Lake Agassiz Research Articles

Paleohydrologic implications of 18O enriched Lake Agassiz water

Lake Agassiz water oxygen isotopic compositions inferred from sediment core organics and pore waters provide some additional insight into the paleohydrology of the Great Lakes and their drainage into the North Atlantic during the late glacial and early Holocene. Isotopically enriched Lake Agassiz water supports the hypothesis that high Huron Basin lake (Mattawa) phases, during the early Holocene (9600–9300 and 9100–8100 years BP) resulted from an influx of Lake Agassiz water and suggests that low lake (Stanley) phases (9800–9600, 9300–9100, 8100–7400 years BP) were influenced more by regional influxes of isotopically depleted glacial melt water. Eastward drainage of enriched early Lake Agassiz water supports an active Port Huron outlet between 11000 and 10500 years BP and also helps to explain the absence of an 18O depleted interval in North Atlantic foram records. This may be the result of a balance between the opposing isotopic effects of depleted Lake Agassiz water and lower sea surface temperatures on carbonate precipitation between 11000 and 10000 years BP.

Erratum: Paleochemistry of Lakes Agassiz and Manitoba based on ostracodes

Erratum: Paleochemistry of Lakes Agassiz and Manitoba based on ostracodes

Northern Redistribution of Freshwater Pearly Mussels (Bivalvia: Unionoidea) During Wisconsin Deglaciation in the Southern Glacial Lake Agassiz Region: A Review

-The chronology of the reinvasion of freshwater pearly mussels (Bivalvia: Unionoidea) into the southern Glacial Lake Agassiz region is reconstructed from the published record of the latest Pleistocene (Wisconsin) deglaciation of the region and known spatial and temporal distributions. The earliest invading unionoidean fauna entered the region above the driftless area of southeastern Minnesota and western Wisconsin between 14,000 and 11,000 years before present; the remaining species colonized the region postglacially. The formation of St. Anthony Falls is correlated with the drainage of Lake Agassiz from the Herman to the Campbell Beach level as an estimate of its date of formation, and a new chronology and mechanism of mussel dispersal into the Red River of the North basin

Using Standards in Simultaneous Renewal

Committed to the progressive tradition with its emphasis on learner-centered curriculum and learner-grounded assessment, faculty in the teacher education programs of the University of North Dakota and its affiliated Lake Agassiz Elementary School articulated program goals and implemented processes of portfolio assessment and group demonstrations of learning. These practices are evolving into assessment systems which are useful in drawing together appreciative audiences for student work, benchmarking student progress, and evaluating and reframing programs and the standards that inform them. The development of assessment processes which place primary value on student reflection has promoted student ownership of learning. The simultaneous development of parallel processes in a teacher education program and its professional development school has the potential to renew a powerful aspect of teaching practice.

Climatic effects of glacial Lake Agassiz in the midwestern United States during the last deglaciation

Stable isotope and pollen analyses of a sediment core from Deep Lake, Minnesota, provide new information on the climatic effects of glacial Lake Agassiz in Minnesota and insights into the cause of the prominent Picea recurrence during the Younger Dryas in the southern Great Lakes region. Bulk-carbonate δ18O exhibited large fluctuations between 12.0 and 9.1 ka (dates are in calendar years throughout this paper unless indicated otherwise), probably reflecting the effects of glacial Lake Agassiz superimposed on climatic warming related to large-scale climatic controls. In particular, a 3‰ decrease in δ18O 11.2–10.2 ka interrupted the δ18O enrichment of 1‰ from 12.0 to 11.2 ka and 3.5‰ from 10.2 to 9.1 ka. This δ18O decrease coincided with the expansion of Lake Agassiz. We interpret this decrease as a result of decreased summer temperature and increased precipitation derived from the cold and isotopically light meltwaters of Lake Agassiz. During this δ18O decline, Pinus pollen continued to increase at the expense of Picea pollen at Deep Lake, as at other Minnesota sites, providing evidence that climatic cooling induced by Lake Agassiz did not cause a reversal to a Picea -dominated vegetation. The absence of such a vegetational response implies that the prominent Picea recurrence during the Younger Dryas in the southern Great Lakes region was not caused solely by climatic cooling due to increased flux of meltwater from Lake Agassiz into the Great Lakes. Instead the Picea recurrence might have been driven primarily by the westward penetration of the Younger Dryas cooling.

Reconstruction of glacial Lake Hind in southwestern Manitoba, Canada

Glacial Lake Hind was a 4000 km2 ice-marginal lake which formed in southwestern Manitoba during the last deglaciation. It received meltwater from western Manitoba, Saskatchewan, and North Dakota via at least 10 channels, and discharged into glacial Lake Agassiz through the Pembina Spillway. During the early stage of deglaciation in southwestern Manitoba, part of the glacial Lake Hind basin was occupied by glacial Lake Souris which extended into the area from North Dakota. Sediments in the Lake Hind basin consist of deltaic gravels, lacustrine sand, and clayey silt. Much of the uppermost lacustrine sand in the central part of the basin has been reworked into aeolian dunes. No beaches have been recognized in the basin. Around the margins, clayey silt occurs up to a modern elevation of 457 m, and fluvio-deltaic gravels occur at 434–462 m. There are a total of 12 deltas, which can be divided into 3 groups based on elevation of their surfaces: (1) above 450 m along the eastern edge of the basin and in the narrow southern end; (2) between 450 and 442 m at the western edge of the basin; and (3) below 442 m. The earliest stage of glacial Lake Hind began shortly after 12 ka, as a small lake formed between the Souris and Red River lobes in southwestern Manitoba. Two deltas at an elevation of 450 were formed in this lake. At the same time, the Souris Lobe retreated far enough to allow glacial Lake Souris to expand farther north along the western side of the basin from North Dakota into what was to become glacial Lake Hind. Three deltas were built at an elevation above 460 m in the Canadian part of this proglacial lake. Continued ice retreat allowed the merger of glacial Lake Souris with the interlobate glacial Lake Hind to the east. Subsequent erosion of the outlet to the Pembina Spillway allowed waters in the glacial Lake Hind basin to become isolated from glacial Lake Souris, and a new level of glacial Lake Hind was established at 442 m, with 5 deltas built at this level by meltwater runoff from the west. Next, a catastrophic flood from the Moose Mountain uplands in southeastern Saskatchewan flowed through the Souris River valley to glacial Lake Souris, spilling into Lake Hind and depositing another delta. This resulted in further incision of the outlet (Pembina Spillway). A second flood through the Souris Spillway from glacial Lake Regina further eroded the outlet; most of glacial Lake Hind was drained at this time except for the deeper northern part. Coarse gravel was deposited by this flood, which differs from previous flood gravel because it is massive and contains less shale.

Paleochemistry of Lakes Agassiz and Manitoba based on ostracodes

The ionic composition and salinity of Lake Manitoba and its late-glacial precursor, Lake Agassiz, changed significantly over the past 11 000 years. The paleochemical record reported here is based on modern analog environments of ostracodes identified in a new 14.5 m core from southern Lake Manitoba. The ionic composition of Lake Manitoba today is dominated by Na+, Cl−, and HC03−, with much less Ca2+, Mg2+, and K+. Evaporative concentration of modern Lake Manitoba water would lead to greater salinity and the near depletion of Ca2+due to continued precipitation of calcite. During periods of highest salinity in the Holocene, however, Lake Manitoba supported Limnocythere staplini. Today this species inhabits waters in which [Ca2+] > [HCO3−], including springs associated with groundwater in Paleozoic bedrock discharging into Lake Winnipegosis (and eventually, after much dilution, into Lake Manitoba). Further complicating the Holocene record are intervals containing Limnocythere friabilis that suggest periodic influxes of dilute water, probably from the Assiniboine River, which bypasses Lake Manitoba today. The variations in Holocene paleochemistry indicated by the ostracode record imply changes in the proportion of overland flow plus precipitation relative to groundwater inputs to Lake Manitoba, independent of changes in evaporation relative to precipitation.

Late Quaternary Upper Mississippi River alluvial episodes and their significance to the Lower Mississippi River system

Late Quaternary Upper Mississippi River alluvial episodes and their significance to the Lower Mississippi River system

Sand-wedge and ventifact palaeoenvironmental indicators in North-West Saskatchewan, Canada, 11 ka to 9.9 kaBP

Exhumed sand wedges and ventifacts in the upper Churchill watershed of north-western Saskatchewan indicate that aeolian and periglacial processes were dominant from c. 11 to c 10.5 ka BP. Nineteen sand wedges were identified that display vertical foliation, form polygonal nets, and penetrate either till, glaciolacustrine sediment or glaciofluvial diapiric sand. The sand wedges, and sand lenses within the till and glaciolacustrine sediments, indicate permafrost aggradation after glaciation and lake drainage. Ventifacts that overlie the sand wedges have preferred keel orientations that correlate with palaeo-katabatic wind directions interpreted from stabilized parabolic dune orientations. Rat-tail lineaments on faceted surfaces of coarse-grained, granitic ventifacts also indicate palaeo-wind flow direction. Strong and weakly developed lineaments on the same einkanter are explained by opposing flow from prevailing winds, katabatic winds and anticyclonic air flow from the adjacent Laurentide Ice Sheet, or by flow separation at the ventifact keel under unidirectional air flow. Two geomorphic surfaces are identified: (1) an older surface with sand wedges and weakly developed ventifacts indicative of permafrost and brief aeolian activity; and (2) a lower and younger segment of the former Lake Agassiz basin, without permafrost indicators but with well-developed ventifacts. The upper surface, developed in association with the Beaver River Moraine and glacial Lake Meadow-McMurray, is 11–10.5 ka BP in age, synchronous with the Younger Dryas cold period. The younger, lower surface was last modified beginning at 9.9 ka BP, when the north-west outlet of Lake Agassiz opened, was susceptible to greater aeolian reworking than the upper surface, and postdates the Younger Dryas. © 1996 John Wiley & Sons, Ltd.

12 600 years of lake level changes, changing sills, ephemeral lakes and Niagara Gorge erosion in the Niagara Peninsula and Eastern Lake Erie basin

Over the last 12 600 years, lake levels in the eastern Lake Erie basin have fluctuated dramatically, causing major changes in drainage patterns, flooding and draining ephemeral Lake Wainfleet several times and widening and narrowing the Niagara Gorge as the erosive effects of Niagara Falls waxed and waned. The control sill for Lake Erie levels was at first the Fort Erie/Buffalo sill, before the Lyell/Johnson sill in Niagara Falls took over due to isostatic rebound. This sill, in time, was eventually eroded by the recession of Niagara Falls and the Fort Erie/Buffalo sill regained control. The environmental picture is complicated by catastrophic outbursts from glacial Lake Agassiz and Lake Barlow-Ojibway, changes in outlet routes, isostatic rebound and climatic changes over the Great Lakes basins. Today, the flow of water into Lake Erie from the streams and rivers surrounding it only accounts for about 13% of the flow out of it, therefore, the importance of flow from the Upper Great Lakes, specifically the flow from Lake Huron, has a great effect on Lake Erie levels. While the changing control sills, Lyell/Johnson and Buffalo/Fort Erie would affect Lake Erie levels, overall they are mostly input driven by the amount of waters received from the Upper Great Lakes. Since Lake Erie's water level changes are so closely tied to Lake Huron's water level changes we have decided to use names assigned to Lake Huron such as the two Mattawa highstands and three Stanley lowstands rather than inflict a whole new set of names on the public. While the duration of each high and lowstand in Erie and Huron may not always be the same, they always happen within the same time frame. The datum elevations used for Lake Huron (175.8 m) and Lake Erie (173.3 m) are historically recorded averages. The Lake Erie levels proposed in this paper reflect Lake Hurons effects on Lake Erie and the levels occuring at the eastern end of the Erie Basin throughout the last 12 600 years. All dates in this paper are uncorrected 14 C dates unless the date was obtained from shells, then the date has been corrected for hard-water effects. Also, all heights are given as modern day elevations and are not adjusted for isostatic rebound.

AFTER THE ICE AGE : THE RETURN OF LIFE TO GLACIATED NORTH AMERICA

Review by James M. Peek, Ph.D. Professor of Wildlife Resources, University of Idaho, Moscow, ID 83844, USA. TEL: 208-885-7120. Pielou, E.C. AFTER THE ICE AGE : THE RETURN OF LIFE TO GLACIATED NORTH AMERICA. Chicago: University of Chicago Press, 1991, 1992 paperback ed. 366 pp. US$13.95 paperback ISBN: 0-226-66812-6. E.C. Pielou's descriptions of events on earth following the last maximum glacial advances of 20,000 years ago should dispel the notion that we live in a static world. It should cause the reader to ponder whether our contemporary society is structured to adapt as we inexorably advance towards the next ice age. As our lake shores change and our climate changes, are we short-sighted humans ready to adjust our lives to accommodate the future? And those of us who concern ourselves with conservation of natural resources: where do our activities, which are often centered on the short-term, fit into the long-term picture? In AFTER THE ICE AGE : THE RETURN OF LIFE TO GLACIATED NORTH AMERICA, such thought-provoking issues are raised as Pielou guides us through the geological evidence, using examples from little-known and seemingly inconsequential species, extinct and extant. We learn of the Malankovich Cycle describing the earth's orbit around the sun, the climatic warming of 10,000 years ago, and the Little Ice Age of 1600 A.D. The Glacial Lakes Agassiz and McConnell are delineated on a map, and their influences on current landform and vegetation pattern, are narrated eloquently. Also, we discover how species like the northern pike are distributed in North America across many major drainages because of their ability to migrate when lakes at headwaters ebbed and flowed, causing them to drain in different directions. We marvel at the careful observation of such seemingly insignificant species as the foraminifera, floating single-celled organisms which have shells that become fossilized and sink into the lake sediments. Forams with right-coiled shells live in warm waters while the left-coiled varieties live in cold waters. Abrupt changes in the presence of these different forams in sea sediments signals changes in climate. The events surrounding the evolution of Glacial Lake Missoula, and the catastrophic floods that modified so much of the terrain in eastern Washington when the ice dams failed, are also covered. The volcanic eruptions and the huge deposits of ash, the major extinction of the large mammals and birds, and how aboriginal man must have occupied the land as it changed through long time spans, are all discussed in sufficient detail to enhance our understanding. Each chapter of AFTER THE ICE AGE... is documented and a section on footnotes is included in the back. This book is highly recommended for those who are interested in the natural world and who wish to acquire a bit of humility and a unique perspective on how forces beyond our control will ultimately dictate the condition of our environment and our future.

Archaeological geology of the Rustad Quarry site (32RI775): An early archaic site in Southeastern North Dakota

AbstractAt The Rustad Quarry Site (32RI775), in southeastern North Dakota, Early Archaic artifacts, bison bone, and hearth features were found in the lowest of three Boroll‐like buried soils formed in alluvial fan sediments. Alluvial fan sediments were deposited from 8000 to 4925 B.P., and were then buried by eolian sand. The alluvial fan sediments (mudflows) bury lacustrine sediments (Sherack Formation deposited from 9900 to 9500 B.P. in Glacial Lake Agassiz II), both of which overlie a Moorhead Phase fluvial terrace deposited from 10,900 to 9,900 B.P. Cultural remains were associated with five radiocarbon ages, three on archaeological charcoal (7180 B.P., 7240 B.P., and 7550 B.P.) and two on organic carbon from two welded A‐horizons containing the cultural remains (7370 B.P. and 7675 B.P.). A well‐developed Aquoll‐like soil formed in the lacustrine sediments from 9500 to 8000 B.P. Soil morphology and chemistry of the Boroll‐like fan soils, the Aquoll‐like lacustrine soil, and other Boroll‐like soils formed in eolian and deltaic sediments nearby suggest a subhumid, cool continental climate with riparian woodland and mixed prairie vegetation at the site and surrounding area from 11,400 B.P. to the present. This partially contradicts paleoenvironmental reconstruction from pollen sites nearby that suggest the area was covered by a closed canopy spruce forest from terminal late‐Pleistocene to 10,000 B.P. Alluvial fan formation and eolian activity on the adjacent Sheyenne Delta occurred from 8000 to 5000 B.P., which indicates greater landscape instability and drier conditions during the mid‐Holocene (Altithermal).

Temporal variations in dissolved methane deep in the Lake Agassiz Peatlands, Minnesota

A study (August 1990 to July 1991) of profiles of dissolved CH4 concentrations, diffusive flux of CH4, and CH4 production rates of 45 sites in the Lake Agassiz Peatlands in northern Minnesota shows that dissolved CH4 deep in the peat (> 1 m depth) mobilized easily to the vadose zone. During August 1990 the dissolved CH4 concentrations at some depths at some sites were supersaturated with respect to one atmosphere partial pressure of CH4. At one site (2.5 m depth) the concentration of dissolved CH4 in the peat pore‐water was 140 mg L−1. In July 1991, at no site did the concentration of dissolved CH4 in the peat pore water exceed 40 mg L−1 in the peat profile. The average calculated diffusive flux of CH4 decreased from 95 to 45 mg m−2 d−1 between 1990 and 1991. Gaseous CH4 was more in evidence in 1990 than in 1991. In 1990, CH4 at many depths bubbled vigorously when peat pore water was sampled. At some sites there was sufficient pore pressure to eject slugs of water forcibly from piezometers. Similarly, dissolved inorganic carbon (DIC) consisting of H2CO3, CO2, HCO3− and CO32− decreased between the sampling times from an average for both bogs and fens in 1990 of 62 mg C L−1 to 38 mg C L−1 in 1991. A dynamic mechanism must exist which traps CH4 deep in the peat column allowing gaseous CH4 to build up, increasing dissolved CH4. Other times, CH4 passes freely from deep peat to the vadose zone. We suggest as a hypothesis that a confining layer of trapped CH4 bubbles forms at depth in the peat, trapping gaseous CH4. The duration of the “bubble confining layer” is uncertain. We propose two hypotheses. (1) The confining layer is usually present and deteriorates after a major climatic event such as a drought, or (2) the confining layer forms and collapses seasonally with seasonal variations in the water table elevation.

Great Lakes response to catastrophic inflows from Lake Agassiz: some simulations of a hydraulic geometry for chained lake systems

Simulations (216) were undertaken to evaluate the impact of typical Lake Agassiz outbursts on the upper Great Lakes under plausible variations in lake surface areas and sill widths. Flows over sills out of lakes are modelled using the equation for a broad-crested weir, with the model time increment set to one day. The model was evaluated for Lake Agassiz outlet sill widths of 1, 4, and 10 km and with outbursts ranging from 100 000 m3 s−1 to 600 000 m3 s−1. The surface area of Lake Agassiz was evaluated for 182 000 km2 ±20%. The surface area of the upper Great Lakes were modelled as either Lake Algonquin (Superior, Huron and Michigan basins =200 000 km2) or Lake Minong (Superior basin 87 000 km2) with sill widths of 0.5, 1.5, and 3 km. Downstream peak discharge modelled at the outlet sill of the upper Great Lakes, was normally between 20 and 60% of the initial outburst, with a lagtime to peak usually between 80 and 280 days. Upper Great Lakes water level rises of between 2 and 20 m are calculated with rises to 36 m for some configurations. Rise magnitude is inversely related to the width of the outlet sills at both lake systems and to the surface area of the receiving lake. The modeling implies that measuring outflow from the upper Great Lakes, or water level rises, does not in itself determine peak or total outflow from Lake Agassiz unless the dimensions of the Lake Agassiz and upper Great Lakes outflow sills are also known. Lake level rises probably coincided on the upper Great Lakes with meltout from the winter freeze-up. Lake levels re-attain equilibrium values with respect to through flow within three years of an outburst. Substantial episodic lake level rises in the upper Great Lakes may have had severe impacts on the lake biota, for example via the affect on spawning grounds.

Climate-driven flushing of pore water in peatlands

NORTHERN peatlands can act as either important sources or sinks for atmospheric carbon1,2. It is therefore important to understand how carbon cycling in these regions will respond to a changing climate. Existing carbon balance models for peatlands assume that fluid flow and advective mass transport are negligible at depth3,4, and that the effects of climate change should be essentially limited to the near-surface. Here we report the response of groundwater flow and porewater chemistry in the Glacial Lake Agassiz peat-lands of northern Minnesota to the regional drought cycle. Comparison of field observations and numerical simulations indicates that climate fluctuations of short duration may temporarily reverse the vertical direction of fluid flow through the peat, although this has little effect on water chemistry5. On the other hand, periods of drought persisting for at least 3–5 years produce striking changes in the chemistry of the pore water. These longer-term changes in hydrology influence the flux of nutrients and dissolved organic matter through the deeper peat, and therefore affect directly the rates of fermentation and methanogenesis, and the export of dissolved carbon compounds from the peatland.

Glacial Lake Saskatchewan and Lake Agassiz deltas in east-central Saskatchewan with special emphasis on the Nipawin delta

The Nipawin delta extends eastward from the town of Nipawin in east-central Saskatchewan. The Prince Albert and Fort à la Corne deltas formed in Lake Saskatchewan at the mouths of the South and North Saskatchewan rivers, whereas the Nipawin and Mossy vale deltas formed in Lake Agassiz at the mouth of the Saskatchewan River. These deltas became lower in elevation and younger eastward. Clays beneath the Nipawin and Mossyvale deltas can be traced beneath the Prince Albert and Fort à la Corne deltas with little change in thickness, suggesting that most of the clays beneath the Nipawin and Mossyvale deltas were deposited during the formation of the Prince Albert and Fort à la Corne deltas. The Nipawin delta formed between 10 200 and 9500 BP. The Campbell strandline (Emerson phase) was the shoreline of the Nipawin delta. The Mossyvale delta formed about 9500 BP. The surface of the Mossyvale delta (McCauleyville level) is about 40 m below the surface of the Nipawin delta (Campbell level).

Isotopic Composition of Old Ground Water from Lake Agassiz: Implications for Late Pleistocene Climate

A uniform oxygen isotope value of -25 per mil was obtained from old ground water at depths of 20 to 30 meters in a thick deposit of clay in the southern part of the glacial Lake Agassiz basin. The lake occupied parts of North Dakota and southern Manitoba at the end of the last glacial maximum and received water from the ice margin and the interior plains region of Canada. Ground water from thick late Pleistocene-age clay deposits elsewhere, a till in southern Saskatchewan, and a glaciolacustrine deposit in northern Ontario show the same value at similar depths. These sites are at about 50 degrees N latitude, span a distance of 2000 kilometers, and like the Lake Agassiz sites, have a ground-water velocity of less than a few millimeters per year. The value of -25 per mil is characteristic of meltwater impounded in the southern basin of Lake Agassiz. This value corresponds to an estimated air temperature of -16 degrees C, compared with the modern temperature of 0 degrees C for this area.

A post-Lake Minong transgressive event on the north shore of Lake Superior, Ontario: possible evidence of Lake Agassiz inflow

Modification of an ice-contact delta built on the margin of Lake Minong (9500 BP) is ascribed to a transgressive event. Reworking of fluvial sediments by wave action and the infilling of the lower end of a distributary valley demonstrate a post-Minong transgression and reoccupation of the lower portion of the delta. Estimated to be in the order of 18 m, this water-level oscillation may represent evidence of one of several catastrophic discharges of Lake Agassiz into the Superior basin, proposed to have occurred between 9.5 and 8.0 ka BP.

Comparison of Measured and Calculated Diffusion Coefficients for Iodide in Compacted Clays

AbstractAn understanding of the behaviour of contaminants in compacted clays is important in assessing the effectiveness of clay-based barrier materials used in many waste containment strategies. Here the diffusion and sorption behaviour of I−with selected compacted clays is examined (129I is a relatively long-lived radioisotope present in high-level nuclear fuel waste.) Diffusion coefficients,D, and distribution coefficients,Kd, were measured for I−with four clays: bentonite, Lake Agassiz clay (a glacial lake clay composed mainly of smectite, illite, kaolinite, and quartz), interstratified illite-smectite, and kaolinite. For the diffusion experiments the clays were compacted to a dry bulk density, ρ, of ∼1.2 Mg m−3. The mean measuredDvalues,Dm, were as follows: bentonite, 310 µm2s−1; Lake Agassiz clay, 0.62 µm2s−1; illitesmectite, 190µm2s−1; and kaolinite, 74µm2s−1. The measured values were compared with those calculated,Dc, from the following model:D=Doτan/(n+ρKd), whereDois the diffusion coefficient in pure bulk water,τathe apparent tortuosity factor, andnthe solution-filled porosity of the clay. Except for the Lake Agassiz clay, whereDmwas about an order of magnitude lower thanDc, theDmandDcvalues agreed within a factor of about two. The Lake Agassiz clay has a markedly higher organic carbon content than the other three clays, and this could affectτa, which may be overestimated in the model.

Deglacial Flood Origin of the Charleston Alluvial Fan, Lower Mississippi Alluvial Valley

Large-magnitude flooding of the Mississippi River from proglacial lakes Agassiz and Superior most likely occurred between 11,300 and 10,900 and 9900 and 9500 yr B.P. The Charleston alluvial fan, a depositional remnant of one of these floods, is located at the head of a wide alluvial plain near Charleston, Missouri. The fan is an elongate, convex-up sand body (16 × 24 km) composed of medium- and fine-grained sand at least 8 m thick. This sand contrasts with the older coarse-grained sand of the braided stream surface to the west and south and younger silty clay of the meandering stream level to the north and east. A weakly developed soil separates the underlying braided steam deposits from the alluvial fan. A bulk-soil radiocarbon date of 10,590 ± 200 yr B.P. from the contact between the fan and clays of the meandering stream system indicates that the Charleston fan was deposited near the end of the early interval of flooding from Lake Agassiz about 10,900 yr B.P. If the Charleston fan is the last remnant of deglacial flooding in the lower Mississippi Valley, then deposition of significant quantities of sediment from largemagnitude floods between 10,000 and 9500 yr B.P. did not extend into the lower Mississippi Valley through Thebes Gap.