Journal of Paleolimnology19: 335–342, 1998. 335c 1998Kluwer Academic Publishers. Printed in Belgium.

Lake Winnipeg coastal submergence over the last three centuries�

Erik NielsenManitoba Energy and Mines, 1395 Ellice Avenue, Winnipeg, Manitoba, Canada R3G 3P2(e-mail: enielsen@em.gov.mb.ca)

Received 25 February 1997; accepted 21 May 1997

Key words:large lakes, coastal recession, submerged trees, beach ridges, barrier beaches, water levels

Abstract

Radiocarbon dating of marsh facies peat and drowned trees along the barrier beaches at the south end of LakeWinnipeg, indicates water levels are presently rising. Lagoonal sediment and associated trees are being buried asthe barrier islands move landward in response to rising water levels. Estimates based on radiocarbon dating suggestthe water level has been rising 20 cm/century over the last three hundred years. This estimate is consistent withlake level records and models of isostatic uplift which suggest the level of the lake should be rising between 6.7 and12 cm/century along the south shore. However, additional radiocarbon dates on submerged trees from ObservationPoint, at the north end of South Basin, and the Spider Islands, near the northern outlet, indicate that at least part ofthe water-level rise is basin wide.

Southward transgression of Lake Winnipeg, throughout the Holocene, is believed to be the result of isostatictilting of the basin, whereas the recent basin-wide water-level rise is more likely the result of a combination ofisostatic tilting and increased precipitation associated with climate change.

Introduction

In 1990, investigations along the barrier island systemwhich forms the south shore of Lake Winnipeg (Fig-ure 1) between Matlock and Beaconia Beach (Figure 2)revealed evidence of both historic and pre-historiccoastal submergence (Nielsen & Conley, 1994). Theresults of those investigations form the basis of thisreport. The purpose of the study was, in part, to developa water-level curve for the south end of the lake basedon radiocarbon dating of inundated terrestrial organicmaterials. The 1990 investigations were augmented in1993 and 1996 by additional observations and radio-carbon dates from similar sites on the east shore, atObservation Point and near the north end of the lake,

� This is the ninth in a series of eleven papers published in thisissue on Paleolimnology of Lake Winnipeg, Canada. The paperswere selected from presentations made at the Special Session: ‘Phys-ical Environment and History of the Lake Winnipeg Basin’ held at theGeological Association of Canada meeting in Winnipeg, May 27–29, 1926. These papers were collected by W. M. Last. Additionalmanuscripts submitted as part of this Special Session will appear infuture issues of Journal of Paleolimnology.

at Spider Islands. Wind set-down and set-up can causedaily water-level changes of up to 2 m dependingon thewind direction, while seasonal water-level fluctuationsmay be as much as 1 m (Penner & Swedlo, 1974). Keyoutcrops were exposed on the upper foreshore due toa combination of seasonally low Lake Winnipeg waterlevels and wind set-down.

Methods

Seven sites where organic marsh sediments and asso-ciated logs, tree stumps and tree roots were exposedon the upper foreshore of the lake were radiocarbondated. Cross-sectional profiles, measured relative tothe lake level and the position of organic samples,were surveyed at each site. The present-day zonationof vegetation was also noted. A curve showing relativelake-level rise at the southern shore was constructed byplotting calibrated radiocarbon dates from West Chan-nel, Pruden Bay, Patricia Beach and Beaconia Beachagainst the water-level datum represented by the sam-

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Figure 1. Lake Winnipeg showing the areas investigated for evidenceof shoreline submergence.

ples. The radiocarbon dates were calibrated to calendaryears using the method outlined by Stuiver & Reimer(1993).

The water-level, using modern vegetation zonationas an analogue, is estimated to have been at least 0.5 mbelow each sample. This estimate is difficult to substan-tiate as the dated trees comprise only willow, tamarackand poplar. Willow and tamarack have known affinitiesfor water and while they may not grow closer than halfa metre above the water level on the foreshore, primar-ily a result of shore ice action, it is possible that theymay occur closer to this level on the lagoon side of thebarrier island. Poplar does not have the same affinityfor water as willow or tamarack and may be a betterindicator of low water levels. Unfortunately only onedated sample was identified as poplar.

Results

Five of the seven sites are on the southern shore. Twoadditional sites, one in the South Basin and one in theNorth Basin, were examined for comparison. At thefive southern shore sites, nine radiocarbon dates wereobtained. Seven of the nine were obtained from wood,which most effectively provides an upper constraint onlake level. A date on soil defines a subaerial landscapewith no affiliation with the lake, while dated lagoonsediments mark inundation.

Sans Souci

Desiccation cracks, infilled with wind-blown topsoil,occur in Lake Agassiz sediments exposed on the fore-shore at Sans Souci. Though not directly related towater level changes in the last few centuries, thesefeatures clearly indicate that the site was subaeriallyexposed. A radiocarbon date of 3885 BP (calibrated)was obtained from the soil (Table 1). Organic muck,unconformably overlying the Lake Agassiz sedimentsand dated at 1080 BP (calibrated), records marsh sed-iment accumulation in a lagoon environment (Fig-ure 3A). The stratigraphy and associated radiocarbondates at Sans Souci indicate that a lagoon and barri-er island were established at the site between 3900and 1100 years before the present (1835 B.C. and870 A.D.), in response to southward transgression ofthe lake.

West Channel

Approximately 0.8 km east of West Channel, a mudflat, in places more than 100 m wide, overlain by athin discontinuous sheet of nearshore beach sand, isexposed at times of low lake level. Logs, in varyingstates of decay, may be found on the surface of themud flat. Tree roots protrude from the underlying clay,but in situ tree stumps are absent, perhaps due to icescouring to a level below the root collar of the stumps.Tree stumps, in growth position, were thus removedleaving only the roots. The radiocarbon dates from thissite may therefore represent a datum slightly higherthan that indicated by the elevation of the samples.

Calibrated radiocarbon ages of 320 and 290 BP(1630 and 1660 A.D.) were obtained on two root sam-ples from this site. The older dated sample was col-lected 17 m further offshore than the younger sample.Although little significance can be placed on the differ-ence between these two dates because the error margins

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Table 1. Radiocarbon dates from the shores of Lake Winnipeg.

Site Lab. No. 14C Age Calibrated Calendar Material Elevation

Yrs. B.P.� Yrs. B.P.� Yrs. A.D. (m)

Sans BGS-1477 3600�80 3885 (1835 B.C.) soil 216.9

Souci BGS-1478 1200�70 1080 870 organic muck 216.9

West BGS-1439 335�65 320 1630 salix sp. 217.2

Channel BGS-1448 230�70 290 1660 wood 217.2

Pruden BGS-1440 290�70 310 1640 Salixsp. 217.3

Bay BGS-1450 210�70 280 1670 wood 217.3

Patricia BGS-1442 190�65 275 1675 Populussp. 217.4

Beach

Beaconia BGS-1443 260�80 295 1655 organic muck 217.4

Beach BGS-1444 185�65 275 1675 Salixsp. 217.4

Observation BGS-1632 160�70 265 1685 Larix sp. 217.4

Point

Spider BGS-1906 390�70 475 1475 tamarack 217.7

Islands BGS-1907 255�75 295 1655 tamarack 217.7

�Radiocarbon years are expressed as years before 1950.

overlap and the samples were collected from the sameelevation, their relative positions and radiocarbon agesare never the less, in agreement.

Pruden Bay

The foreshore to the east of the mouth of Pruden Bayis relatively narrow and steep compared to most otherareas along the south shore. East of Parisian Lake thebeach becomes narrow and indistinct and, in places, isreplaced by foreshore marsh. The eroding foreshore atthe mouth of Pruden Bay exposes rooted tree stumpsand organic detritus in places interbedded with sandand associated with numerous bison bones. Two root-ed stumps, one willow and one unidentified from thissite dated to 280 and 310 BP (calibrated) (1670 and1640 A.D.) (Table 1). The radiocarbon dates and theassociated bison bones are in agreement. As bison isknown to have become extinct in the wild in Manitobain the 1820s the association of bison bones with theradiocarbon dated samples confirms the dating of thesite to before the middle of the nineteenth century.

Patricia Beach

During periods of low water, the foreshore at Patri-cia Beach exposes numerous rooted tree stumps andfibrous organic material interbedded with clay. Anin situ poplar stump, rooted in the foreshore at anelevation of 217.4 m dated to 275 BP (calibrated)(1675 A.D.) (Table 1).

Beaconia Beach

The foreshore at the south end of Beaconia Beachexposes the most extensive drowned forest encounteredalong the entire barrier island system (Figure 3B). Atotal of 24 stumps are rooted in marsh clay and peat atan elevation of 217.4 m asl. A single rooted stump wasdated to 275 BP (calibrated) (1675 A.D.).

At the north end of the beach, a prominent blackfibrous organic muck layer, exposed on the lower fore-shore at an elevation of 217.4 m asl, was dated to295 BP (calibrated) (1655 A.D.) (Table 1). The fibrousorganic layer is, in places, overlain by pebbly graveland sand of the present foreshore beach.

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Figure 2. Barrier islands and radiocarbon dated sites at the south end of Lake Winnipeg.

Observation Point

A single radiocarbon date of 265 BP (calibrated)(1685 A.D.) (Table 1) was obtained from a rootedtamarack stump exposed on the foreshore at Obser-vation Point near Manigotagan, on the northeasternshore of the South Basin (Figures 1, 3C).

Spider Islands

Two new radiocarbon dates of 475 and 295 BP (cal-ibrated) (1475 and 1655 A.D. respectively) (Table 1)were obtained in 1996 on rooted tamarack stumps fromthe foreshore at Spider Islands on the northeast shoreof Lake Winnipeg (Figures 1, 3D).

Discussion

The seven sites examined in this study exposein siturooted tree stumps, logs, roots and organic sedimentson the present-day foreshore and all record a similarsequence of events. Fine-textured organic-rich sedi-ments, of undetermined thickness, exposed at the baseof the stratigraphic succession are interpreted to bemarsh sediments similar to those currently deposited inthe lagoons and marshes behind the barrier islands.Thepresence of marsh sediment on the foreshore clearlyindicates that the barrier islands have moved landwardin response to transgression of the lake. Rooted treestumps, roots, fallen logs and fibrous organic sedimentassociated with the fine-textured marsh sediments atthe various sites represent trees and marsh plants thatpreviously grew in and along the edges of the marsh orlagoon behind the barrier islands.As the lake level rose,the barrier islands transgressed landward, burying the

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trees and underlying marsh sediments (Figure 4). Withcontinued transgression, the tree stumps and marshsediments became exposed on the foreshore and arenow being eroded by waves and drifting lake ice.

The elevations and age of the 6 wood samples fromWest Channel, Pruden Bay, Patricia Beach and Beaco-nia Beach were used to construct a water-level curvefor the southern shore (Figure 5). Elevations of thesamples were estimated to vary from 217.2 and 217.4metres above sea level (asl) (Table 1) or a mean of217.3 m asl. The 1918–1967 mean water level of thelake was 217.4 m (Penner & Swedlo, 1974), 0.1 mhigher than the mean elevation of the dated samples.It is acknowledged that the interpretation of radiocar-bon dates younger than 200 years and between 300and 360 (radiocarbon years) BP is problematic (Stu-iver & Reimer, 1993). The fact that several dates fallbetween 200 and 300 BP and that the samples are allfrom approximately the same elevation suggest theydate a single event. In addition, it clearly emphasizesthe need for multiple radiocarbon dates. The sevencalibrated dates cluster around 300 BP (1650 A.D.).Hence, if it is accepted that the trees grew 0.5 m abovelake level, a rise from 216.8 m asl at 1650 A.D. to217.4 m asl at 1950 A.D., or 20 cm/century, is indicat-ed. If the trees grew closer to the water level, a lesserrate would be implied.

Analysis of 1914 to 1971 water level records byPenner & Swedlo (1974) suggests tilting and south-ward transgression of the Lake Winnipeg basin. Theyconcluded that Berens River is rising relative to Gimliat a rate of 6.7 cm/century. Prior to the present study,geological evidence for long-term changes in the lakebasin consisted of two radiocarbon dates from a sin-gle core taken in shallow water near Victoria Beach.Radiocarbon dates of 1660�60 BP (GSC-1977) and1060�210 BP (GSC-1980), were obtained from thebottom and top of a 40-cm thick peat layer, approxi-mately 3 m below the long term mean lake level (Teller,1980), and suggest a water level rise of approximately20 cm/century through the last millennium and a half.Land-based studies on raised beaches at the north endof Lake Winnipegosis by Nielsen et al. (1987) furtherindicate the north end of that lake has been uplifted anestimated 6 m, relative to the southern outlet, in the last5000 years as a result of isostatic tilting. This suggestsan average rate of tilting of 12 cm/century for LakeWinnipegosis. The new geological data and radiocar-bon dates are thus in agreement with the available dataand suggest the water level at the south end of LakeWinnipeg is rising at a rate of about 20 cm/century as

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Figure 5. Plots of elevation of wood samples from the southernshore of Lake Winnipeg vs. age in calendar years. The inferred20 cm/century lake level rise is indicated by the arrow.

the result of isostatic uplift of the outlet at the northend of the lake.

The discovery of similar inundated stumps far-ther north at Observation Point in 1993, at WashowBay in 1994 (Forbes, pers. com. 1994) and at Spi-der Islands in 1996, indicate that at least some of therecent transgression is basin-wide. A lake level rise of20 cm/century at the southern shore, driven by differ-ential uplift of the outlet, may correspond to an approx-imately 15 cm/century water level rise at Washow Bayand Observation Point but will probably not accountfor the transgression observed at the Spider Islandsnear the northern outlet. Consequently, the water-levelrise in Lake Winnipeg during the last three centuriesmay be, at least in part, due to climatic variations,a conclusion supported by the work of Clark (1993)in Minnesota. Clark concluded that the climate of theLittle Ice Age, between approximately 1640 and 1920,was colder and wetter than the period either beforeor after that time. These conditions would lead to rel-atively higher water levels in Lake Winnipeg as thelake receives nearly 50% of its water from northwest-ern Ontario and the adjacent parts of Minnesota viathe Winnipeg River (Brunskill et al., 1980). Interest-ingly, Clark found high concentrations of charcoal insediments dating between 1440 and 1660 A.D., imme-diately before the Little Ice Age, suggesting this was atime of relatively low precipitation. Trees growing nearthe shores of Lake Winnipeg, during the low water fif-teenth and sixteenth centuries, would subsequently beinundated by the rising lake levels of the Little Ice Age.

Alternatively, transgression in the North Basin andelsewhere may be in part the result of shoreface pro-file down-cutting not related to water-level rise (DonForbes, pers. com., 1996).

Conclusions

Marsh sediments and associated rooted stumps, logsand tree roots on the foreshore of Lake Winnipeg indi-cate the level of the lake has risen over recent geologicaltime. Radiocarbon dating of wood associated with themarsh sediments suggests that the rate of this rise hasbeen 20 cm/century during the last three hundred years.This is comparable to similar rates determined previ-ously by direct water level measurements and upliftrates for Lake Winnipegosis (Penner & Swedlo, 1974;Nielsen et al., 1987). Isostatic uplift of the outlet at thenorth end of Lake Winnipeg accounts for this change inthe water level at the south end. However, the discov-ery of similar submerged shoreline features at Obser-vation Point, Washow Bay and Spider Islands farthernorth indicate that at least part of the water level rise isbasin-wide. Basin-wide transgression is not believedto be the result of differential isostatic uplift but isprobably due to down-cutting of the shoreface profileor climate change. The onset of generally cooler andmoisture conditions in A.D. 1600–1640, at the begin-ning of the Little Ice Age (Clark, 1993), may havebeen reflected in an increase in Lake Winnipeg waterlevels. Lowering of water levels in Lake Winnipegosisover the last 5000 years can not, however, be attribut-ed solely to climate fluctuations. The relative roles ofisostatic uplift, down-cutting of the shoreface profileand changing inflow into the lake as a result of cli-mate change during the last three centuries remain tobe resolved.

Acknowledgments

I would like to thank Harvey Thorleifson, of the Geo-logical Survey of Canada, for many helpful discussionsand his advice over the last few years regarding water-level fluctuations in Lake Winnipeg and Lake Win-nipegosis. Critical reviews by Gaywood Matile (Mani-toba Energy and Mines),Harvey Thorleifson,Dan Kerr(Geological Survey of Canada), David McLeod (Man-itoba Historic Resources), Robert Gilbert (Queen’sUniversity) and Rudy Klassen greatly improved themanuscript.

References

Brunskill G. B., S. E. M. Elliot, P. Campbell, 1980. Morphometry,hydrology, and watershed data pertinent to the limnology of Lake

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Winnipeg. Canadian Manuscript Report of Fisheries and AquaticSciences No. 1556: 32 pp.

Clark J. S., 1993. Fire, climate change, and forest processes duringthe past 2000 years. In Bradbury, JP & WE Dean (eds), Elk Lake,Minnesota: Evidence for rapid climate change in the north-centralUnited States. Boulder, Colorado, Geological Society of AmericaSpecial Paper 276: 295–308.

Nielsen E. & G. Conley, 1994. Sedimentology and geomorphic evo-lution of the south shore of Lake Winnipeg. Manitoba Energy andMines, Geological Report GR94-1: 58 pp.

Nielsen E, D. H. McNeil & W. B. McKillop, 1987. Origin andpaleoecology of post-Lake Agassiz raised beaches in Manitoba.Can. J. Earth Sci. 24: 1478–1485.

Penner F. & A. Swedlo, 1974. Lake Winnipeg shoreline erosion,sand movement, and ice effects study; The hydrologic, hydraulicand geomorphic studies, technical report appendix 2, v. 1-B,Water Resources Branch, Department of Mines, Resources andEnvironmental Management.

Stuiver M. & P. J. Reimer, 1993. Extended14C data base and revisedCALIB rev 3.0.314C age calibration program. Radiocarbon. 35:215–230.

Teller J.T., 1980. Radiocarbon dates in Manitoba. Manitoba Depart-ment of Energy and Mines, Mineral Resource Division, Geolog-ical Report GR80-4: 61 pp.

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