StowAway XTI

Temperature Logger. This logger was originally placed by another researcher on

May 20 of 1998 and left to record temperature over the fall and winter seasons at an interval of 2 houn. During the 1999 field season, the data from the logger were retrieved and the interval set to 5 minutes to allow for greater resolution.

The logger was placed in the shade at the base camp. located near to the lake

(Figure 3.1). However, an error in starting the logger occurred and data were not collected from June 9 to July 11. The logger was started correctly and during the remainder of the summer until August 4, 1999, temperature data were collectecl without incident.

3.2.1.2 Precipitation

Precipitation fell only as rain during the field season making collection in a simple funnel-based rain gauge possible. A funnel with a 19 cm diameter was attached to a water bottle using simple construction methods. This system was placed in the base camp, elevated 1.5 m off the ground in an open region unobstmcted by trees or other structures. During the summer. rain was measured daily by removing the water bottle from the structure and measured using a 25 ml graduated cylinder. Estimated error associated with this method is i 1 ml due to water residue in the rain bottle and accuracy associated with reading measurements on the graduated cylinder. Using the known area of the funnel, the volume captured by the rain gauge was calculated as millimetres of rainfall.

3.2.1.3 VVind and Waves

Daily measurements of wind direction were taken when researcherç were on the lake carrying out other analysis, or from the base camp. At this time. the height and direction of And-waves were recorded. Observations were also made from the base camp location; however. these were restricted due to the shape of the lake and. therefore, were more representative of stom or large- scale changes in wind direction. 3.2.2 Fluvial Observations

3.2.2.1 Stage Recording and Discharge Measurements

Stage was recorded using a Leopuld-Stevens Type F analogue recorder, positioned above a prefabricated cylindrical metal stilling well Wh dimensions

1.2 m height and 0.4 m diameter. The location of the well within Strohn Creek was based on the ability to place and maintain the well in swift moving water and a judgernent of the maximum and minimum heights the stream might experience during the season (Figure 3.1). The well was secured within Strohn Creek using rope suspended from the highway bridge above as well as several large rocks positioned at the base. No movement of the well was noted during the season.

The stage of the strearn was measured against a metre stick perrnanently secured to the side of the stilling well. The stage was measured and recorded every time the stilling well was inspected and the chart paper changed. The paper used for recording fluctuations had a maximum duration of eight days.

The recorder was checked every seven days and the paper replaced such that no data were lost. The data recorded were digitized at an intervai of two hours.

This was camed out at Queen's Univenrty, where information was put into

spreadsheet files for further use and analysis.

Discharge was measured by taking integrated measurements across the

Strohn Creek channel using an AOtt curent meter. The stream channel was divided into 1 m intervals. perpendicular to flow direction. using a fixed rope. An average of water velocity for each 1 rn section was calculated from

measurements at two-tanths and eight-tenths the measured depth of the interval. Veloaty was calculated from the number of revolutions per minute of a 4 cm propeller. The averages were then multiplied by the area represented by the 1 m sections. and the results summed to provide a measure of instantaneous discharge. A total of seven discharge measurements were made over the course of the summer, during pends of both low and high stage. These discharge measurements were plotted with stage data to produce a rating cuwe that was used to determine a discharge hydrograph from the continuous record of stage.

3.2.2.2 Suspended Sediment lntegrated Samples

Suspended sediment was sampled from Strohn Creek. Surprise Creek.

Hanna Creek. and Tintina Creek using a OH49 integrating sediment sampler

(Figure 3.1). This instrument was lowered fmm the highway bridges over the respective creeks into the deepest part of each channel. established prior to sampling using a weighted rope. During the summer, the deepest point rnigrated due to shifting channel material; the sampling position was relocated accordingly.

Sampling accurred at least once a week and more often during the freshet period. In total, sampling occurred on 18 days, providing 118 integrated samples.

An integrated sample of the water column was taken. nearly filling the sample boue with one profile (one descent and ascent to have the bottle near, but not wrnpletely, full). If the bottle was filled cumpletely. the sample was discarded and a new sample taken to ensure that overestimation of suspended aiment concentration did not occur (Le. continued deposition of sediment in the sample bottle after the maximum water volume had been attained). Frequency of sampling depended on the timing of other field work, availability of sample bottles, and observed changes in stage andlor sediment content of the creeks.

In total. Strohn and Surprise creeks were sampled 30 tirnes. Hannah and Tintina

Creeks were sampled 18 tirnes. Sampling was diçcontinued eariier for Hannah and Tintina Creeks as the observed sediment collected reached wnsistently low values as the field season progressed. The samples were filtered through 0.45 pm cellulosic paper using a hand operated vacuum pump attached to an

Erlenmeyer flask.

3.2.3 Acoustic Surveying

Acoustic records of the Meziadin Lake basin were taken during the 1998 field season using a Datasonics SB? 5000 3.5 kHz profiling system. Operating

from a small boat, multiple cross-lake and one longitudinal transects were made

in an attempt to cover the geographic extent of the lake. Sound velocity, for the

acoustic signal, through sediment was assumed tu be equivalent to water and,

therefore, set at 1460 mls. Positions were recorded directly onto the analogue

read-out from hand-held GPS (Global Positioning Systern) at one-minute

intervals. The potential positioning error associateci with this technique is k 50 m.

Analysis of acoustic transects involved digitizing the recorded GPS

coordinates with observable characteristics of sedimentary fill. Initially, depth of

the water coiumn (surface to sediment-water interface) was recorded to allow

construction of a bathymetric map (Figure 5.1). Bathyrnetry was constructecl by

plotting the values recorded for depth according to their GPS coordinates in

Grapher 2.0 software. A file mntaining the digitiued boundary of Meziadin Lake was then overlain. This illustration was exported to CorelDraw 9.0 to allow for the construction of isobathç using linear interpolation methods, based on th8 observable depth values.

Second, the thickness of sedirnentary fiIl was digitized, including multiple depositional facies (sediment-water interface to observable or calculated bedrock) according to GPS coordinates. In some areas of Meziadin Lake, the thickness of sedimentary fiIl exceeded the range of instrumentation.

Extrapolation of the shape of Meziadin Lake was accomplished by cavering the original transect prînt-outs with clear plastic laminant. and visually extrapolating the bedrock side slopes until they converged. A cornputer simulation of shape was attempted by placing GPS coordinates and bedrock depths into Grapher 2.0 software, using a low order spline equation to extrapolate a parabolic shape.

However, within the distal am of Meziadin Lake. near to the outftow of Hannah

Creak, there is a large bedrock shelf. which the spline curve was not able to take

into account. thereby causing signifiant error. A map of sediment thickness was created using these digitized points in the same manner as described above for

lake bathymetry (Figure 5.3). It should be noted that due to the impenetrability of

inflow-proximal sediments, interpolation of thickness isopleths would introduce a

signifiant amount of assumption. As a result, the inflow proximal region has

been assigned a 'no data" label.

Third, features of the subbottom record such as glide planes. or the

character of acoustic refledors, were observed to detemine the status of

physical processes within Meadin Lake. 3.2.4 Limnological Observations

3.2.4.1 Lake Stage

A record of lake stage was kept over the field season using a metre stick positioned within the lake near the field camp. This represented a relative measure of lake level change throughout the sumrner. This rdwas attached to a permanent dock in approximateiy 1.5 m or water. Although the lake level never fell enough to have the stick cornpletely exposed, lake level did reach a point higher than the measuring device. submerging the permanent dock. The metre stick was raised above the lake level with the bottom of the stick overlapping the previous position to allow for cornparison of level. When lake level had gone dom, the metre stick was retumed to its original location.

Lake level was recorded at least once a day dunng the summer. During the spring rnelting period. lake level was tecorded up to 4 times in 24 hour period.

3.2.3.2 Conductivity~emperatureITurbidityProfiling

Profiles of Meziadin Lake were taken at 18 sites throughout the lake using a Hydrolab Datasonde3 TernperaturelConductivilyTTurt>iditysensor. A single profile consisted of both lowering and raising the sensor with the rneasurement

interval set at 3 seconds (average of 1.5 m in accordance with practiced rate of ascentldescent), thereby giving an instantaneous reading of al1 of the above

mentioned variables for recorded depth in the water colurnn. lnertial error during sampling, introduced by the sensors. may have produced a dampend response to variation within the water colurnn, thereby slightly blurring sharp physical boundaries. The most significant error was associated with the thermal inertia of the temperature thennistor; error associated with turbidity and wnductivity data was likely minimal, associated with the downward or upward draw of water corresponding to the movement of the instrument. Optical turbidity, originally recorded by the instrument as NTU, was later calibrated to mg\L using an equation established for the instrument by Gilbert et al. (1997) for glacilacustfine systems. Turbidity, T in NTU. is converted to suspended sediment concentration, C in mgiL, using the equation:

C = -1.825 + 0.7557 T

Profile site selection was based on an attempt to cover the areal extent of the lake (Figure 3.2). This allowed an examination of proximalldistal trends and cross-lake trends in the measured variables. Nine sites were located in the

inflow-proximal am, with nine sites in the outfiow am of Meziadin Lake. This

separation is slightly skewed since the total area of the oumow am is

significantly greater than inflow-proximal am; however. the variability of

temperature/conductivity/turbidity was assumed to be more significant in the

inflow-proximal am, necessitatirtg greater coverage. The position of each site

was recorded using a GPS. The error associated with the GPS for the duration

of the field season (1999) was +1- 50 m.

Profiling was camed out on 13 days. The instrument was lowered and

raised from a winch attached to the boat, while attempting to maintain a constant

speed for ascents and dexents. Damage to the instrument made consistently BASE CAMP TEMPERATURE/CONDUCTIVIr/TTURBIDITY * PROFlLlNG SlTE O SEDIMENT TRAP MOORING SlTE TlDBlT UNDERFLOW TEMPERATURE RECORDER LOCATION ISOBATH INTERVAL - 10 and 20 rn

Figure 3.2:Location of TemperaturelConductivity~urbidity(CTD) profiling sites and \J sediment trap rnoorings within Meziadin Lake. spaced profiling days impossible. Instead, days of profiling represent days when the instrument was functional. Profiling was also govemed by weather; in addition to boating safety issues, rough water made consistency in the speed of asœnt and descent dificult.

3.2.3.3 Underflow Monitoring

Evidence of undeflow activity in the delta-proximal region was examined using Onset Computer Corporation's Tidbit StowAway Temperature Logger.

This submersible logger was placed near the delta in a region that was observed to be in the direct path of Strohn Creek fiowing into Meziadin Lake (Figure 3.2).

The logger was submerged in approximately 70 rn of water attached to a buoyed line, such that the logger was easily located. The logger's position was recorded using a GPS. Due to the error associated with the GPS. the position of the tidbît was also triangulated using landmarks on shore.

The range of the Tidbit Temperature logger is 4OC to 37OC according to the manufadurers specifications (Onset Computer Corporation Web Site, as of

Jan. 2001 ). While the resolution of the instrument is posted to be 0.16OC. its

accuracy is kO.Z°C; therefore, 0.2% is the minimum resolution value recognized

in the examination of underfiow events.

The initial sampling interval was 5 seconds; however. the tidbit data logger

filled every 4 days and needed to be replaced with another prepared logger. This

interval was deemed too short and later changed to 10 seconds, allowing 9 days

between replacement.

The logger record& temperature successfiilly until June 14 at which point the logger was found Roating in the middle region of the inflow am. It was

retumed immediately to its correct position with time and date recorded. On two

other occasions when the mooring appeared out of place (July 1 and July 25),

the logger data were examined and it was detemined that the Tidbit mooring had

been disturbed. The recorded temperature spikes were in the range of +20°C;

only removal from the lake could cause a disturbanœ of that magnitude, thereby

indicating disturbance by people using the lake. Since the logger was moved

from its correct position, data recorded after the disturbance, until the logger was

changed, were deemed unusable.

3.2.5 Sedimentological Observations

3.2.4.1 Suspended Sediment

Suspended sediment depoçition in Meziadin Lake was examined using

sediment traps positioned in the water column. Eleven sediment traps moorings

were established prior to spring melt and recorded suspended sediment

deposition from the water column for the entire lake over the course of the

surnmer. The traps were located dose to temperature/conductivity/turbidity

profiling sites to allow for direct cornparisons between recorded data and

recovered sediment, and to ensure good wverage of the areal extent of the Me. Sediment trap moorings mnsisted of two pairs of funnel-shaped traPs (Figure 3.3). One pair was located 40 m below the surface and the other 1 m

above the lake flwr, thereby distinguishing suspended sediment flux by depth.

The depth of 40 rn was decided on as an estimation of the maximum Iikely depth SURFACE ALOAT

1 FLOAT

Figure 3.3: A. A schematic of the sediment trap moorings used in Meziadin Lake.A pair of funnel traps were located 40 m below the water surface. and 1 m above the iake floor. Two Roats were used: the surface float for locating the mooring and the float at 20 m to keep the mooring line ngid and vertical B. A photo of a pair of funnel-shaped traps attached to aluminum rod supports. of the epilimnion and represents an atternpt to differentiate sediment deposition from this region. The depth 1 m was decided on in atternpt to catch sedimentation for the entire water column, including possible undediow activity.

Funnels were mnstmcted from sheet metal. cut and shaped by hand.

These funnels were attached to 500 ml Nalgene inert plastic bottles, allowing sediment to pass from the funnel to a thin replaceable receptacle. Baffies were used on the funnels to keep sediment from king resuspended or recycled within the trap, under the influence of currents, thereby destroying any recorded stratigraphy. The funneUbottle system was attached to the mooring rope using crosseû 6 mm thick aluminum rods, allowing the funnel and bottle to stay 50 cm from the mooring rope, avoiding the collection of algae or sediment that might have temporarily attached to the rope.

Pain of funne Wbottles were used to provide a back-up in the event of loss or damage. Some trap bottles were unusable Men recovered as the funnel was no longer attached to the collection bottle, and the timing of this loss was not known. Moorings 1 and 2 were placed in the lake on June 4, moorings 2, 3.4.8. and 9 were placed on June 5. and mooring 5,6.7,10, and 11 on June 6. The five moorings dosest to the point of inflow were recovered and examined on July

3. shortly after a period of high discharge. Moonng 1 required a change of al1 trap bottles. while rnooring 2 required a change of only the lower trap bottles. Ail traps were taken from the lake on August 8. The upper traps for rnooring 6 were lost. and trap mwring number 9 was lost completely and was believed to have falfen victirn to curious fisherrnen. 3.2-4.1 Ekman Samples

An Ekman sediment sampler was used to remove samples of lacustrine sedirnent from Meziadin Lake. The sampler was attacheci to the research boat winch and allowed to drop to the bottom of the lake, upon which a rnessenger was sent to dose the jaws. Sediment was raised to the boat and carefully examined. If the jaws did not close completely, or if the flaps on the top of the sampler did not close, the sarnple was discarded. Tin cans were used to preserve the sarnple stratigraphy. Ali samples were allowed a pend of approxirnately 36 hours to dry before plastic wrap was applied to both ends of the tin cans. thereby sealing the sarnple completely.

In total, 8 samples were removed from the lake. Sampling location was based on an attempt to cover the areal extent of the lake and. therefore. provide information on sedimentation in ail parts of Meziadin Lake. In addition. the placement of the suspended sedirnent traps was taken into consideration to allow for direct cornparison between the sensor profiling data, sediment trap rnoonng data. and the Ekman samples. Samples from the deeper parts of Meziadin Lake

(e.g. CTD sites 9, 10. 11, 12). and inflow proximal regions (CTD site 1) although desirable, were not logisticaliy possible due to the difficulties associated with excessive depth or coarse material not allowing Ekman penetration. 3.3 LABORATORY ANALYSIS

3.3.1 Suspended Sediment Fiiter Sarnples

Pnor to any field work, al1 0.45 pm cellulosic paper filters were placed in a desiccator to remove excess water and then weighed. These sample were weighed to 5 significant digits in The Queen's Geology Isotope Lab. All integrated samples taken were filtered in the field using a hand operated vacuum pump. After use in the field. these filter samples were oven dried in the Queen's lab at 60 OC for at least 12 houn and weighed to four signifiant digits. Some of the sarnples from early in the season showed negative values for the weight of sediment. The reason for this error is due to pre-season drying procedures in which fiiter papers were placed in a desiccator to remove water from the cellulosic filter paper. This procedure was not as effective in removing water as oven drying; therefore, initial weights (pre-field weights) contain ex- water. In

çorne samples, this excess water makes the desiccated filter heavier than the weight of the oven dried filter plus sediment (e.g. sarnples with little sediment accumulation). To correct for this, ten filter papes were weighed. oven dried, and re-weighed. An average for the weight of these papers was established and this value is recognized as the standard initial weight of al1 filten used during the field season- There was also some error believed to be associated with static electncity affecting the scale dunng the pre-season weighing of the filter papers, although this error is difficult to quantify. 3.3.2 Ekman Samples

Ekman samples were rernoved from their containers and cut into two sections using a wet knife. One section was sealed with plastic wrap and archived Mile the other was consumed by analysis. After cleaning the surface to a smooth finish, the samples were logged and photographed. A portion of the sediment measuring approximately 2.5 cm X 2-5cm X total length of Ekman sample was rernoved and placed on a fiat surface. Using a fine wire. the sediment was sampled at a minimum intemal of 2 mm in accordance with observable laminae. The maximum interval used was 4 mm in circumstances when no observable stratigraphic change was observed. The total number of

samples per wre ranged from 24 to 43, depending on the length of the sample.

The separated sediment was then plunged with a 1 cm3 syringe to provide a

known volume for each sample.

3.3.2.1 Organic and lnorganic Carbon

The detemination of organic and inorganic content in Ekman and

sediment trap samples was camed out by Loss on Ignition procedures (modified

from Dean, 1974). Each 1 cm3plug was placed on a pre-weighed foi1 sheet and

dned overnight at 60% to detemine water content. The sample was then re-

weighed, providing the percent of water and approxirnate bulk density of the plug.

At this time. each plug was broken up into ioose sediment. A small portion was

rernoved for grain size analysis. Sediment samples were transferred to pre-

weighed cnicibles, weighed. and heated for a minimum of 5 hours at 550'~to

remove any organic carbon. The sarnples were weighed after a short cooling period, and were then heated to 950% for a minimum of five hourç to remove any inorganic carbon. allowed to cool for a brief period and re-weighed.

3.3.2.2 Grain Sue

Grain size was examined using a Coulter LS 200. laser scattering particle size analyzer. During LOI experiments. a small amount of sediment was removed and placed in a pre-weighed test tube. Preparation for çamples was done according to the procedure outlined in Beierle et al. (2001 submitted). The arnount of sedirnent required for the analysis depended on the grain size. Finer materials required approximately 100 mg of sediment, while approximately 1 50 mg was used for sand grain sites. Two scattering runs were done for each sample.

3.3.2.3 Thin Sections

Thin slabs of sediment were removeà frorn the Ekman samples and frozen using liquid nitrogen to avoid the growth of large ice crystals. which could disturb sediment st~ctures.Sarnples were then freeze-dried and embedded with epoxy, under vacuum. according to the procedure outlined in Lamoureux (1994).

Samples were aR with a rock saw, sanded using progressively finer grit, and mounted on glass slides. The coarçe sediments from samples near the point of infiow (Ekman 3) becarne disturbed under vacuum and did not show clear sedimentary structures. The sarnples were examined by high-resolution scanning (1600 dots per inch) uçing a Hewlet-Packard Slide scanner. Enlarging the scanned picture allowed the examination of otherwise unobservable sedimentary structures, textures, and particle orientations.

3.3.3 Sediment Traps

3.3.3.1 Mass Accumulation Rates

Two laboratory procedures were used to detemine the Mass

Accumulation Rates (MAR) for the 500 ml sediment trap bottles; one method for bottles containing observable stratigraphy, and another rnethod for bulk sediment samples. Sediment from mooring numben 1 and 2 were sub-sampled for stratigraphy. Sedirnent fmm traps 3 - 11 were treated as bulk samples. Once the dry mass was established, the MAR for the summer was calculated using the known area of the funnel(248.8 cm2), and the number of days each trap was in place in the water column.

3.3.3.2 Stratigraphie Sampling

Samples that displayed intemal stratigraphy were marked for volume prior to opening. Each plastic bottle was cut on both sides to allow for wet knife splitting of the collected sediment. Half was wrapped with plastic and archived mile the other was used for analysis. The sediment was cleaned to a fiat surface, logged and photographed. Sediment was sub-sampled at 1 cm intervals with 1 cm3 of sediment removed using a syringe. Water content, and organic and inorganic carbon, were detemined on the plugs as describecl above for the

Ekman samples. Based on the water content and the initial volume of the sample, mass of the sample was calculated. This maswas then used to determine the MAR of the sample using the known area of the funnel, and the length of time the mooring was in the water column.

3.3.3.3 Bulk Sarnpling

Samples with no observable stratigraphy were transferred from the 500 ml

Nalgene bottles to pre-weighed plastic beakers. These beakers were placed in drying oven at 60'~until al1 moisture was removed from the sample. For some of the larger samples. up to 72 houn was required to remove al1 moisture. Samples were then weighed to 4 decimal places.

3.3.3.4 Grain Size

Grain size for sedirnent trap sarnples was done using the Coulter LS200 in the same manner as described above for the Ekrnan samples. For those bottles examined stratigraphically, the interval used for grain size sampling was consistent with organidinorganic carbon sampling interval. For those bottles examined as a buik sampk, each beaker was stirred periodically during water removal to ensure homogeneity of grain sue. Large portions of dry sedirnent were broken into loose partides for sub-çarnpling, also to ensure hornogeneity. CHAPTER 4 - BASIN HYDROLOGY AND SEDIMENT TRANSPORT

4.1 INTRODUCTION

Meziadin Lake. northern British Columbia, provides an opportunity to examine pst-glacial sedimentation at a waterçhed scale. In this chapter. components of the Meziadin drainage basin hydrologie system are examined as they relate to the mobiluation and transportation of sediments to Meziadin Lake.

Desloges and Gilbert (1994) suggest that the character of lacustrine deposits in a glaciolacustrine system cmbe related to the balance between intemal and extemal forcing mechanisms. The mobilization and deposition of sediments is attributed to a feedback relationship between the boundary conditions (lithologic. glacial and nival, morphometric. human) of the drainage basin and extemal forcing mechanisms such as precipitation and temperature. Together. these cm control water and sediment yields from the basin and. therefore, further influence the nature of glaciolacustrine depositional processes.

In this chapter a review of the physiography of the Meziadin basin outlines the development of the fluvial geomorphology of the drainage basin in the wake of Quatemary glaciation. Stream morphology and basin boundary conditions are examined as they relate to sediment mobility and funoff. The climate of the northem British Columbia Coast Mountains is also dowmented to understand the scale of past and present meteorological forcing on drainage basin hydrology. Finally, the hydrology of the Meziadin basin is examined using instrumental data aquired over a petiod of one melt season (spring and summer 1999). From these recorded data. a quantitative understanding of the basin hydrologie system cm be established, allowing correlations to limnologica! and sedimentological observations made during the same time period. The relationship between air temperature and precipitation and nival and glacial melts is of particular importance.

4.2 PHYSIOGRAPHY OF MEZlAOlN LAKE WATERSHED

4.2.t Regional Quaternary Glacial History

Geomorphic conditions in the Meziadin basin are partly a function of the development of the region's post-glacial fluvial system. Meziadin Lake is located on the eastem fnnge of the northem Coast Mountains. a region dorninated by glacial landforms and significant present-day glaciers.

During the maximum extent of the Cordilleran Ice Sheet. flows in the

Stewart region onginating from accumulation centres in the Coast Mountains moved predominantly west towards the Portland Canal and southwest along the fiord towards the Pacific Ocean (Clague,1989) (Figure 2.1). In the Meziadin region. on the eastem side of the Coast Range, the dominant fiow was southeast. This Row continued dong the Nass Depression towards what is now the Skeena River drainage system, eventually heading west towards the Pacific approximately 250 km south of the Meziadin basin (Figure 2.1) (Clague. 1989;

Ryder 8. Maynard, 1991). Since ice in the Meziadin basin likely originated from higher elevations and walescd in lower regions. ice fiow in the Meziadin basin may have reversed its direction several times dunng the expansions and retreats of the Cordilleran lcesheet (Ryder 8 Maynard, 1991). ln situ disintegration of stagnant ice, as well as the retreat of individual ice tangues marked the deglaciation of the Cordillera Ice (Ryder 8 Maynard, 1991; Fulton. 1991).

Glacier fluctuations during the Holocene in the northem Coastal

Mountains. particularly the StewartlGranduc region have kendescribed as periods of episodic advance and retreat (Clague and Mathews. 1992). A large number of the glaciers in the southem Coast Mountain range (e.g. Tidemann

Glacier) did not reach their maximum Holocene extent until the midHeoglacial.

2600-2700 yr BP (Ryder and Thompson. 1985). a pend also that corresponds to the advance of glaciers in the StewaNGranduc region (Frank Mackie and

Berendon Glaciers; Clague and Mathews. 1992). Although glaciers in the

StewarUGranduc region presentiy extend well below tree line from higher elevation centres of accumulation, since the Little Ice Age (LIA), glaciers here and in the Meziadin region have retreated significantly . exposing LIA lateral moraines and unconçolidated sediments at the fwt of glaciers (Figure 4.1). The

LIA maximum for the Berendon glacier, approximately 50 km northwest of

Meziadin, has been dated from tree rings at 1660 A.D. (Clague and Mathews.

1992). This glacier presently demonstrates significant retreat of ice from its LIA

lateral moraines (observed dunng field work). Glacial retreat since 1940 has

been particularly notable in the Meziadin basin, causing significant changes to

hydrologie regime as fluvial and lake drainage patterns change in response to ice

damming and lowered lake levels (Mathews, 1965). Church and Ryder (1972) Figure 4.1: Photographs of the Meziadin drainage basin from a small plane. A. The red arrow in the photograph highlights the Little Ice Age (LIA) lateral moraine created by this glacier, thereby illustrating the amount of recession in the Meziadin Basin. B. The red arrows highlight significant reg ions of unstable drift matetial deposited at the toe region of the receding glaciers. These represent significant sources of sediment to the postglacial streams cutting throug h this outwash region and, therefore, to Meziadin Lake (these streams eventually join to become Surprise Creek). Meziadin Lake can be seen in the background in the top centre of the photo (green arrow). suggest the foot of receding glaciers represents a region of unstable sediment storage that is available for fluvial redistribution by glacial or nival mnoff. This unstable drift represents a paraglacial sediment source for the postglacial fluvial systems of the Meziadin basin.

4.2.2 Site Description

4.2.2.1 Hannah and Tintina Creeks

Hannah and Tintina creeks are meandering streams that drain the northeastem and eastem portions of the Meziadin mtchrnent (Figure 3.1). The total area drained by these streams is approxirnately 155 km2, representing 30% of the total Meziadin drainage basin. Hannah and Tintina creeks drain low relief foothitls of the Boundary Range that are presently not glaciated and densely vegetateû. The morphology of both creeks includes gravellpebble point bars, exposed and eroding cut banks. and flood chutes. Fens and bogs are cornmon in this portion of the Meziadin catchment. The wiâths of Hannah and Tintina are approxirnately 10 m at the entry point to Meziadin Lake.

4.2.2.2 Strohn and Surprîse Creeks

The primary source of water and sediment to Meziadin Lake is Strohn

Creek, which is the confluent of Surprise and Lesser Strohn Creek. the major fluvial systems that drain the glaciated portion of the Meziadin catchment. The

Meziadin basin has two glacier accumulation centres; the iargest with 8.5% basin average is drained by Surprise Creek in the northwest. The total area drained by Surprise Creek is approximately 190 km2, representing 35% of the total drainage basin. The second center of accumulation, with 1.5 % basin average, is located south of, and is drained by, Lesser Strohn Creek. The total area drained by Leser Strohn Creek is approxirnately 50 km2,and represents 1196 of the total drainage basin. The cartographie representation of these streams on the Energy. Mines & Resoum 150, 000 scale topographic map Meziadin Lake:

Cassiar Land Distnct is inaccurate because it shows Lesser Strohn Creek to be the larger channel. This error may date to 1958, when Strohn Lake (Figure 3.1). an ice-contact lake outside the eastem edge of the present drainage basin boundaries, drained east into Meziadin Lake. In that year, a catastrophic drainage of icedamrned Strohn Lake occurred to the west beneath the retreating

Bear River Glacier (Mathews, 1965). Since that time. Strohn Lake has drained west through the Bear River towards the Portland Canal. Therefore. the topographic map may represent a time when fiow through lesser Strohn Creek was greater than at present.

Surprise Creek is a braided system,with large channel ban consisting of cuarse bedload material (Figure 4.2-A). Large debris (tree trunks, large branches) remains stranded on these bars from periods of higher flow. The dimensions of the Surprise channel are not known because high water velocities prevented transects from being wmpleted. However. integrated sample measurements for suspended sediment suggest an approxirnate depth of 2 m at the thalweg. Lesser Strohn Creek is also a braided river systern of reduced magnitude, thereby allawing transect measurements (Figure 3.4). The width of

Lesser Strohn Creek was 10 rn for al1 readings. except for June 18 and June 20. Figure 4.2: Photographs the Surprise Creek, facing southeast towards Meziadin Lake. Lesser Stmhn and Surprise merge to form Stmhn Creek, which flows into Meziadin approximateiy 600 m east of this point. A. Photograph was taken on June 7; The braided morphology of Surprise Creek can be observed mth large channel bars, some containhg signiftcant vegetative growth, other with large stream debris (arrow indicates the presence of a large tree tnink within the channel. B. Photo was taken on June 14; this pend represent the initial stages of freshet in the Meziadin basin. hma cornparison to Photo A, a significant rise in stage is observable (highway bridge pilings for sale). it is also intetesthg to note that the tree in Photo A has been rernoved by increaçed discharge of Surprise Creek. when. due to higher stage and faster currents. the location of transect measurements was shifted to a wider, shallower area (widths of 12 rn and 11 m respectively). An average depth for the 1 rn measured intervals over the course of the summer was 0.58 m, with a maximum depth of 0.94 m.

Seasonal climatic variations in hydrologie conditions in the drainage basin introduce the potential for lacustrine sedimentary processes to demonstrate a seasonal signal. Seasonal control of allochthonous sedirnentation can originate from several sources, with some acting on the entire basin and some acting only on the lake (O'Sullivan, 1983). The following examination of the climate of northern British Columbia detaiis hydrological controls on montane sedimentary processes specifically as they relate to the delivery of sediment to lacustrine basins.

4.3.1 Northem Coast Mountains, Briüsh Columbia

The regional climate of northwestem coastal British Columbia results from the interaction of Pacific and continental air masses (Cayan, 1989). Because the

Meziadin basin is at the margin of two climatic systems it is a challenging region in which to work but provides a unique setting to examine the relative strengths of continental and maritime synoptic systems through the Holocene.

No previous instrumental ciimate data exits for the Meziadin basin; however, climate nomals from three Environment Canada dimate stations surrounding the Meziadin region are shown in Figure 4.3. These data provide regional dimate information for cornparisons with the Meziadin drainage basin.

Temperature range differs significantly between the three locations based on elevation and physiographic setting. Stewart (coastal; elevation 7 m asl) has the smallest temperature range at 17.9 OC; Srnithers (interior; elevation of 523 m asl) has a range of 23.9 OC, and Dease Lake (interior; elevation of 816 rn asl) has the largest range of 30.3 OC.

Further examination of these normals highlights the signifiant reduction in precipitation experienceâ by the interior regions of BC (Smithers and Dease

Lake) campared to the western wast (Stewart). Located at the western edge of the North Amencan continental rnargin. the Coast Mountains are dominanted by eastward movement of wan, moist Pacific air masses (Cayan. 1989). Winter precipitation in the region is dominated by cyclonic systerns. developed in the

Gulf of Alaska by the Aleutian Low (Ryder, 1989). Precipitation in the northem

Coastal Range is dominated by orographic systerns. particularly as winter snowfall, as previously noted in the StewarVGranduc region (Gilbert et al.. 1997).

The Pacific High, which dominates summer in the northern Coast Mountain region, bnngs fair weather, with precipitation occuning most cornrnonly as convective storms.

The climate of the British Columbia lnterior System is a balance between the Pacific mantirne air masses (described previously) and continental air masses (Fufton, 1991). These synoptic weather patterns are modified by the cornplex physiography of the Cordillera (Ryder, 1989). Maritime air masses have generally lost a large portion of their moisture through orographic precipitation en route (as described above for Coast Mountains), causing a rainshadowing effect

(Bum. 1994). This west to east reduction in precipitation is illustrated in the climate nomals in Figure 4.3. Physiographic controls on precipitation have also been suggested to be an important factor in the establishment and growth of the extensive glaciers and icefields of the Coast Mountains (Fulton. 1991).

Interactions between maritime aimass and continental air masses from the

Arctic (winter) and frorn the tropics (summer) can be regionally specific in nature with local rain-shadowing and orographie effects contrasted with intermontane valleys acting as maritime air conduits (Spear and Cwynar, 1997).

It shoutd be noted that normals from the three stations do show some sirnilarity in their seasonal distribution of precipitation. There is a rnarked reduction of precipitation in March and April in al1 three regions, wrth increased amounts during the summer rnonths. Maximum values occur in the late summer at Dease Lake, versus late fall at Stewart and Srnithers. Eariy autumn is generally the period of highest precipitation for the Meziadin region (Mathews.

1965).

4.3.2 Meteorological Observations from Meziadin Lake Watershed

The rneteorology of the Meriadin Lake basin was monitored from May 25 to August 10, 1999. The following section is a summary of those meteorological observations and their relevanœ to Meziadin drainage basin hydrology.

4.3.2.1 TEMPERATURE

As noted in Chapter 3, some error was associated with the collection of 7- STEWART - €LN. 7 m m

--- JAK FEB mi APFi MAY- ai JUL AUC SEP OCT NQ\I DEC DATE SMlTHERS - ELEV. 523 rn

DEASE LAKE - ELEV. 816 rn

DATE

FIGURE 4.3 Climate nomals for Stewart, BC(1965-1998);Srnithers, BC (1942-1994) and Dease Lake, BC (1944-1 990). (Source: Environment Canada, Canadian Meteorological Centre, Canadian Climate Nonnals 8 Stewart Mean Daily Data) temperature data during the 1999 field season. As a result. important temperature data comesponding to signifiant changes in basin hydrology

(changes to be discussed in later sections) from the eariy portion of June were lost. Temperature records from surrounding regions were examined to detemine the existence of a suitable analog record. Records from Stewart, BC

(59.93*N, 129.889N), located 56 km west of Meziadin Lake, do not correlate well with recorded temperature from the Meziadin basin. The Boundary Range, which includes the Cambria Icefield, separates the Stewart and Meziadin basins.

This range is likely the dominant barrier to the effective translation of temperature.

Smithers, BC (54.8I0N. lZi.189N) is located approximately 250 km south of Meziadin Lake on the western margin of the Skeena Mountains. Although located significantly further from Meziadin than Stewart, Srnithers is located within the Nass Depression, a southern continuation of the Nass River drainage basin. Mean daily temperature at Smithers correlates well with recorded

Meziadin temperatures (Figure 4.4). Therefore, the cornplete Smithen temperature record is uçed to fiIl in gaps in the Meziadin record and correlate to

other measureâ hydrologie variables in the Meziadin drainage basin (Figure 4.5).

The relationship between air temperature and discharge is examined further in

Section 4.4 - Hydrology.

During late May and early June 1999 lower than normal spring

temperatures were recorded at several Environment Canada climate stations in

the Nass Basin. as well as southern interior BC. This cool weather had a DATE

4 1.23 StandardM St~WersTemp.

Figure 4.4: A. Standardized mean daily temperatures for Meziadin and Srnithers. The inset box highlights temperature data from the summer field season of 1999 that is examined at higher resolution in Figure 4.5. The period of temperature data loss (June 9 - July Il)can be observed in the above figure. Important to note is the significant temperature increase in early June, within the period of lost Meziadin data. B. The scatter plot demonstrates the strength of the relationship between the temperature records. DATE

Figure 4.5: A. Continuous records of Meziadin Basin precipitation, Meziadin Lake level, Strohn Creek dischargeland mean daily Smithers air temperature. B. The scatter plot demonstrates the strength of the relationship between standardized Lesser Strohn Creek discharge and standardized mean daily Smithers air temperature. The r2 value displayed represents the relationship including al1 dates. The r2 value increases to 0.54 if the highlighted dates (outliers) are removed. significant impact on the character of nival melt in the Nass Basin. Melting of the heavy winter snowpack was delayed, leading to a large and sudden release of water at the first significant increase in temperature. Severe flooding was reported in several regions of interior BC at the onset of the delayed spring freshet (press releases in Appendix l),in the Nass and Skeena Rivers in the

Terrace BC, region.

4.3.2.2 PRECIPITATION

Precipitation occurred only as rain at lake level in the Meziadin basin during the field season. Between May 22 and 28. low temperatures in the

Meziadin region allowed snow to fall at higher elevations. The maximum recorded 24 hour precipitation value for the summer field season was 15.87 mm on June 16 (Figure 4.5).

There are no extended precipitation records in the Meziadin basin; therefore; because precipitation in the Coast Mountains is regional in nature

(Ryder, 1989), only broad cornparisons can be made to other basins. Total precipitation collectecl at the MeAadin Lake meteorological station during June was 51 mm, a value consistent with the nomals for both Srnithers and Dease

Lake, but less than half that of Stewart (Figure 4.3). Total precipitation collected in July was 13 mm, a low value for Srnithen and Dease Lake, and an order of magnitude lower than expected for Stewart. Two months does not represent a significant period of measurement and, therefore, does not allow for close correlation between the Meziadin basin and the three dirnate normal sites.

However. it can be inferred that precipitation in the Meziadin region is more similar to Srnithers and Dease Lake than to Stewart.

Although wind in the Meziadin basin was studied only on a qualitative level, daily patterns of katabitic wind development and resulting wave action created a significant vector for sediment dispersal.

During the summer of 1999. wind direction was examined from the lake daily or several times daily, depending on the schedule for other lake process analysis. During wam days, with clear or partially cloudy skies, a strong katabatic wind developed in the northwestem glaciated portion of the Meziadin basin in the late moming, and fiowed east toward Meziadin Lake. Although not quantified by instrument, winds were estimated to reach 30-40 kmih in the inflow proximal am by late aftemoon (14:OO - 15:OO) on a regular basis. The resulting wind-generated waves grew during the afiemoon from small waves less than 15

- 20 cm high at midday (1 1:00 - 13:OO) to over 50 cm by late aftemoon (16:OO).

On several hot, clear days in mid July wave heights of 60 - 70 cm were recorded at 14:OO near the southeast bend in the lake. A significant difference in wind strength and direction existed between the two amis of Meziadin Lake, with wind and wave strength decreased signficantly within the infiow distal am. By late aftemoon (16:OU - 1TOO), wind generally reached its daily maximum, Mile wave height continued to increase. Waves began to re-orient themselves in a suutheasterly direction and pmceeded through the distal am. There was a significant reducüon in wave height, with waves rarely exceeding 30 - 40 cm in the distal am. 4.4 HYDROLOGY

This section examines Meziadin basin hydrology as it relates to the rnobilization and delivery of sediment to Meùadin Lake from the four inflowing cree ks.

4.4.1 Discharge

4.4.1.1 Hannah and Tintina Creeks

Stage was not measured continuously at Hannah (H) and Tintina (T) creeks because they were not initially wnsidered to be significant sources of water or sediment to Meziadin Lake. Instead, stage observations were made when suspended sediment samples were collected. Their depths changed synchronously over the field season, reaching a maximum (at the thalweg) of 1.7 m (H) and 1.O rn (T) in late May, shallowing to approximately 1.O m (H) and 0.6 m

(T) later in the summer. At the highest stage in the 1999 field season (May 25 -

June 4), both streams overRowed portions of their banks in response to increased lower elevation nival melts in the Meziadin basin.

4.4.1 -2 Strohn and Surprise Creeks

Strohn Creek is the primary water and sedirnent source for Meziadin Lake.

It is the confluent of Lesser Strohn Creek and Surprise Creek, two proglacial streams that drain the northwest portion of the MeUadin basin. Gauging of

Surprise Creek (the larger of the two) was not possible due ta strong currents and depths greater than 2 m. making transect measurements of water velocity impossible. Lesser Strohn Creek is smaller and has dimensions and flow characteristics that allowed river gauging and velocity measurements. While, the sirnilarity of the lesser Strohn and Surprise catchments (i.e. Quatemary deglacial history). and close proximity suggest that basin boundary conditions and meteorolugical forcing would be comparable. the magnitude of hydrologie response should be considerable less for Lesser Strohn Creek. Since Lesser

Strohn and Surprise are both proglacial. braided streams, it is assumed that glacial and nival melt signals are also consistent. Therefore, the discharge record from lesser Strohn Creek is likely a reasonable proxy for the total drainage into Meziadin Lake.

Using a rating curve, an equation for the conversion of stage to discharge was calculatecl (Figure 4.6). Using this equation. the continuous record of stage was converted to a record of dixharge for lesser Strohn Creek (Figure 4.5).

Seven measurements of water discharge were taken in total (june 1, 7, 11 , 18,

20, 26. and July 14) thereby providing rneasurernents oefore. during, and after

high stage to ensure the stability of the rating curve. The maximum discharge

recorded for lesser Strohn Creek for the summer of 18.35 mb-' was recorded at

14:OO, June 16, while the average discharge for the entire field season was 9.28

m3s-'. As mentioned above. Surprise Creeks drains 5.5 times the glacial

average, and 3.5 times the total drainage basin area of lesser Strohn Creek;

thus. for the purposes of correlation to other proglacial systems. the mean

discharge of Strohn Creek, king the sum of Surprise and Lesser Strohn creeks,

should be in the range of 40 m3s". This value is likely an underestirnation as it

indudes a conservative multiplication factor for the contribution of Surprise Creek. On the basis of this estimate. the maximum discharge from Strohn Creek

(on or about June 16) into Meziadin Lake was of the order of 80 - 85 m3s-'.

4.4.1 -3 Discussion

In the Meziadin basin, rneteorological forcing in the fom of precipitation and air temperature influences the timing and magnitude of nival and glacial rnett, mile physiographic conditions modify these signals.

The most important influence on discharge in the Meziadin basin, dernonstrated by an value of 0.45. is air temperature because it represents the available energy for nival and glacial melt (Figure 4.5-6). Leemann and Niessen

(1994) also documented a positive wrrelation between air temperature (Aphl to

October) and ninoff with a wrrelation coefficient of 0.70 (? = 0.49).

The air temperatureldischarge relationship in the Meziadin basin cm be made more statistically significant if the relative influences of precipitation and nival melt are excluded (Figure 4.5-8). The dates of these outliers in the scatter plot correspond with the two largest precipitation events for the summer and the most significant penod of nival melt (June 14-1 9). The removal of these outlying values (Le. dates associated with peak precipitation and timing of nival melt) increases the * value to 0.54. Diumal peaks in temperature are followed by diumal discharge peaks with an average lag of six hours (Figure 4.7). This diumal variation is not well pronounceci in eariy June, but is significant in the later part of the summer when discharge variation is almost completely a fundion of glacial mett with little or no nival component. Discharge (m3 s -1 ) Figure 4.6: Rating Curve for Lesser Strohn Creek with equation for the conversion of stage (s) to discharge (Q) and the calculated strength of the relationship

- Discharge 1999 --- Air Temperature Figure 4.7: A cornparison of Lesser Strohn Creek discharge with air temperature at lake level in the Meziadin basin. Peaks in air temperature are followed by peaks in discharge, with an average calculated lag of 6 houn. Rising trends recordeci in air temperature (from July 16 - July 18) are followed by nsing trends in discharge over the same period. Therefore, air temperature controis the magnitude of diumal variation, as well as base flow. Discharge records were transcribed at an interval of 2 houn; temperature logger was set at a recording interval of two hours. 4.4.2 Suspended Sediment

The focus of this section is to examine suspended sediment flux to

Meziadin Lake and its relationship with discharge (Figure 4.8-A & B). Aeolian inputs to Meziadin Lake are not addressed due to their minor role in the overall sediment budget.

In the previous section, the relationship of air temperature and precipitation on nival and glacial melt was discussed with respect to discharge. A sharp increase in discharge began on June 13, reaching a maximum on June 16, and dedined by June 19. The maximum suspended sediment concentration for al1 inflow sources (Surprise,Lesser Strohn. Hannah, and Tintina), over the entire field season, were recorded on June 16 (Figure 4.8). Mean and maximum concentrations (rnglL) for the four fluvial systems are surnmarized in Table 4.1.

Table 4.1: Mean and maximum suspended sediment concentrations for the Meziadin drainage basin fluvial systems

NAME MEAN (mm) MAXIMUM (rngn)

SURPRISE 336.48 1425.40

LESSER STROHN 42.04 184.52

TINTINA 38.80 117.53

Fluvial sediment load is a function of discharge (velocity & stage) and sediment availability (Desloges, 1995; Deslogeç & Gilbert, 1995). When the Row velocities of the Meziadin fluvial systems were reduced at the end of the pend of nival melt-enhanced discharge, shear stress acting on channel sediments is reduced. The ability of the streams to erode and transport sediment was reduced. resulting in decreased suspended sediment concentrations.

Another trend in suspended sedirnent concentration, illustrateci in Figure

4.8 (A&B), is the difference between the nonglacial meandering streams in which suspended sediment concentration decreased through the sumrner and the proglacial streams in which suspended sediment concentration stabilizes in

late summer. Suspended sediment in Hannah and Tintina Creeks approaches zero in the later stages of the summer, while suspended sediment stabilizes within the Meziadin proglacial systems (Surprise and Lesser Strohn creeks).

4.5 CONCLUSIONS

In summary, during the spring and sumrner of 1999. coder May and early

June temperatures delayed the melting of snow packs in the Meziadin basin.

High precipitation throughout May saturated soils and snow pack at lower

elevations. A significant rise in temperature from June 11 to 17 provided the

necessary energy for high-elevation nival melt and the initiation of glacial melt.

The amalgarnation of precipitation events, nival and glacial melt produced a large

and rapid increase in discharge on June 16. This maximum discharge was more

than double the mean calwlated for the entire summer. The maximum

suspended sediment concentrations were recordad on the June 16 for al1 fluvial

systems, corresponding to the period of maximum discharge.

The timing of highest dixharge in Maadin basin corresponds to a significant rise in Meziadin Lake level (Figure 4.5). Although no records exist for long-tenn lake level variation, long-terni local residents in the Meziadin area cornmented that the speed of lake level rise, as well as the absolute lake level, associated with the 1999 melt season was among the highest they had ever seen, breaching raised shorelines and flooding private docks. CHAPTER 5 - MORPHOLOGY & LIMNOLOGY

5.1 INTRODUCTION

The morphology and the physical limnology of Meziadin Lake play a significant role in determining patterns of seùimentation. The fom of the lake bottom influences the direction and down-lake extent of underfiow events.

Circulation and stratification determine the extent to which inflowing river currents are modified by, and incorporated into. Meziadin Lake.

In this section. the morphology of the Meziadin Lake basin is examined using amustic records to detennine the distribution. thickness. and character of sedimentary fill. Three physical parameten (temperature. conductivity, and turbidity) were monitored throughout the 1999 field season to allow the evaluation and reconstruction of density stratification and circulation patterns in

Meziadin Lake. A continuous record of lake bottom temperature, near to the point of Strohn Creek inflow, denotes the timing and character of underfiow activity.

5.2 M€ZIADIN LAKE MORPHOLOGY

Meziadin Lake exists in a glacially scoured basin that likely contained a larger, deeper proglacial lake dunng the retreat of the Cordilleran Ice Sheet. This overdeepened basin is a resuit of erosion by a southeastward flowing ice Stream originating in the Coast Mountains. Wfih morphology and physical processes similar to those of coastal fiords. MeMadin and a large number of similar postglacial lakes within the Canadian Cordillera cm be defined as fiord lakes

(terni originally coined by Nasrnith, 1962)(Figure 2.4).

5.2.1 Acoustic Record

Details of the procedure used in the Meziadin lake acoustical survey are located in Section 3.2.2. The depths obtained from this acoustic survey, as well as depths sounded using other lake monitoring equipment throughout the 1999 field season. were plotted according to their GPS coordinates. In total, 25 transects of the Meziadin Lake were completed and a bathymetric map was wnstructed by interpolating isobaths between recorded points (Figure 5.1).

Meziadin Lake deepens rapidly from the delta front reaching 50 m depth within 150 m distance of the delta front with a mean dope of 18'. Further from the point of inflow this slope is reduceâ to 5' from 50 m to 100 m depth over a distance of 600 m. These angles are slightly lower than those rewrdeâ for the

Bowser Lake delta, a comparable fiord lake located less than 100 km north of

Meziadin (Gilbert et a/., 1997). Hummocks 0.5 to 1m in height exist on the delta foreset slope and toeset regions. indiwting the siurnping of material (Figure 5.2-

A), although no glide planes are evident.

The inflow-proximal ami of Meziadin Lake has a U-shape at the sediment- water interface (Figure 5.2-B), established 0.8 km from point of inflow, reaching a maximum depth of 133 m that extends as far as the mouth of Hannah Creek.

Wdhin the distal amof Meziadin Lake, a strong reflective valley-bottorn, below __---ACOUSTIC TRANSECT

-- - .- ISOBATH interval 310 and 20 rn

FIGURE 5.1: Bathymetry of Meziadin Lake, constructed frorn the acoustic survey of the 1998 field season. Figure atso illustrates the multiple transects, covering the areal extent of Meziadin Lake, frorn 18 3.5 kHz profiling system operated from a srnall boat. Profile , 11 0.5 km EAST i

41 I

rd. V.E. = 10

1 IML V.E. = 8.75 Figure 5.2: Subbottom acoustic records (3.5kHz) from several transects in Meziadin Lake. Depths are plotted for a sound velocity of 1460 rnls. For run locations, see Figure 5.1 A - Profile of the delta region taken from a portion of transect 18.Hummocks can be seen between 90 and 100 m. B - U-shaped proximal arrn taken from transect 11. Lake bottom is beyond the range of instrumentation, therefore, only Facies 1 is observable (cf. Figure 5.4), with a large number of laterally continuous reflectors. C - V-shaped distal arrn of Meziadin taken from transect 1. Undulations in Facies 1 (acoustically stratified upper layer) controlled by topography of lower facies (2 & 3) (observable in Figure 5.4). $ the sedimentary fill. becornes evident in the amustic cross section at transect 8. and at the same GPS coordinates in transect 18 (long profile), and persists to the distal end of Meziadin Lake (Figure 5.2-C). Corresponding to the presence of this surface within the awustic record is the shift from the U-shaped sedimenüwater interface tu a V-shape. Gentle undulations in sediment fiIl are controlled by this irregular bottom surface topography (Figure 5.2-C).The depth of the lake is reduced significantiy from 120 m adjacent to Tintina Creek to 50 m over a distance of 2 km southwest.

5.2.1.2 Sediment Thickness

Sediment thickness in the inflow anof Meziadin Lake extends beyond the range of instrumentation. Transect 11 represents an eastlwest cross-section showing the upper 50 m of sedirnent fiIl (Figure 5.2-8). Calculations for total thickness at a given transect were produced by placing clear plastic laminant over the original acoustic readout, tracing of the observable bedrock. thereby allowing visual extrapolation of the bedrock side slopes. Computer simulation of shape using a low order spline equation was also attempted; however, this method was unable to account for the broad shelf on the northeastem side of the distal amand, therefore, introduced signifiant error. Extrapolation placed the bottom of the valley at transect 11 at 284 rn below present lake level. where there is 152 m of sediment fiIl, focused in the rniddle of the lake channel. A maximum total thickness of 185 m was extrapolateci for the inflow am, located along transect 15 (Figure 5.3). This is comparable to a sediment thidmess of 234 m calculated for Bowser Lake (Gilbert et a/., 1997). Focusing of sediment along the Figure 5.3: Thickness of sedimentary fiIl in Meziadin Lake constructed from the acoustic subbottom record, including al1 three sedirnentary facies (as described in text). Sandy, acoustically impenetrable sediment in proximal region (black out region - no data) prevented assessrnent of thickness. Thickness assumes that the sound velocity in sedirnent is the same as in water (1460 mls). thalweg of the lake channel may be a result of 1) quasicantinuous slope failure along the steep valley sides (Desloges and Gilbert, 1998) and 2) significant deposition from turbidity currents that follow the deepest part of the lake

(Chikita. 1989).

Sediment accumulation is minor on the south and southeastern side of the lake due to steep topography, while more significant accumulations exist on the shelf on the north and northwestem side (near the Hannah Creek outflow).

There are no visible slump depusits at the edges of the U-shaped trough within the inflow am: buried slump deposits of this nature were rewgnized in the Stave

Lake acoustic record, indicating the build-up and failure of lateral deposits

(Gilbert & Desloges. 1992). There are. however. several srnall, localized slumps, induding glide planes, in the distal am near to the Hannah Creek point of outflow.

Lake depth on the southem (right-hand) shore of Meziadin Lake is 5 m lesthan the northern shore, recordecl 2 km from point of inflow. A 3 rn difFerenca in sediment thickness along the southem shore is also noted in transects 4 km from point of inflow. This accumulation of sediment along the right-hand side of the inflow am is likely due to the Coriolis effect deflecting turbid plumes (Pharo and Carmack, 1979; Smith, 1981). No such right-hand shore increase in sediment thickness is visible in the distal am. possibly due to a significant reduction in sedimentation, thereby rnaking northlsouth variation dificult to measure (Pickrill and Irwin. 1983; Desloges and Gilbert. 1991)(cf

Figure 6.2). 5.2.1.3 Character of Sediment

The coarse texture of sediment in the delta region does not allow acoustic penetration and, therefore, the acoustic record does not provide data on foreset progradation. The presence of coarse material is typical of active delta regions and has bennoted in similar postglacial lacustrine systems including Bowser

Lake. Chilko Lake. Lillooet Lake. and Stave Lake (Gilbert, 1975; Gilbert and

Desloges, 1992; Gilbert et ai., 1997; Desloges and Gilbert, 1998).

in the distal am. acoustic penetration to the bedrock surface occurred throughout this region, indicating the existence of three sedimentary facies

(Figure 5.4). Facies i appean in all acoustic transects, throughout the lake

(proximal and distal arms), as the uppenost zone of acoustically stratified sediment. Although the bottom of Facies 1 is not observable in the proximal am. the thickness of this deposit in the distal arrn is generally consistent across the lake width (thinning at the shorelines). Within this facies. multiple acoustic reflecton lie horizontally in the proximal am (Figure 5.2-B) but confomi to the undulating topography of the underîying surface in the distal am(Figure 5.2-C).

Reflectors remain unintempted and do not pinchout in those proximal am transects that demonstrate increased thickness along the southem shore due to

Coriolis deflection. The character of these surfaces is not consistent with depth; there is a trend to stronger reflectors doser to the waterlçediment interface. This trend was recorded in the acoustic profiles of Chilko and Bowser lakes, where changes in the acoustic irnpedence (Le. stronger reflecton) of the most recently deposited sediments may indicate a change in depositional regime andlor a change in sediment source from deeper sediments (Gilbert et al.. 1997; Deloges

& Gilbert. !998). Changes in the character of acaustic reflectors may also be partly a function of sampling procedure as time-variable gain of the acoustical signal cannot be ramped staeply enough to fully account for attenuation in more deeply buried sediment (Gilbert. 2001 ; personal communication).

The repetition and character of reflectors shows a significant proximal- distal variation. indicating a variation in the nature of sedimentation. In the distal am (Figure 5.3). a single major refiector at a depth of approxirnately 53 m eventually splits into several reflectors moving northwest towards the proximal atm. Due to the splitting and merging of reflectors. it is diffwlt to follow an individual reflector over a significant distance across several transect recordings.

Facies 1 is likely a post-glacial lacustrine deposit consisting of mainly silt and clay rhythmites deposited from suspended sediment and turbidity currents.

In the proximal amturbidity currents have likely been the primary agent of deposition, producing strong (coarse-grained), laterally continuous reflectors

(similar to facies seen in Bowser Lake by Gilbert et ai.. 1997). while in the distal am. the role of turbidity currents is reduced, consequently reducing the frequency and strength of reflectors.

Facies 2 is a dense. laterally inconsistent layer that is observed to have its greatest thickness at, or near. the thalweg of the lake channel. The top surface of Facies 2 is generally fiat wmpared to the lower boundary. which is highly irregular, following the undulations of the lower surface. Some basic layering or grading of this facies can be observed but is dificuit to follow consistently across individual transects.

The lack of wherent reflectors and poor penetration due to the denselcoarçe nature of this facies suggests deposition in a glacilacustrine or high-energy ice-proximal environment. The focusing of this facies near the thalweg suggests powerful, persistent turbidrty currents. The lateral extent of these deposits throughout several transects and the generally conforniable upper surface also suggests deposition from large turbidity currents from a retreating ice mass (Gilbert and Desloges. 1987). Therefore. Facies 2 is likely deposited in a higher energy environment than present, possibly when ice existed in the proximal arm or in the nearby Stnihn and Surprise river valleys.

Facies 3 is the lowest recognizable layer recurded in the acoustic profiles.

Due to its density and. therefore, la& of consistent acoustic penetration in many places, this layer cannot be continuously distinguished from the bedrock beneath the lake across an individual transect. Although difficult to categorize in al1 transects, this facies tends to be focused in the centre of the lake channel but closely follows the undulations of the lower bedrock surface. There does not appear to be any intemal structure to the facies, and its thickness, where observable, is inconsistent.

The coarse, unsorted nature of this deposit suggests that this facies is a water-lain till or ice-contact sedirnents. Poorly sorted diamict bedding can be related to deposition from highenergy sub-glacial meltwater streams (Eyles, et al., 1987). Lake stratification, intemal currents, and patterns of sedimentation can be influenced. or controlled, by the character of inflowing river water. Density. which changes through the year with temperature change, suspended sediment and dissolved sedirnent loading of the rivers and lake, is the primary control on the interaction of river and iake water masses; thus. the relationship is predicted to be dynamic. In this section temperature, conductivdy (dissolved sediment concentration). and suspended sedirnent measurements are used to examine interactions between Strohn Creek and Meziadin Lake. and the properties of dom-lake water masses derived from inflow'ng fluvial systems. Hypolimnic temperature is monitored using a submersible logger to establish whether dense river currents piunge at the point of inflow to becorne turbidity cumnts and, if so. to examine the characteristics of these temperature anomalies.

5.3.1 Temperature

Temperature profiles of the water column. from stations covering the areal extent of the lake (Figure 3.2), were used to recognize the initiation and development of thermal stratification and the presence of river-induced currents.

Sites 3, 5 and 7 are within the infiow proximal am of Meziadin Lake. site 16 is within the distal ami, and site 10 is considered a transitional station between the No.

Weak stratification of Meziadin Lake began prior to the date of fint measurements (June 2), with a small rise in temperature in the top 20 m of the water column at al1 sites (Figure 5.5). Mean air temperature for May was 9.4 OC. SlTE 7

TEMPERATUFE (OC) 2 4 6 8 10 12 14

SlTE 5

SlTE 10 SlTE 16

Figure 5.5: Temperature profiles from several CTD stations over the course of the summer of 1999 illustratirtg temporal and spatial variations in the themal structure of the water column. There is no June 28 profile for site 7. with considerable cloud cover and rnild. but recurring, precipitation. and fairly

significant wind from the south, conditions that likely delayed the onset of any

signifiant thermal stratification. On June 10, inflow proximal sites undewent

what appears to have been seasonal overtum, with temperatures at al1 depths at

3.9'C, while the distal sites appeared unaffected. The distal anof Meziadin

Lake may have expenenced overtum eartier in the melt season. or may possibly

have exhibited a more gradua1 mixing of the water column, suggesting a regional

separation of mixis behaviour through the lake.

The depth and character of the thermocline changed significantly both

temporally and spatially over the course of the summer in Meziadin Lake.

Stratification in the ouMow am (site 16) developed unifomly over the summer

with a weak themocline centred at 10 rn, and lake-wide maximum epilimnal

temperatures of 17.3OC. Disruption of the themocline at site 16 is attributed to

the westeriy katabatic winds that regulariy produced waves originating within the

infiow am, which then migrated southeast through the lake (see Section 4.3.2.3)

and prevented a strong tripartite structure from developing. Thermal structure

development in large alpine lakes is typified by strong heating of surface water

without the development of a distinct epilimnion (cf. Gilbert, 1975; Desloges and

Gilbert, 1998). Any development of a thermocline is deepened and weakened

within the inflow arm and at transitional site 10 by these katabatic winds.

A second source of thermal stratification distuption at inflow proximal sites

is the inflowing river curent from Strohn Creek. Although the density of these

currents, and the resulting depth at which they move through the lake, is controlled primarily by suspended sediment concentration (discussed in section

53.4). inflowing water masses play a large role in determining the depth, development. and stability of the themocline (Gilbert, 1975; Hamblin & Carmack,

1978). At site 3 development of thermal structure in the upper 20 on June 17 m was disnipted by a period of maximum fiuvial discharge from Strohn Creek

(Figure 4.5) and, therefore, the presence of inter- and underfiowing water masses, causing disniption of the entire 105 m water culumn. The Meziadin

Lake water mass continued to interact inflowing water masses and by July 5 the water column at site 3 had established false themioclines (not true tripartite water column structure based on density difference) at 60 rn and 15 m respectively, with these depths representing dynamic boundaries for interfiow.

These trends were recorded at reduœd thickness and higher elevations in the water column at sites 5, 7, 10 on August 6, possibly indicating a reduction in the strength of inflowing current or an retum of stability to the water column.

A rise in the temperature at the lake bottom indicated the presence of an underfiow current on June 17 at sites 3. 5 and 10, with the magnitude of the recorded temperature change reduced with increasing distance from point of inflow. This disniption corresponded to the pefiod of maximum fluvial discharge from Strohn Creek (Figure 4.6). Warmer surface water was drawn to the bottom as part of plunging river flow due to the higher concentration of suspended sediment (Canack et aL. 1978; Pickrill & Irwin, 1982). with temperature disniption at the lake floor providing one means of recognizing the movernent of this underflow through the lake (Gilbert, 1975), with the eventual incorporation of the underflow into the ambient water approximately 10 km from point of infiow.

Site 7 does not record an underfiow, but demonstrates temperature variation on the çame date as an interRow from 40 to 70 rn depth. This and other underflow events are discussed in more detail in section 5.3.4.

5.3.2 Conductivity

Patterns of conductivity closely mirror those of temperature with inflowing water masses being the prirnary control of variation w-thin the lake, particularly in the proximal am. Precipitation represents a secondary influence on conductivity restncted to the upper water column.

Higher conductivity values were recorded in the hypolimnion where the rnixing of the lake and inflowing water bodies is lirnited. while lower values were recorded in the epilimnion, which was reduced by inflowing water masses and precipitation (Figure 5.6). Lower conductivities from water originating from

Strohn Creek were predicted as its discharge is primarily derived from ice and snow melt and has not had an opportunity to incorporate dissolved solids (Gilbert et al., 1997). Low conductivities in the upper water column recarded early in the season (June 2) may also be a result of melting ice cover, particularly in the outflow am (site 16) where wind-blown ice collected during late April and early

May, as noted by local residents.

As previously described using temperature data. inflow proximal sites recorded a localized mixing of the water colurnn on June 10-12, with conductivities throughout the vertical extent of the water column reaching 0.96 mS cm". The depth at which condudivity recorded the deveiopment of weak SlTE 3 SlTE 7

OATE JUNE 2-8 JUNE 10-1 2 JUNE 17

JUNE 21 SITE 5 I JULY 5 AUGUST 8

SlTE 10 SlTE 16

Figure 5.6: Conductivity profiles from several CTD stations over the course of the summer of 1999 illustrating temporal and spatial variations in the conductivity characteristics of the water column. There is no June 2-8 profile for site 7. thermal stratification was also consistent with temperature data. Conductivity transitions (false chernoclines) in the proximal am represented the dynamic upper and lower boundaries of infiowing water masses of lower conductivity from

Strohn Creek (e.g. site 3 between 20 and 70 m). The effect of these river-borne currents is also observed at sites 5 and 10, where reductions in conductivity are reduced in magnitude higher in the water colurnn.

During the field season. water colurnn conductivities recorded at site 3 became progressively reduced due to the incorporation of glacially derived rneltwater from Strohn Creek. This water column trend was recorded at site 5, but becomes difficult to discern at sites 7 and 10, and was cleariy reversed at site

16. The conductivity increases seen in the distal regions of Meziadin Lake are attributed to the inputs from Hannah and Tintina Creeks (which drain fens and bogs) and subsequently migrate southeasteriy due to wind and waves, as well as infiltration from nonglaciated regions within the basin such as the slopes that bound the southwest side of the lake.

The presence of underfiow activity is recorded on June 17 at sites 3. 5. and 10 by reductions in conductivity in the lowest regions of the water column.

By June 21. conductivity profiles at sites 3 and 5 have re-established intedow charactenstics, Mile the underfiow is still visible at site 10 as late as June 21, at reduced magnitude.

5.3.3 Turbidity

The primary control over the spatial variability of turbidity (concentration of suspended sediment in mg Tl) in MeUadin Lake is the density dnven interactions of fiver-induced wrrents with the lake body. Inflowing water masses. iaden with suspended sediment, exhibit a cornplex relationship with the lake body in that they can potentially disrupt, or be influenceci by, varying stability of water column stratification. The conversion from turbidity to suspended sediment concentration

in mg/L is outlined in section 3.2.3.2.

Profile data from June 2-8 and June 10-12 illustrate turbidity to have been

neariy uniform throughout the water colurnn in the proximal am, regardless of the weak initial thermal stratification developing within the lake (Figure 5.7). The

inteflow of suspended sediment from Hannah and Tintina creeks was significant

to the suspended sediment budget of the distal amduring the early stages of

nival meit (note the slight bulging of the interfiow plume at a distance of 12 km.

from inflow at Station 11. the point of oufflow for Hannah Creek); however,

slightly higher concentrations of suspended sediment in the lake were recorded

in the inflow amassuciated with the initiation of the freshet period in the

StrohnlSurprise fiuvial system (Figure 5.8-A).

Turbidity profile data from June 17 indicate the presence of a significant

underflow event associated with plunging river flow at sites 3. 5 and 10 (Figure

5.7). The suspended sediment concentrations recorded for the June 17

undetfiow activity were the highest lacustrine values taken during the summer

field season, with the maximum concentrations of 75.6 mglL recorded at site 4 on

this date. Examination of the June 17 CTD turbidity data in a longitudinal profile

(Figure 5.8-8)suggests that several discrete underflow events are recorded

throughout the lake rather than a single lake-wide underfiow as suggested by the O' I 1 1 I

40- A E SITE 5 :8 80:

AUGUST 6 iKm) Olt3

SlTE 10

Figure 5.7: Turbidity profiles from several CTD stations over the course of the summer of 1999 illustmting temporal and spatial variations in suspended sediment concentration within the water column. There is no June 2-8 profile for site 7. SUSPENMO SEDIMENT CONC ENTFTION (mgl- 1 1 0-5

1 1 5-10

I -I ] 10-15 .: 1 15-20 20-30

30-40 40-50 50-60

v I I 1 1 I 1 1 I 604 O 4 8 12 16 A) DISTANCE (km) Figure 5.8: Long profiles of Meziadin Lake constructeci from CTD profile sites on June 10 and June 17 demonstrating variations in suspended sediment. Overflows, interflows, and underflows are recorded d O in different regions of the lake and at different times throughout the summer field season. V.E. = 45 d profile data (Figure 5.5). There are four high suspended sediment concentration underfiow 'bulgesn that can be traced from the point of inflow at site 1. as far as site 13 in the distal am, 12 km dowklake. On June 17, site 7 recorded a strong interfiowing current in temperature, conductivity, and turbidity profiles from 30-70 rn depth in the water colurnn (Figure 5.5, 5.6, & 5.7). Variation in the character of inflowing water masses in the proximal amwas likely due to Coriolis deflection, promoting higher suspended sediment concentrations on the south side of the lake as the underfiowing plume impinged on, and continued along, the right-hand boundary of the lake (Hamblin & Canack, 1979; Pickrill and Irwin, 1983). A transect of the proximal am illustrates the presence of the undeflow current along the south margin and the interflowing material located along the northem margin; however, these phenornena appear to be diçcontinuous at this time

(Figure 5.9). The northern shore interflow may be a remnant of continuous inteflow that was teminated when the infiowing current increased in suspended sediment concentration and plunged to the lake bottom to become an underflow.

Site 16 turbidity profiles over the course of this period (June 2-21) remained unchanged, indicating a signifiant proximal-distal difference in limnologie response to inflowing suspended sediment.

Profiles from the end of the freshet pend (June 21) and late in the summer (July 5 and August 6) illustrate a spatial and temporal change in the character of turbid interfiows and, therefore, changes to the character of the thenocline. Suspended sediment concentrations recorded at site 3 indicate that turbid interflows shift from a thickness of 50m on June 21 to 30 m on July 5, and Distance (km)

SEUSED IN # TRANSECT PROFILE # SITE NOT USE0 IN TRANSECT PRORLE - AXlS FOR TRANSECT PROFILE

5.9: Transect of suspended sediment concentration constructed from CTD profiles taken on June 17 across the width of Meziadin Lake. approximately 20 rn by August 6 indicating a thinning of interfiow currents at a single location over the coune of the summer (Figure 5.7). The thinning of interflow currents over tirne was also recorded at sites 5, 7, and A0. A spatial trend in interRow character was illustrated by an interfiow on July 5, which was recorded decreasing in thickness as it progressed dom-lake from 50 m at site 3, to 30 rn at site 5, and 20 rn at site 10. Longitudinal profiles of Meziadin Lake for

July 5 and August 6 (Figure 5.10-A & B) illustrate the thinning of turbid plumes at each profile site over time, as well as the tapering of individual plumes as they move down-lake. It is important to note that the recorded thickness of these

interffow currents from suspended sediment concentration are consistent with temperature and conductivity variations in the water column previously examined in this section (Figures 5.5 & 5.6). The thinning of interflow thickness at the point

of inflow is Iikely related to a decrease in Strohn Creek discharge and suspended

sediment load (Figure 4.6), thereby allowing river water to enter the lake as a

cohesive unit. Therefore, the structure of the water column can be considered a

fundion of the inflowing plume's density (pnmarily a function of çuspended

sediment concentration), which detemines interfiow thickness and position in the

water wlurnn, rnodified over tirne and space by the development of a weak

thermal stratification within the lake body. This development of water culumn

stratification may allow interflows to be more eficiently transported down lake

later in the summer as site 16 demonstrated the presence of interfiowing

sediment on July 5 and August 6 (Figure 5.7). 0- # StTEUSEOIN LONG PROFILE 8 SITENOTUSEDIN 10- LONû PROFILE AXlS FOR LONG

80-

110-

I 1 7 1 I I I . 1 SUSPENDED O 4 8 12 16 P SEDIMENT USTANCE (km) CONCENTpATTON (ml-1 i 1 0-5

160 Jr T I . I . 1 I 1 . 1 O 4 8 12 16 20 DISTANCE (km) Figure 5.10:Long profiles of Meziadin Lake constructed frorn CTD profile sites on July 5 and August 6 demonstrating variations in suspended sediment. Overflows, interflows, and underflows are recorded - O in different regions of the lake and at different times throughout the summer field season. V.E. = 45 ui 5.3.4 Underflow Events

Underfiow activity in Meziadin Lake is attribut& to plunging river flow. wntrolled primarily by the greater density of the inflowing water (Gilbert, 1975;

Wierich, 1986) and influenced by the density stratification of the lake body

(Carmack et al., 1979). A record of underflow activity is, therefore, a proxy record of the periods when sediment is disperçed and deposited by turbidity current flow. The Tidibit temperature logger located on the lake floor (Figure 3.2) recorded the passage of turbidity currents as a nse in lake bottom temperature.

Anomalies in the hypolimnic temperature are related to the plunging of dense river water, wamed during its routing through the drainage basin fluvial systems

(Weirich, 1986) and, additionally, through the entrainment of warmer surface water (Lambert and Giovanoli, 1988). The presence of underflows, identified as positive temperature anomalies, has been measured and studied by several authors (Gilbert. 1975; Lambert and Giovanoli, 1988; Lewis etal, submitted).

Mean hypolimnic temperature at the point of inflow, for the duration of the surnmer field season, was 4.69 OC, with a total range of 2.66 OC. The minimum resolution of the instrument is 0.2OC;therefore, fluctuations smaller than this value should not be wnsidered a record of actual environmental phenornena

(Figure 5.1 1: examples of values above and below this minimum are shown for

June 28 & 29). A cornparison of the hypolirnnic temperature record to the Lesser

Strohn Creek discharge record shows that the pedod of maximum underflow temperature variation corresponds to the freshet period for the Meziadin basin fluvial systems (Figure 5.1 1). The freshet perïod also corresponds to the penod of maximum-recorded suspended sediment concentrations in Surprise Creek.

From June 10 to June 21, underfiow activity appearç to have becorne quasi- wntinuous, suggesting that the plunging of river water was extensive across the delta region, thereby preventing colder hypolirnnic water from reaching the sensor and reducing the recorded temperature (Gilbert, 1975). Temperature spikes are visible on June 10, June 12, the end of the largest peak on or about

June 14 (period of lost data), June 17. June 19, and June 20. The recording of four temperature peaks by the underfiow monitor on and prior to June 17 corresponds to the turbidity profiles taken by the CTD sensor (Figure 4.6-B), which outiines four underfiowing sediment bulges moving through the lake.

There appears to be some variability in the character of temperature anomalies during this period of quasi-continuous underflow. from the earliest recorded undeffiows (June 9-10) to those at the end of the freshet period.

Hypolimnic temperature fluctuations do not appear to correlate well with diumal cycles of discharge in the early part of the freshet period (Figure 5.1 1: W6- 1 Y6).

Temperature spikes near the end of the freshet pend (June 17, 18, and 19), however, correspond well with the discharge curve, as peaks in undertlow temperature follow diurnal discharge peaks for each of the three days. In section

4.4.1.3 nival melt was show to reduce the strength of the relationship between air temperature and discharge by reducing the resolution of diumal discharge fluctuation. This shift to increased diumal variation in plunging river Row near the end of the freshet period is, therefwe, likely a result of the reduced influence of nival rnelt on strearn discharge and, therefore, over stream competency. After June 21. temperature anomalies are generally reduced in magnitude, and are recorded as discrete interruptions of lake bottom temperature rather than a quasi-continuous period of flow. The maximum increase in temperature recorded within the hypolimnion after June 21 was 1.1 OC on July 20. with the majority of recorded anomalies in the range of 0.25 - 0.5OC. Increases in hypolimnic temperature and diurnal fluctuations of discharge show some covariance. with several recorded underfiow events closely rnatching discharge peaks. Over the entire pend of recorded underflow data, however. a cornparison of discharge verçus temperature has an F value of 0.34, indicating a weak statistical relationship between the dixharge of Lesser Strohn and

Meziadin Lake underRow activity (Figure 5.643). Temperature change of the inRowing current at different times may partly explain the low correlation value between data sets. Lateral shifts of turbidity currents may have introduced discrepancy between the records as the temperature logger may have recorded only a portion of the total underflowing current (Lambert and Giovanoli, 1988).

Examinations of several temperature spikes, from the later part of the summer. show multiple peaks (particulariy evident on July 9; other events include June 27.

July 1, July 20, July 24 and July 31) al1 of which are greater than the minimum resolution of instrument. These multiple peaks likely indicate a change in fiow direction of turbidity currentç: an initial increase in temperature componding to underfîow, a drop in temperature as the curent shifts lateraliy away from the logger, and a second increase in temperature as the current shifts back towards the logger. Discrepancy between the stage record acquired for Lesser Strohn Creek, and the true stage for Surprise Creek. the larger source of water and sediment to Strohn Creek, may also have reduced the strength of the relationship between underfiow temperature and discharge records.

There is further evidence of cuntinued underflow activity down-lake, as recorded in turbidity longitudinal profiles from the mid to later summer (July 5 and

August 6). An examination of the dischargelunderRow graph (Figure 5.1 1-A) shows that spikes in underfiow temperature were recorded shortly before the July and August dates of CTD profiling. Subsequently, on July 5 and August 6 the presence of undertlowing turbid water was recorded by the turbidometer. at concentrations that were considerably less than during the freshet peirod.

5.4 CONCLUSIONS

The rnorphology of Meziadin Lake is typical of fiord lakes found within northem and southern Canadian Cordillera. Sediment thickness, character, and distribution, detenined from awustic surveying, provide insight into the postglacial depositional history of the region. High energy, ice-proximal sedirnents (Facies 3) are followed by stratified highenergy turbidity current deposits (Facies 2), followed by distal lacustrine deposits (Facies 1). This stratigraphy suggests that the tongues of Meziadin basin glaciers retreated. depositing signifiant amounts of sediment as they went. with the largest proportion focused at the thalweg of the lake channel. A signifiant proximal basiddistal basin inequity exists in the quantity of sediment deposited during retreat. as well as a significant distal reduction in the importance of deposition from turbidity currents. Significant refiectors in both arms. dernonstrate the past significance of turbidity currents as a source of sediment to the central regions of the lake channel.

Monitoring of temperature, conductivity, and turbidity provides an indication of the inf uence Strohn Creek has on circulation, thermal stratification, and sedirnentation. As sediment-laden water entered Meziadin Lake as an undiffetentiated plume. the weak stratification of the summer on 1999 does not provide density boundaries strong enough to immediately dictate the depth at which interfiows w.ll travel through the lake. Initially, the significant suspended aiment load, originating from freshetdorninated discharge, created several diurnal pluses of underflowing sediment. Following this. as discharge and suspended sediment decreased. the plume entered the lake in a thick doud and eventually moves through the upper water column. thinning and penisting as a nbbon plume until further down lake. Coriolis deflection has a significant

influence in the proximal arm; however. suspended sediment concentrations are too low to note a significant right-hand shift in the distal arm.

Turbidity currents. although reduced in strength since glacial retreat. exist throughout the summer. appearing as two dominant foms. The first fom. charged with freshet-induced suspended sediment. exists as a large quasi-

continuous underfi~~ngplume, raising the temperature of the hypolimnion by as

much as 2%. The second fomi. related to diurnal variations in discharge and

suspended sediment. are smaller. laterally shifting currents, dernonstrating multi-

modal peaks from 0.25 - AOC. CHAPTER 6 - SEDIMENTOLOGY

6.1 INTRODUCTION

Meziadin Lake represents a continuation of rnontane lacustrine research focused on fiord lakes of the Canadian Cordillera, specifically, lakes of the mid and northern Coast Mountains (Gilbert et al.. 1997; Desloges and Gilbert. 1998).

An examination of the character and thickness of Meziadin postglacial sedimentary fiIl places the basin within a spectrum of Cordilleran lacustrine basins and provides detail on regional Quatemary glacial retreat, rates of sedimentation and basin inundation, and sediment routing controlled by basin hydroclimatology (cf.,Gilbert. 1975; Eyles et aL, 1990. 1991; Gilbert and

Desloges. 1992; Desloges and Gilbert. 1991; Mullins et al. 1990).

During the 1999 spring and summer field season, sediment trap moorings and Ekman grab samples were recovered to allow analysis of depositional environments within Meziadin Lake. Sediment trap moonngs provide mass accumulation rates (MAR) over the areal extent of the lake differentiated by height in the water column, as well as recording the stratigraphy of a single season deposit in some samples. Ekman samples allow analysis of depositional trends at the water-sediment interface. as well as recovering several previous seasons of sedirnentation for cornparison. with the number of yean rewvered dependent on the depth of sediment sampled and accumulation rates for that region.

The goal of this work is to establish an understanding of the distribution patterns and rates of accumulation for sediment in Meziadin Lake. This includes an examination of the physical processes that are active within the Meziadin drainage basin and within Meziadin Lake. The importance of turbidity currents as a mechanism of coarse-grain sediment deposition is of particular significance to an estimation of Quatemary accumulation rates.

6.2 SEDIMENT TRAP ANALYSE

6.2.1 Mass Accumulation Rates

Sediment trap moonngs were placed in the lake in the fint week of June

(June 5-7) prior to spring freçhet. which was established from discharge data to have begun on or about June 10 and continued until June 21 (Figure 4.6). at which tirne discharge decreased. On July 3, infiow proximal trap mwrings (1, 2.

3, & 4) were brought to the surface and examined (Figure 6.1),Wh the upper and lower bottles Rom mooring 1. and the lower bottles from moonng 2 replaced.

Therefore, proximal trap moonngs 1 and 2 rewrded temporal changes in MAR. contrasting the initial 29 days with the subsequent 36 days of the summer. MAR was calculated using the dry weight of the sample (detemined using bulk or stratigraphie sampling methods; see section 3.3.3.1) and the known area of the funnel, and the number of days the trap was in the water wlurnn.

6.2.1.1 Spatial Variation

Accumulation of sediment in Meziadin Lake demonstrates significant

inflow proximalfdistal, and northlsouth shore variation (Figure 6.2-A). Sediment aTidbi! Logger Trap 4

7 River

,,.?.. ,'. TlDBlT UNDERFLOW TEMPERATURE 'Y DATA LOGGER LOCATION INSET 2 ISOBATH INTERVAL - 10 and 20 m Figure 6.1: Meziadin Lake, British Columbia. The bathymetric map displays the locations of several aspects of field work from 1999 including CTD profiling, sediment traps, and underflow monitoring. lnset 1 shows the location of Meziadin Lake, specifically, where the lake is located with respect to physiographic systems of BC. lnset 2 shows the location of Meziadin Lake within the drainage basin with respect to glaciers and fluvial systems. 181.88 180 Mass Accumulation p 160 Rate (MAR) for entire 2 120 N UPPER TRAP season so 4E * 60 Missing Trap 30 O 1 Upper 1 Lower 2 Lower Trap MAR from June 4 to July 3 = 29 days

" ' 1 Upper ' 1 Lower ' 2 Lower ' T~P MAR from July 3 to Aug. 9 = 36 days B Figure 6.2. A. Mass accumulation rates (MAR) for sediment traps moorings. Values illustrate the significant variation in MAR based on proximalldistal and northlsouth positioning, as well as vertical distribution in the water colurnn. There is no upper trap at mooring 11 due to the shallow water depth. B. Temporal variations in MAR associated with the first 29 - days of the summer field season, versus the last 36 days. Ul accumulations near to the point of infiow are two orders of magnitude larger than the accumulations in the distal regions. MAR in rnoonng 1 was relatively similar for upper and lower traps (181.88 and 208.47 glm2/d respectively) as eariy spring fluvial currents entered the lake and progressed approximately 300 rn lacking significant vertical differentiation (also demonstrated in the longitudinal turbidity profiles of Figure 5.1 0-A&B). lnflow circulatory patterns were established within

1.5 km from the mouth of Strohn Creek. as MAR for mooring 2 illustrates significant differentiation in deposition from the top 40 rn of the water column

(42.15 glm21d in the upper trap) versus the entire 110 m water column (168.86 gim21d in the lower trap).

The proxirnalldistal trend in sediment accumulation is partially reversed within the proximal anwhere upper trap accumulations for sites 5 & 6 near the southeastward tum of the lake were 1.2 times greater than mid-proximal am traps 3 & 4. This trend was more significant in lower traps, as traps 5 & 6 demonstrated an averaged increase in accumulation 2.1 times greater than traps

3 & 4. One explanation for this MAR increase is the shape of the lake may introduce resistance to the continued dom-lake progress of sediment plumes, thereby providing a region of deposition at traps 5 & 6, while the mid-proximal am represent areas of continued transpartation energy (at traps 3 & 4). Recall from section 4-2.2.3, that katabatic winds and their resultant waves were significantly reduced in magnitude after being reoriented southeast. Traps beyond the southeastward bend (numbers 7-1 1) reestablish progressive reductions in MAR with increasing distance from point of oufflow, with the most distal traps (10 & 11) accumulating sediment in the range of 1-2 g/m2Id.

Cross-lake variations in MAR were also observed in moorings 3 versus 4, and 5 versus 6; southem accumulations were 2.4 time greater in upper traps and

1.8 times greater in lower traps. Greater MAR values from upper and lower traps of moorings 3 8 5 are attributed to greater concentrations of suspended sediment in over, inter. and underfiowing plumes moving preferentially dong the southern shore. deflected by Coriolis forces (Figure 5.9; also documented by Smith, 1978).

However. greater proportional accumulation in upper traps (Le. greater Coriolis deflection evident in upper 40 m of water column cornpareci to total water column) suggests significant deposition by turbidity currents. which are influenced by gravity, bottom topography, and direction of inflow, and less by

Coriolis forces (Lambert et al., 1976).

6.2.1.2 Temporal Variation

Trap moorings 1 & 2, which recorded accumulations before and after July

3, recorded intraseasonal variation in MAR. Sediment accumulations recurded

by the upper trap of moonng 1 prior to July 3 represented 80% of the total

sediment accurnulated over the entire summer. with a MAR over the 29day

period (45% of the total time in lake) of 324.32 glm21day. The lower traps of

moorings 1 and 2, accurnulated 86% and 92% of their respective total sediment

mass over the same 29 days, producing MAR values of 400.08 and 346.19

glm21day (Figure 6.1-8). This trend of early summer sedimentation is simiiar to

Pharo and Camiack's study (1979), where these authon found that 90 % of

river-borne material entered Kamloops Lake during the freshet period from May- June (aha fiord lake with one major inflw; Figure 2.1). This cornparison is significant since the Kamloops Lake basin is presently unglaciated, hilethe

Meziadin contains 53 km2of glaciers in its basin. This similarity in sedimentary accumulation reinforces the notion that the strongest control over discharge and, therefore. stream competency for Strohn Creek is nival melt. Discharge variation associated with glacial melt, and its resultant suspended sediment. is a secondary source.

MAR values after July 3, for the remaining 36 days of the summer. show significantly reduced values. It should be noted that the traps were removed in

August of 1999. but sedimentation likely occurred for at least another month, perhaps longer. However, an examination of the fluvial suspended sediment concentrations displayed in Figure 4.8 suggests that the transportation of sediment to. and accumulation of sediment within. Meziadin Lake after August 10 would have been insignificant to the total annual budget. It is important to note, however, that MAR for the upper trap of mooring 1 is greater than the lower trap in the later part of the summer, while MAR in the lower trap was greater during freshet. This indicates a temporal shift in the depth at which sedirnent enters

Meziadin. This vertical increase in the depth of interfiowing plumes over the course of the summer was also recorded by the turbidometer at site 3 (Figure

5.7; discussed in section 5.3.3).

Although as much as 80-90% of the total sediment accumulation for the summer for proximal sites occurred in the first 30 days of the field season, this trend is not necessanly applicable for sediment accumulation in distal ragions. Deposition in distal amtraps beyond 12 km (CTD site 10) likely occurred almost exclusively from suspension. Sedimentation may have, therefore. taken longer to move down-lake within the turbulent epilirnnion, settling in distal regions, than observed for the highly turbid fiow demonstrated in proximal regions. In addition, continued development of thermal stratification through the summer appeared to increase the efficiency of interflows (turbidity profiles in Figure 5.7, and longitudinal profile of interRow in 5.10). transporting sediment further into the distal regions of the lake over the duration of the field season.

6.2.2 Character of deposits

Upper and lower trap bottles from rnoorings 1 & 2 collected sufficient sediment that the internat lithostratigraphy of the deposits could be sub-sarnpled.

Upper and lower traps indicated a shifk in texture from a wane base. with a very dark gray Munsell colour of 5Y 311, fining upwards throughout, with colour shifting towards olive tones to 5Y 414 (Figure 6.3.1-A and 6.4.1A). The texture of upper and lower traps of mooring 1 are more coarse than the mooring 2 traps, hich is a function of increasing distance from point of inflow and reduœd energy available for transportation.

The Partide Size Distribution (PSD) plots (after Beierle et al.. subrnitted) illustrate that, in addition to an upward fining trend. variations in the particle distribution symmetry suggest the deposlionai energy associated with each trap varied significantly over the accumulation pend (Figure 6.3.1-8and 6.4.1-8).

The lower trap of mooring 1 fluctuates between discrete penods with a wide distribution of finer particles sites. to peflods of coarse sand (maximum particle

size of 950 pm) with high percent of volume values (Figure 6.2.1-8). Similar fluctuations are recorded in the upper trap of mooring 1. although partide size does not exceed very fine sand (maximum particle size of 120 FM). ln both

sample's PSD plot there appear to be 3 periods of higher energy, mrse

sedirnentation pnor to the change in trap bottle (Le. under the black line) with the

intervals centred at 105. 37, and 22 mm in the lower trap and 70,48, and 30 mm

in the upper trap. The asymmetrical distribution seen in these traps suggests that the coarse grain sizes (medium to fine sand) are lirnited by fiow wmpetency

(8eierle et al., submitted), while the penods in which the distribution expands into

finer particles sizes (including fine silts and coarse day) may indicate settling in

standing water. The upper 15 mm of the lower sarnple, deposited after July 3

(above the black line), indicates a pronounced shift to lower energy, finegrain

deposition, while this shift to lower energy occurs before July 3 in the upper

sample (Le. fining trend established below line indicating replacement date).

An examination of frequency distribution for these samples (Figure 6.3.2)

suggests that the variations displayed by the PSD charts are significant. The

graph of mean particle size correlates very well with intervals associated with

shifts in kurtosis. These match closely to the PSD plot of changing percent

volume, with 2 significant perioâs of increased grain size and leptokurtic

behaviour at 105 mm and between 45 and 25 mm respectively (this larger

intewal encompassing two reg ions of high percent volume). Skewness

dernonstrates an inverse relationship to these trends, with distribution more

negatively skewed within the above - mentioned intervals. This behaviour reinforœs the likelihood that periods of increased volume percent from the PSD charts (higher mean grain size, increased kurtosis, and more negative skewness) represent periods of higher energy and the prefetential deposition of coarçe particles; followed by periods of lower energy deposition including wider distribution symmetry of finer grain sizes.

The traps of rnooring 2 (Figure 6-4-14)do not demonstrate the same PSD variability, nor are their percent volume values as high as those illustrated in the more proximal mooring 1. This reduced distribution variability likely indicates that the method of deposition in this region is more consistent through time. These traps do, however, illustrate changes to the energy associated with deposition, or sediment availability. in this region occumng pnor to the July 3 replacement.

Upper and lower traps demonstrate similar trends in PSD with high volume

percentages of more coarse sediment (very fine sand) grading nomally upwards with broadening of the particle size distribution, thereby reducing percent volume

values. The mean distribution of grain size (Figure 6.4.2) shows significantly

reduced variation compared to mooring 1; kurtosis also initially demonstrates

only very slightly leptokurtic values cornpared to the more proximal mooring 1.

The symmetry of the rnooring 2 traps is relatively stable (excluding the lower trap

spike at 18mm). shom by a fairiy consistently negatively skewed distribution at

approximately 4.8and standard deviation at 3.8, cornpared to variable

asymmetry of mooring 1 traps; this likely indicates that sedimentation from still

water was more predominant in regions further from point of inflow.

The trends of organic and inorganic carbon are mirrored in nearly al1

Standard Deviation (lm Kurtosis Skewness

a -4, Upper ~1:: Trap w-PO-.

Lower Trap

Figure 6.4.2 : Statistical data (mean, standard deviation, kurtosis, skewness) demonstrating changes to the distribution syrnmetry of individual grain size samples from sediment trap mooring 2. All data shown in this figure correlate to the stratigraphy examined in Figure 6.4.1. sediment trap profiles, with an organic carbon trend of higher to lower percentages through the summer, while inorganic carbon increases over the summer at comparable rates (Figures 6.2.1 & 6.3.1 - C). This is likely related to an increase in the proportion of glacially denved inorganic sediment in Strohn

Creek discharge and. therefore, a relative reduction in organic material introduced by the Strohn Creek channel or Meziadin Lake body. This trend is disturbed in the lower trap of rnooting 1 by increases in organic carbon values at the intervals corresponding to more coarse sediment.

6.3 EKMAN SAMPLE ANALYSE

In total, 8 Ekman grab samples were taken from Meziadin Lake.

However, similarities between samples from neighboring sites suggest that, to avoid redundancy, only 4 samples need be examined in this chapter. Samples were treated as cores and, therefore. split. cleaned, and allowed to dry for a short period of time to allow for irnproved contrast of light and dark sediment prior to photographing. Cores were sub-sampled at srnaIl intervals (between 3 - 5 mm), deterrnined by observable changes in texture or wlour, to ensure that fine variations in sedimentation were recognized.

Ekman sample 3, from the same infiow proximal location as CTD site 3

(Figure 6.4.1-A). shows significant variation in texture frorn dark reddish brown sands (Munsell: 5YR 2.512) to reddish gray coarse silt (5YR 512) and yellowish red (5YR 518) clay. This change in colouration according to grain size is consistent through the remaining 3 Ekman samples. irii

Li- Laminations in Ekman sample 3 appear to be rhythmic in nature. These laminae range from 15 to 20 mm thick, with a basal layer of coarse sediment grading nomally, to finer silt and clay size particles. with a sharp upper boundary between fine sediment and the next overlying cuarse bed (Figure 6.5.1 A).

Figure 6.1 shows the location of core pc-1 b, taken from the lake during the summer of 1999. Analysis of this core (total length of 1.24 m), by another researcher, suggests that the sediments of this region are varved. with mean varve thickness at 10.2 mm (standard deviation of 4.15 mm) (Scott Bames, personal communication; 2001). There are 6 full couplets in this sample. with a warse deposit on the surface from the 1999 field season, and approximately 20 mm of distu- sediment at the bottom of the core (dated as 1992). Intrusions of coarse material occur within the silt and clay haifwuplet of every rhythmite observable in the core sarnple. These coarse grained intrusions are interpreted to represent highenergy stom events during the generally quiescent autumn and winter depositional pend (Shaw & Archer, 1978; Gilbert, 1975). Recall from section 4.3.1. autumn is a penod of significant rainfall in the north Coast

Mountains, which aids in explaining the consistency of these intrusions throughout the Ekman sample. The thickness of the warse sand half-couplet compared to the finer silt and clay halfcouplets appearç to be generally equal in most rhythmites, although the presence of intmding sands in the fine deposits causes some discrepancy. The PSD of Ekman sample 3 illustrates significant shifts in grain size distribution (Figure 6.4.1-8). with the coarse fraction centred at approximately 95 pm (very fine sand) and the finer fraction centred at approximately 8 pm (fine to very fine silt). Between 10mm (bottom of 1999 coarse sediment) and 130 mm (sample becornes disturbed) there are 6 significant variations in PSD that can be wrrelated to the 6 full rhythrnite couplets in the are sample. Correlation between observed structure and grain size measurement data is illustrated by the cuwes of other statistical measures of frequency distribution (skewness, kurtosis), recognizable in the annual variation of distribution syrnmetry. The mean grain size illustrates the presence of varves with the coarse half-couplet recognizable as a peak in the curve (Figure 6.5.2). 1

It is interesting to note the presence of additional peaks in mean grain size relating to interruptions of the fine portion of the varve by the stom-event laminae seen in Figure 6.5.1-A. Leptokurtic and negatively skewed trends also correlate well peaks in mean grain size, suggesting that the periods of

highest available energy for deposition (coarse sediment) correlate well with

periods of increased sorting. Fluctuations in organic and inorganic carbon

content do not demonstrate any significant trends in response to PSD variation.

nor do the values change significantly through the length of the core.

Laminated sediments are also observable in Ekman sample 5

(curresponding to CTD site 5). with cdour and some texture variation

comparable to Ekman sample 3. Laminae are significantly thinner in sample 5.

where the thickest consistent sections measuring approximately 2-5 mm (Figure

6.5.1-A). Multiple thin sections of Ekman cores 5, 7, and 16 were required for

high-reçolution observation of sedimentary structures and organization

(examples shown in Figures 6.5.1, 6.6.1, & 6.7.1-8). Thin sections for Site 3 are 3% y') 4 Vol.

-% Organic

Figure 6.6.1: A - Photograph of sediment stratigraphy from cleaned Ekman grab sample. 6 - Thin section of part of Ekman sample allowing higher resolution examination of sedimentary structures. C - Partlcle Size Distribution (PSD) displayed as a percent of volume over a given vertical interval (mm). D - Organic and lnorganic Carbon content as percent. E - Map of Meziadin Lake and location of Ekman sarnpling site and key for PSD chart. Mean Standard Deviation (w-0 (w-0 Kurtosis Skewness 2 3 4 5 6 71.52 2.53 3.544.5-1.2 4.8 4.4 O 0.4-0.8 -04 0 0.4 0.8 I jIl] 1

--

Figure 6.6.2 : Statistical data (mean, standard deviation, kurtosis, skewness) demonstrating changes to the distribution symmetry of individual grain size samples from Ekman sample 5. All data shown in this figure correlate to the stratigraphy exarnined in Figure 6.6.1. not show due to the poor reproduction of structure within the thin section (see section 3.3.2.3.for explanation). Under high-resolution examination. 1 bewmes difficult to follow a consistent rhythmicity throughout Ekman sample 5 (i.e. there are not dear or consistent rhythmites within the Ekman sample suggesting the lack of a varved record). What appears to be an altemation of coarse and fine textured material in the lower portion of the core (centred around 110 mm) shifts to irregulariy larninated sediment in the upper regions (centred around 60 mm; also the upper region of the thin section; Figure. 6.5.1-8). The PSD plot supports this apparent lack of rhythmic structure with a highly symmetrical distribution. centred at approxirnately 8.5 pm (fine silt). showhg minimal variation through the

are. However. shifts in mean grain size. kurtosis, and skewness suggest that

changes to distribution symmetry do occur at a fine scale. although peaks in data

do not clearty match with observable sedimentary structures. It is possible that

the sampling interval us4for grain size analysis was not small enough to pick

up very fine variation in particle distribution. Due to distance from point of inflow.

there is only irregular deposition of coarser sedirnents (very fine sand); therefore.

coane grain deposition cannot provide an annual marker for varve recognition.

Without chronological control, it is not possible to say with certainty wtiether the

laminae are varved.

Organic and inorganic carbon percentages in sarnpie 5 are greater than

recorded values for the more proximal Ekman sample 3 and trap moonngs 1 and

2. Organic carbon values are consistently higher than inorganic carbon, with

perxntages demonstrating a trend of opposition. Ekman sample 7 shows reduced variation in wlour and texture. consisting rnainly of fine massive sediments with thin, irregular larninations (Figure 6.7.1-A).

The dark tone of the surface sediments (top 25 mm) is a function of oxidation of the upper sediments during storage, not a depositional feature. The most signifiant disturbance to the simple sedimentation pattern is a section between

45 and 65 mm in which the fine sediment no longer represents a cohesive mass.

Thin section analysis also highlights the la& of structure or organization within sample 7 (Figure 6.7.1-6). with thin. intermittent larninations in the lower portion of the thin section. and irregular shaped particles located in the upper regions. some of which appear fibrous. This material is likely buried organic debns as the values for organic carbon are raised significantly to 7 % over this interval (Figure

6.7.1-D), while inorganic carbon is relatively unaffected. The tack of variation in the PSD plot over this interval (Figure 6.6.1-C) lends weight to the theory of organic matter deposition since the pre-treatment of Coulter LS200 samples includes the destruction of organics with hydrogen peroxide (H,O,) pnor to laser scattering (Beierle et al.. submitted). The PSD plot shows very regular symmetry and almost no variation over the length of the are, with volume percentage spikes in the fine siit grain site (approximately 7-8 pm). Mean grain size and skewness show minimal variation throughout the sample; platykurtic behaviour is also generally consistent. It is interesting to note at Ekman site 7 that fluctuations in organic and inorganic carbon are reduced in magnitude compared to variation seeri in the more proximal traps (Figure 6.4.1-C), possibly indicating better sorting of organic and inorganic carbon-bearing sources pnor to

de position.

Ekman sample 16 contains very fine, massive sfC;,xi,i :hat does not appear to have any significant laminations (Figure 6.8.1-A). Thin section analysis corroborates this observation and illustrates a cornpiete lack of structure within the sarnple, with parücle size falling primarily in the ciay and very fine silt size classes (Figure 6.8.1-8 8 C). The PSD plot illustrates the dominance of fine-grained particles throughout the sample, with some birnodality in the lower regions (85 - 135 mm). Bimodality may be a result of temporal variation in sedimentation, with silt size grains deposited from turbid plumes during the summer. with the finer clay fraction settling during winter. Similar to Ekman sample 7. mean grain size, and skewness show little variation, with the total distribution showing mainly leptokutic behaviour. suggesting significant sorting of matenal pnor slow deposition from suspension. Organic and inorganic carbon percentages are similar lower in the me,but diverge in more ment sediments

(Figure 6.8.1-D).

6.4 DISCUSSION

6.4.1 Cornparison of Trap and Ekman Data

The above examination of sediment trap and Ekman sample data highlights spatial and temporal patterns of sediment distribution, as well as comparing highly variable rates of accumulation. While deposition of sediment proximal to the point of Strohn Creek inflow (tramp rnooring 2) has been 10 100 1000 D -% Organic - % lnorganic Fiaure 6.8.1: A - Photograph of sediment stratigraphy from cleaned Ekman grab sample. V B - Thin section of part of Ékman sample allowing higher resolution examination of sedimentary structures. C - Particle Size Distribution (PSD) displayed as a percent of volume over a given vertical interval (mm). D - Organic and lnorganic Carbon content as percent. E - Map of Meziadin Lake and location of Ekman sampling site and key for PSD chart. suggested to be annual in nature, it is unclear whether laminae recovered from mid-inflow proximal anlocations (Ekman sample 5) are varved. Using localized

MAR from sediment traps and bulk density established from Ekman sampling, it is possible to hypothesize the thickness of annual deposition on the lake floor, to corroborate previously detemined rhythmite thickness.

Using the MAR data from trap mooring 2. total sediment accumulation in the lower trap was 208.47 g/m2Jd. Bulk density for Meziadin Lake sediments was detemined by averaging the dried mass of total of ten sediment plugs, from

Ekrnan samples, of known volume (1 cm3). Using the value 1.21 glcm3as the bulk density of Meziadin sediment, dividing MAR by bulk density resulted in a value of 0.174 mmld of sediment accumulation at the site of trap mooring 2. The mooring was in the water column for a total of 64 days (June 4 - August 9), thereby collecting, in the funnel-shaped traps, the equivalent of 11-1 2 mm of lake floor sediment. While Ekman sample examination suggests that varves in this regions range from 15-20 mm, the value of 11 .12 mm over only 64 days provides a reasonable estimation of the total 1999 season lake fioor accumulation, with approximately 20 % of laminae thickness yet to be deposited (Gilbert, 2001; personal communication). Sedimentation from suspension continues in Meziadin

Lake past August into the autumn months, likely providing a minimum of 50 additional days (into late September early October) of fine sediment deposition.

Also important is increased thickness of annual hythmites associated with winter stom intrusions, which were observed to be present in every winter half-couplet of Ekman sample 3 (Figure 6.5.1).The additional 20 % should bnng the thickness of sediment, wllected at trap mooring 2 in 1999, within the established range of 15 - 20 mm for varves.

Similar calculations can be made for accumulation at each trap mooring. as well as the location of Ekman sample 5 (Figure 6.1). The MAR of 22.01 g/m2/d at trap mooring 3 represents 1.16 mm lake floor accumulation over the 64- day pend. Considering that a significant amount of deposition from suspension, and winter storm deposition, would likely have occurred later in the 1999 season

(in the range of 20 %) it is likely that the accumulation collected in this area correlates with 2-5 mm observed laminae thickness.

Although no Ekman sarnple was recovered in the region of trap mooring 5

(Figure 6.2), thickness of the 1999 lake floor deposit can be hypothesized from trap data. The MAR of 39.14 glm2/dprovides a thickness of 2.07 mm in 64 days.

Using the same logic established for the above-examined sites, it is likely that laminae are in the range of 4-7 mm.

6.4.2 Meziadin Lake Depositional Environments

6.4.2.1 lnflow Proximal Varves

According to the rhythmic nature of the deposits, bimodal texture. and similanty to the annual sediments of Bowser Lake and other large postglacial montane systems (Desloges and Gilbert. 1987; Gilbert et al. 1997) inflow proximal hythmites are assumed to be varves (Figure 6.9). Sediment deposition in proximal regions in Meziadin Lake is similar to Moose Lake, BC, (Gilbert and

Desloges, 1995) where large nival-melt charged freshets in spring produce eady season sand layers, followed by sustained glacial melt in summer produces which produœs a signifrcant silt layer. This nivalcontrol of spring hydrology is demonstrated in the discharge hydrograph for lesser Strohn Creek (Figure 4.5) as well as the occurrence of significant underflow events recorded in CTD data

(Figure 5.5).

Sediment trap evidence suggests that in the most proximal regions

(moonngs 1 & 2), high sedimentation rates allow the character of deposition to be examined at an intraseasonal scale, with discharge events recorded within the sediment record. Underflow deposits are visible in trap accumulations as discrete changes to particle size distribution; however. it is not possible at this stage to correlate individual undeflow events (monitored by the Tidbit) with a discrete change in grain size within sediment traps or Ekman samples. The PSD plot of Figure 6.1-8 represents the deposits of three coarse-grained turbidity currents. with variations in grain size that are subtle and not irnmediately evident through observation of cieaned trap sample surfaces.

Moving further dom-lake (at moonng 2) deposition by underfiowing currents does not fom multiple discrete events in the lower trap. Rather. accumulated sediment forms a distinctive varve over the year, with a coane base and upward fining material, capped by silt and clay material, likely deposited over the winter in a low energy environment (O'Sullivan, 1983). The existence of a coarse-grained intrusion in every clay cap of Ekman 3 indicates the importance of late autumn stoms in the Meziadin basin, likely a function of the basin's geographic location on the lee side of the northem Coast Mountains (Shaw and Archer, 1978).

The dashed lines in Figure 6.9 represent an area in which the annual nature of laminations is unsure. While a cornparison of Ekman 3 with Core pc-1b suggests that varved sedirnents do occur at greater distances from point of inflow at reduced thickness, Ekman sample 5 does not clearly demonstrate annual differentiation. Wthout better chronological control, it is not possible to say with certainty at what point sediment changes from varved to laminated.

6.4.2.2 Distal and Lateral (Shallow) Laminated Sediments

Sediment deposition on the lateral slopes and distal regions of the inflow proximal amdemonstrates irregular laminations. but is not varved (Figure 6.9).

The side slopes of this region may receive sufficient sediment, temporally differentiated. to form varves. However, due to steep lateral sides and the slow steady slumping of material, facies may be disturbed. Disturbance was obsewed

in Ekman 7 (Figure 6.7.1), which recorded the intrusion of a highly organic layer,

possibly from shallow regions. More likely the shallow lateral regions and the

farthest regions of the proximal amdemonstrate stratification based on episudic

events related to inflow, particularly with unusually high snow melt funoff in early summer.

In the region of CTD site 5 (Figure 6.9) it is difficult to detemine wrth

certainty that laminae in this region are necessarily annual in nature. Ekman

sample analysis shows laminations in the range of 2-5 mm, but it was not

possible to state with certainty that these were annual rhythmites. Trap data

suggests that, during the 1999 season, suficient sediment was collected to produce a recognizable layer. However, the thin nature of laminae in this region, the lack of a consistent coarse grained annual marker, and the lack of chronological control (radio-isotope dating) make consistent varve recognition difficult. Small variations in sediment supply, hydrometeorologic forcing of sediment transportation, or changing strength or direction of turbidity currents wuld cause lack of recognition of an individual rhythmite in any given year.

A proximal to distal reduction in the role of turbidity currents in Meziadin

Lake as a rnechanism of deposition has been identified based on an examination

of acoustic reflectors in the proximal and distal amis (section 5.2.1.3).Ape Lake and Moose Lake are two examples of lacustrine settings where sediment

changed fram varved to diffusely laminated when turbidity currents no longer

affect a given area of the lake (Gilbert and Desloges, 1987; Desloges and

Gilbert, 1995). While turbidity currents are not necessarily a requirement of

varve formation (Smith, 1978). they piovide a rnethod of coarse sediment

deposition that makes varves easier to recognize. A lack of coarse material or a

reduction in the total accumulation of sediment, combined with a lack of thermal

stratification to provide temporal differentiation of sediment deposition (Stum,

1978), wuld lead to a shift frorn varves to irregular larninations.

6.4.2.3 Distal Massive Sediments

Sediments from inflow distal regions of Meziadin Lake are massive

because of low MAR values (Figure 6.2) and the settling of sediment from

suspension year-round, with no coarsegrained summer input. The acoustic

survey of the distal regions of MWadin Lake did not demonstrate a right-hand increase in sediment accumulation, indicating that sediment plumes moving through the distal regions of Meziadin Lake were too diffuse to create variation in sediment distribution (Desloges and Gilbert. 199 1).

The lack of laminated sediments may also be related to the inability of turbidity currents to carry that far. Varved sediments can occur in depositional environments without turbidity currents (Smith, 1978); however, in Meziadin

Lake, without the energy asçociated with turbidity currents, coarse sedirnent does not extent into the infiowdistal arm, thereby allowing recognition of

laminae. The largest underflow events of the summer recorded by CTD profiling

(Figure 5.8). reached as far as CTS site 13 (12 km) but were not energetic enough to deposjt sediment particle sizes greater than silt.

Weak thermal stratification, recorded in by CTD profiling (Figure 5.5),did

not create a diçcrete epilimnion. which could have ensured the suspension of

matenal, and thereby provided a change in the rate of deposition for siit and clay

size particles (Sturm. 1978); therefore. laminated sediment deposition couid not

occur. Sturm and Matter (1978) and Gilbert and Desloges (1987) also

documented the deposition of massive sediments when the difference between

summer and winter deposition becornes indisœmible.

6.4.3 Accumulation History

Calculations of lake bottom topography from acoustic suwey data have

placed the maximum thickness of sedirnentary fiII in Meziadin Lake at 185 rn for

the inflow proximal am, focuseci near the thalweg of the lake channel. Sediment

trap and Ekrnan grab data have dernonstratecl that deposits near the inflow proximal region are annual in nature (Figure 6.9) with an average accumulation rate of 17 mm a" (section 6.3). If this present day accumulation rate is assumed to have been constant since deglaciation, sedimentary fiIl within the proximal am

of Meziadin Lake represents 10.6 x 1O3 years of sediment deposition. Gilbert et

al, (1997) estimated accumulation to have occurred in the Bowser Lake basin for

9800 years. A amparison of the present status of distal glaciolacustrine

processes in these two northem Coast Mountain basins would suggest that

Meziadin Lake was adually ice free a significant period of time before Bowser

Lake; Bowser Lake glaciers, glaciofluvial processes, and glacilacustrine

processes presently indicative of a more active basin. Therefore, the established

date of 10.6 x 1O3 for the Meziadin Basin likely represents a reasonable date for

the initiation of sediment accumulation.

Gilbert et al. (1997) suggest that error associated with their accumulation

history is retated to the unknown character of basal fiIl and early focusing of

sedimentation in the centre of the lake channel. These errors are also likely to be

significant for Meziadin Lake fiIl considering the sirnilarities between the

physiographic, clirnatic, and sedimentologic regimes of the Bowser and Meziadin

basins. Based on this age of accumulation, the weakest assumption made with

respect to estimating the time frame for the deposition of sedimentary fiIl is that

present rates of accumulation were similar throughout the Holocene. Based on

published ages for glacial Cordilleran lcesheet activity in the Coast Mountains

(Ryder and Thompson 1986; Ryder, 1986), adMty is believed tu have decreased

during the hypsithemal(9 - 4 ka BP), and increased significantly dunng the Neoglacial (peaked at approximately 2600-2700 yr BP). Glaciers have since retreated from their LIA extent. Having previously exarnined the nature of acoustic refiectors within Facies 1 of the subbottorn deposits. the reduction of turbidrty currents as agents of deposition over time would suggest that accumulation rates have decreased considerably.

6.5 CONCLUSIONS

An examination of the sedimentology of montane Meziadin Lake highlights several elements of distal lacustrine sedimentation:

1. Sediment deposition in Meziadin Lake dernonstrates significant temporal

variation with 80-902 of inflow proximal region sedimentation occumng

within the freshet pend. This trend suggests that the spring freshet

hydroclimatic response of the Meziadin basin is more similar to the

unglaciated Kamloops Lake basin in the southam Canadian Cordillera.

than to glaciated Bowser Lake basin, approximately 200 km east.

2. Trap mwrings highlight sediment accumulation and distribution patterns

including a) increase southem shore accumulation particularly in the upper

40 m of the water column, b) reduced accumulation wrth increasing

distance from point of oufflow, and c) a signifiant increase in

accumulation at the bend in the lake.

3. lndividual trap accumulations in the two most proximal moonng

demonstrate intraseasonal variation in their sediment stratigraphy,

possibly containing evidence of the passage of multiple underfiowing currents.

Three primary depositional environments can be distinguished within the lake basin: a) lnflow Proximal Varves b) Lateral (Shellow) and Distal

Laminateci Sedirnents and c) Distal Massive Sediments.

Variation in the character of sediment deposition is caused by a) variable

influence of turbtdity currents throughout the lake b) variable accumulation

rates and c) weak thermal stratification.

A cornpanson of the character of amustic refiectors against observeci

underflow activity suggestç that the role of turbidity currents as

depositional agents. in both proximal and distal ansof Meziadin Lake,

has been significantly reduced over time. This has lead to a decline in the

spatial extent within the Meziadin Lake bastn in which varved sediments

are generated.

Calculations of HoIocene sedimentary accumulation based on

sedimentary thickness in the proximal am of 185 m. and a modem

accumulation rate of 17 mm a", provide a reasonable estirnate of

sediment accumulation for 10.6 x 1O3 yean. Error asçociated with this

date is likely due to variable rates of accumulation over tirne, the unknown

character of basal fill. and early focusing of sedimentation in the centre of

the lake channel. CHAPTER 7 - SUMMARY AND CONCLUSIONS

7.1 INTRODUCTION

This work represents an assessrnent of the processes that influence the

hydrology, limnology, and sedirnentology of Meziadin Lake and its basin. The

impetus for this study is to provide a better understanding of the nature of distal

glacilacustrine deposition, particularly in the fiord lakes of the Canadian

Cordillera.

7.2 CONCLUSIONS

7.2.1 Hydrology and Sedirnent Transport

The seasonal nature of the Meziadin basin hydrologic regime is

manifested in climatic control over nival and glacial rnelt-water. representing the

two major sources of sediment transport energy. Hydrologic variation and

sediment mobilw in the Meziadin basin can be summarized by several

conclusions:

1. Climate in the Metiadin basin is similar to other regions located on the

lee side of the Coast Mountains, therefore, demonstrating reduced

precipitation cornpared to coastal sites of the same latitude, late

auturnn and winter representing the period of greatest precipitation,

and greater range of mean monthly temperature.

2. The spnng freshet of 1999 in the Meziadin basin was delayed due to

unseaçonably cool weather, thereby producing a large nival rnelt- induced peak in stream discharge on June 16 in response to the first

significant nse in temperature.

3. Fluvial suspended sediment concentration reached its summer peak

on June 16, in response to increased discharge and, therefore,

increased stream cumpetency.

4. Discharge demonstrated a dear diumal signal, related directly to

diumal air temperature variation. This relationship was improved as

the summer progressed and the nival component of stream discharge

was reduced.

5. Suspended sedirnent values from the post-glacial fluvial systems in the

Meziadin basin gradually decreased after the freshet period, reaching

a relatively constant value, while suspended sedirnent concentrations

for non-glacial streams approached zero over the same time period.

The physical limnology of Meziadin Lake can ba considered typical of large, deep fiord lakes, including those in the southem regions of the Canadian

Cordillera and other montane lacustrine settings. Dominant characteristics of

Meziadin limnology include:

1. There appears to be a regional separation of miMs behaviour in

Meziadin Lake as seasonal overturn occurs at different times in the

spring between the proximal and distal amis.

2. Weak stratification develops in the upper water column as shown by

profiles of temperature, and mirrored by conductivrty. This structure is easily disnipted a) by wind generated wave which depress the

developing themocline and b) infiowing fiver currents. particularly in

the proximal am.

3. lnffowing currents initially enter the lake as a vertimlly undifferentiated

masses, eventually thinning to interfîowing plumes that move dom-

lake. Over the course of the summer. as discharge decreases and

more stable stratification develops, these plumes enter higher in the

water colurnn and progress as thinner interflows.

4. There is significant Coriolis deflection of inter and underflowing plumes

to the right-hand (south) shore of the lake.

Meziadin Lake is similar to other post-glacial depositional systems with one major inflow. in which river currents are recordecl to dorninate circulatory patterns. Although total inflow from Strohn Creek to Meziadin Lake is considerably smaller than other Cordilieran lakes (Gilbert. 1975; Desloges and

Gilbert, 1994), Strohn Creek is observed to impart significant control over lacustrine proœsses.

7.2.3 Turbidity Cunents

The presence and impact of turbidrty currents on sedirnentology of

Meziadin Lake, particularly within the inflow proximal am, can be summarized by several obsentations.

1. Fluctuations recorded in hypolimnic temperature by the Tidbit data

logger indicated the plunging of inflowing river water in two foms a)

high energy quasicontinuous underfiow related to freshet-enhanced discharge and b) discrete lower energy turbidity currents related to

diurnal dischargelsuspended sediment cycles.

2. High concentrations of suspended sediment were recorded by the

turbidometer, within the hypolimnion, as 'bulgesn in suspended

sediment moving dom-lake during penods of known underflow

activrty. The number of observable bulges correlates to the number of

measured temperature spikes before and on date of measurement.

3. The lack of signifiant south-shore sediment accumulation, shown by

acuustic surveying, indicating deposition by turbidity currents under the

influence of gravrty and bottom topography, as opposed to inter and

overflows that are affected by Coriolis deflection.

4. The lateral continuity of acoustic reffectors in the proximal am, and the

down-lake consistency of significant refiectors (which eventually pinch-

out with increasing distance from point of inflow) indicate deposition

from turbidity currents.

5. The presence of coarse-grained particles, fining upwards. were

recorded in proximal sediment traps as multiple, discrete, turbidity

current events. In more distal traps and Ekman samples. coarse

particles deposited by turbidity currents provide a clear marker of

annual deposition.

As in Bowser Lake (Gilbert et aL, 1997) and other post-glacial lacustrine

systems (Gilbert, 1975; Desloges and Gilbert, 1995) higher energy, quasi- continuous turbid undeAows develop in Meziadin Lake during the spnng freshet and eariy summer when sediment laden nival, melt water enters the lake. Due to the increasing strength of thermal stratification and reduced discharge and fluvial suspended sediment concentration, underfiows develop into les energetic tubidity currents later in the summer and are primarily a function of diumai variations in discharge and suspended sediment concentration, both originating from the melting of glacial iœ.

The role of turbidity currents in Meziadin Lake has changed significantly since deglaciation. Tuhidity currents are presently not as significant or extensive

in Meùadin Lake as they likely have been in the past; nor are turbidity currents

as significant to Meziadin Lake sediment deposition as they are presently in

Bowser Lake, according to observations by Gilbert et al. (1997). There is a

reduction in the consistency of recent acoustic reflecton moving into the distal

basin of Meziadin Lake compared to deeper sediments. and cornpared to more

recent Bowser Lake sediments. There is presently no distal thickening of fiIl

associated with the termination of densrty currents as the lake floor rises. as

observed in Bowser Lake and, possibly, in buried sediments of Meziadin Lake

(strong reflectors located in Facies 1 in distal am-Figure 5.4). Bowser Lake rnay

be, therefore, a working exsmple of what the turbidity cuvent sediment regime of

MeUadin Lake may have kenpreviously wtren ice was more extensive mthin

the basin. or more proximal to the lake.

7.2.4 Sedirnentology

Patterns of sediment accumulation and distribution in Meziadin Lake can

be descnbed as characteristic of a post-glacial systern in which the energy associated with deposition, and the availability of sediment has ken reduced over time. The major trends obseweâ in Meziadin Lake sedimentology can be summanzed by:

1. Acoustic surveying demonstrated 3 major facies within the subbottom

record. The most recent sediments represented lowenergy, distal

lacustrine deposition, Mile the earliest sedirnents related to a high-

energy , ice-proximal depositional environment.

2. Accumulation of sediment in Meziadin Lake iç highiy variable spatiaily

and temparally. Signifmnt proxirnaUdistal, northlsouth differences

exist in the mass accumulation rate of sediment. Approximateiy 85 %

of the total annual sediment deposition can occur during the spring

freshet in proximal regions of Meziadin Lake.

3. Three primary depositional environments presently exist within

Meziadin Lake a) proximal, annually laminated sediments, b) shallow

(lateral) and inflow distal, irregularly larninated rhythmites c) distal

massive sediments.

4. Sedimentation in proximal regions cm be quantified at sub-annual

resolution, providing information on undefiow events.

Hydrologic, turbidity current, and sedimentologic evidenœ suggests that the Meziadin basin is presently in a state of reduced energy aççociated with the transportation and deposition of sediment to Meziadin Lake. This reduction in energy is likeiy partly linked to the retreat of glaciers in the basin since the LIA advance. A review of su bbottom evidenœ and Cordilleran-based literature suggests that the Neoglacial and Little Ice Age may have represented periods of increased energy (Le. glacial advance) within the HoIocene. This period would have foilowed after ice retreated far enough into the Meziadin basin to initiated distal lacustrine sedimenMion (e.g. there are two regions of significant acoustic refiection in Facies 1 within the distal an, suggesting increased presence of turbidity currents during this pend of deposition).

7.3 IMPLICATIONS FOR FUTURE RESEARCH

7.3.1 Post-Glacial Depositional History of Canadian Cordillera

The study of post-glacial depositional environments will continue to produce a large arnount of data on the retreat of ice fram the montane

Cordilleran regions of British Columbia. The regional nature of dimate, exaggerated by complex physiography of the Canadian Cordillera, ensures that the timing and characteristics of glacial retreat, the arnount and character of sediment deposited, and even the mechanisms of deposition will Vary spatially.

Continued examination of pst-glacial lacustrine systems will provide information on regional glacial advance and retreat, helping to piece together the complex fluctuations of the Cordilleran Icesheet.

7.3.2 HighResolution Reconstructions of Environmental Events

Lacustrine basins provide interesting environments for glacial retreatladvance reconstructions based on the strong hydrologie and sedimentologic relationship shared by lake and glacier complexes. Recent literature shows that the strength of this relationship can be explored quantitativety. allowing lacustrine sediments to becorne a proxy from climate dnven variations in glacier activity (Leeman and Neissen, 1994; Leonard, 1997).

Evidenœ from this work demonstrates that iniiw-proximal sediments can record discrete variations in basin hydrology, such as nival melt-indu& discharge peaks, as the passage of undeAowing turbid plumes. Çurther work is required in the Meziadin Lake drainage basin. including the establishment of chronological control on laminated sediments. High-resolution examination of sediments from this glacilacustrine setting should. therefore, allow the quantification of

paleohydrologic events. Successtd interpretation of high-resolution changes to

sediment records. as they relate to actual environmental phenornena, will rely on

a strong understanding how weather patterns manifest thernselves within a given

basin, combined with observations of basin geomorphic responses to extemal

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News Release

For lmmediate Release Provincial Emergency Program Flood 1999:01 May 6,1999 SNOWPACK LEVEI REMAIN UNCHANGED AS COOL WEATHER DELAYS MELT

VICTORIA - Snowpack levels through the southem part of the province have changed little over the past month, and the potential for higher than normal spring mn-off water levels remains, according to the latest snow survey data gathered by the Ministry of Environment's River Forecast Centre.

Water run+ff experts say any serious flooding continues to depend upon the rate at which the snow melts in the next six to eight weeks. A warm spell longer than five days could cause many riven to rise to flood levels.

The outlook fiom the weather ofice of Environment Canada is for a waming trend next week, which should increase the rate of melt However, no substantial warmth or precipitation is predicted, and this should result in the gradua1 melt continuing, with riven remaining well below flood levels at least through mid- month.

"While the short-term weather forecast is promising, people should not let their guard dom," said Environment, Lands and Parks Minister Cathy McGregor. "People in flood-prone areas should continue to be vigilant and make al1 necessaiy preparations to protect themselves, their families and their homes should the temperatures head upwards."

Based on mountain snowpack readings, areas that appear to be most vulnerable to high flows include: the Thompson River basin, the middle and Iower Fraser basins and the Similkameen, Okanagan and Kettle basins. While much of the Kootenay and Columbia basin is controlled by hydro dams, areas in the West Kootenay and western portions of the Columbia basins that are on uncontrolled streams and rivers could see flood conditions if there is a rapid rnelt.

In response to the potential for higher than normal run-off this spring the provincial goverment has taken a nurnber of steps over the past few weeks to ensure public safety and protection of the environment, tncluding: providing more than $5 million to €und 68 emergency flood mitigation projects such as dike re-construction and river bank protection in the province; regular monitoring and public reporting of the snowpack throughout B.C. ; working with municipalities, diking districts, and key agencies to ensure that flood planning is coadinated. In the event of flooding, local govemments can cal1 on additional assistance from the provincial government under B.C.'s flood response plan; ensuring more than five million çandbags are ready for deployment to municipalities as they are the first line of response. Also, providing six sandbag-filling machines to key centres near the flood-prone areas; and requesting dam and dike ownes conduct pre-mn-off inspections of al1 water control facilities and conducting aerial su-s to monitor dams in high-risk areas.

The province has a toll-free public information line ( 1-888-336-7378) providing regular updates on flood conditions throughout the province. information about snowpack levels can be found on the Intemet (www.env.gov.bc.ca).Tips on flood-proofing at home and flood precautions for horneowners are also available ( www. pep. bc.ca/).

Printed brochures are also available from government agents' offices, Provincial Emergency Program regional offices. MLA constituency offices and municipal govemment O tXees.

The next Ministry of Environment mow swey update is expected More the end ofthe month. MINISTRY UPDATES PROVINCIAL FLOOD POTENTIAL

For Immediate Release Ministry of Environment, Lands and Parks 330-30:ELP97/980-020 May 7, 1997

VICTORIA - The latest snow swey information shows that the potential for flooding remaios hi@ for many areas of the province, the Ministry of Environment, Lands and Parks said today.

As a result of the cool, unsenled weather last month, there has been little melting of the mountain snowpack. However, melting of Iow eievation snow combined with saturated soi1 conditions has brought many nven to higher levels than are nonnally experienced at this time of year under these weather conditions.

Any warm spell in the next few weeks lasting more than four or five days could bring many snowmeit-fed rivets to flood stage quite quickly. ldeal weather conditions would be altemating 3- or 4-day wmand cool spells to bring the snowpack down in a manageable rnanner. Obviously, any substantial rainfall occurring hile riven are swollen with snowmelt will increase the river levels.

Although almost al\ areas of the province are likely to experience high river levels, areas of particular concem include the Nechako River basin, the Bulkley and Babine riven in the Skeena kver basin, the South Thornpson River basin, the Fraser Canyon, the Nicola River, the Similkameen, Okanagan and Kettle valleys, the Columbia valley south of Revelstoke and south of Golden and al1 the Kootenay River basin.

Cornmunities and individuals in flood prone areas throughout the province should be aware that river and lake levels will rise rapidly with the onset of any warm weather. Srnail watenheds will respond very quickly, but the major riven will take longer to react and, if the warm weather does not last long although bey will rk,will probably not reach damaging flood levels.

The ministry continues to urge local govemments and dylcing authorities to rwiew and inspect al1 existing flood protection works to ensure the adequacy of these structures during a potential flood event. There are a number of other steps people can take to prepare for any possibility of flooding. Parents should ensure their children stay well away from fast-flowing streams. People should observe local ditches and streams, and prevent debris From blocking culverts or Stream crossings. Frigid . 1 spring - shatters records FTHE rcccnt dardwct wçithcr bac VI?IM ex- ctsaivc, it's ùccaasc it is. It oaawcd and mincd more at tbe Tema d Kitimat airport kit munth than cver rca>rdcd tbcn bcforc. In May, the ikpurt brokt a 39-yur-dd prccipitation rrairdtrdarr46yur- dd snowfail record. Tbc airport's mean rnooih- ly pdpitacioa ntc for May is 49.9 millimccrri. Tbc to- tal prccipiltion nte Izst autciih was rnorc dun twm thH amwnt, at 1056 mg- limttrrs. As fa snowtàii nies, rhc airport brdrc aaotbcr record in May wbea 15.4 cenu- mares feu. Tk prcvious rcorrrû wrs 8.4 ccntimcîrcs recordcd in 1965. Il was risa considurbly cbldct in May thrn avengt Thc rvarge tespenture [af iht mont6 is 9.9 d-cs Celais. Last inonth's iver- agc tanpcn'urc wu bcloo; arnarl at 7.6 dcgrets Ce!- riPr Adkgto Gary Myers dimrtt apcfulirt rt En- ofNd&May is Qc kst month IO record slowfaU. He expiiiacd snaw an stüi QU in Jonc, but neva ~lorcconf. Mprs said it brt ncvcr saowed in Temcc in Jdy a A- A- ta 55 ytrn it brs dysnowed oncc in Sep- -. me Tenace Standard, June 9, 1 999. NO author given