Superior Basin Research Articles

Larval identification of the ruffe, Gymnocephalus cernuus (Linnaeus) (Percidae: Percini), in the St. Louis River Estuary, Lake Superior drainage basin, Minnesota

A reproducing population of the Eurasian percid Gymnocephalus cernuus was discovered in the St. Louis River, Lake Superior basin, near Duluth, Minnesota. This study is the first documented evidence of the species occurrence and reproduction in North America. Larvae resemble typical percids and are described with emphasis on meristic, morphometric, and pigmentary characters. Ruffe are distinguished from all native North American percids by the presence of a slightly concave head becoming attenuated at larger length intervals, a pointed snout with teeth on the maxillary and premaxillary by the time a length of 11 mm is reached, a large dorsally pigmented swim bladder, a serrated preopercle, and few postanal myomeres (usually 18–22). In particular, ruffe superficially resemble johnny darters as yolk-sac larvae, and walleye, yellow perch, and logperch at lengths greater than 10 mm, but can be distinguished by the previously mentioned characteristics and a deep shoulder depth and a narrow caudal peduncle.

Aeolian sandstones in the Copper Harbor Formation, Late Proterozoic, Lake Superior basin

Sandstones with cross-sets up to 1.5 m thick occur within the Copper Harbor Formation, most prominently at Five Mile Point. In contrast to intercalated sandstones with smaller scale cross-stratification or horizontal lamination, pebbles are very scarce in the large-scale cross-stratified sandstones and, where present, are restricted to set bases. The large-scale cross-stratified sandstones are better sorted than intercalated sandstones and show a mean palaeocurrent direction at 90° to the mean for the interbedded sandstones.The large-scale cross-stratified sandstones are interpreted as the product of fields of small transverse aeolian dunes that formed on dry parts of alluvial fans bordering the Keweenawan rift. The aeolian palaeocurrent mean is along the rift valley, and tentatively may be interpreted as the result of topographically constrained palaeo-tradewinds.

A geophysical profile of the southern margin of the midcontinent rift system in western Lake Superior

A 70‐km‐long seismic reflection profile in western Michigan provides new insight into the nature, distribution, and structure of the Keweenawan Supergroup volcanic and overlying sedimentary rocks and the controversial Keweenaw fault along the southern boundary of the Midcontinent Rift System in the Lake Superior basin. Interpretation of the 5‐s reflection data constrained by surface geology, magnetics, and gravity modeling shows that volcanic rocks which cropout north of the Keweenaw fault dip northerly to depths of the order of 17 km. South of the fault, volcanic rocks overlain by ∼2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the South Range. The volcanic pile within the basin thickens rapidly to the north of the Keweenaw fault, suggesting that the volcanics were deposted in an extensional fault‐bounded basin. Clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event. The thickness of the volcanic‐filled basin implies that the upper crust was almost completely broken during the rifting event. The available evidence is interpreted to show the Keweenaw fault as a moderate‐ to high‐angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence. There is no evidence from the seismic profiling for major faulting (except for the Keweenaw fault), intrusions, or folding of the Keweenawan Supergroup in this region.

Multiyear trends in snowpack ion accumulation and loss, northern Michigan

From December 1982 to May 1987 the quality and quantity of snowfall and snowpack were measured at four stations in a small (176 ha) gauged watershed adjacent to Lake Superior's south shore. Annual precipitation was less than and winter precipitation greater than observed east and south of the Lake Superior basin. Snowfall averaged 37% of annual precipitation but was quite variable. Snowfall significantly increased (from 21.1 to 35.7 cm, P < 0.001) with 150 m increase in elevation. Hydrogen and SO42− were the dominant ions in precipitation and snowpack while Ca2+ and HCO3− dominated stream water. No significant time trends in winter precipitation chemistry were found. Snowpack ionic retention rarely matched precipitation input. Generally, midwinter freeze‐thaw periods resulted in elevated concentrations of solutes in snow meltwater and greatly reduced snowpack ion loads prior to peak stream water discharge. All ions exhibited pulses in snowmelt each year. Preferential elution from the snowpack of SO42− and K+ was observed. Stream water H+, NH4+ and most NO3− pulses were associated with midwinter thaws. Stream water SO42− pulses were rare. For those years exhibiting the most pronounced spring increase in stream H+, NO3− and SO42− concentration, the magnitude and rate of concentration change was as great in a year of small peak snowpack ion load as it was following a more rapid ion loss from a larger snowpack load. This suggests that the pattern of snowpack melt and especially the pathway the meltwater follows to the stream may be important factors in determining whether stream water chemical pulses occur.

Time resolution of geologic events on the Keweenaw Peninsula and implications for development of the Midcontinent Rift system

Three flood-type lava flows from the Keweenaw Peninsula in upper Michigan have been precisely dated, using U Pb analyses of zircon and baddeleyite. Zircons were extracted from pegmatoid segregations within two basaltic flows. Ages from the Copper City Flow, near the base of the Portage Lake Volcanics, and the Greenstone Flow, near the top, are 1096.2 ± 1.8Ma and1094.0 ± 1.5Ma, respectively (95% confidence errors). U Pb analyses of baddeleyite from these samples are in general agreement with the zircon results although some fractions appear slightly older, possibly due to 222Rn loss. The age of an andesite flow from the Lake Shore Traps, 1000 m above the Portage Lake Volcanics and within the Copper Harbour Conglomerate, is 1087.2 ± 1.6Ma from analyses of zircon. This gives an age for sedimentation within the Copper Harbor Conglomerate. These ages, along with stratigraphic data, indicate a subsidence rate of 1.3 −0.6 +1.8 mm/y during eruption of the Portage Lake Volcanics and 0.2 ± 0.1mm/y (1σ errors) for the transitional period from flood basalt eruption to clastic sedimentation. Magma eruption rates were at least 0.02–0.05 km 3/y during emplacement of the Portage Lake Volcanics. Rates as high as 0.2–0.6 km 3/y are inferred from regional stratigraphic data and currently available precise dates from throughout the Lake Superior basin. These values are comparable to the high rates of magma production estimated for Phanerozoic continental flood basalt provinces. Analyses of three detrital zircon grains from a sandstone at the base of the Copper Harbor Conglomerate give ages from 1101 to 1106 Ma. A fraction consisting of 29 detrital zircon grains has an age of 1104 ± 2Ma indicating that sediments within the Copper Harbor Conglomerate were probably derived almost entirely from lower Keweenawan rocks. Since the Portage Lake Volcanics are paleomagnetically normal, the age of the Keweenawan paleomagnetic polarity reversal is now bracketed between 1096.2 ± 1.8Ma and1097.6 ± 3.7Ma, the age of a reversed flow within the Osler Group. From this and other studies it appears that apparent polar wander rates are different for reversed and normal poles and that normal and reversed paleomagnetic poles of comparable age have different geographic positions. This suggests that the asymmetry between reversed and normal Keweenawan poles for igneous rocks may be due to a non-geocentric dipole.

Fecundity of Spring- and Fall-Run Steelhead from Two Western Lake Superior Tributaries

Abstract We examined the relationship between egg number and body size for 70 steelhead Oncorhynchus mykiss (formerly Salmo gairdneri) collected during 1985–1987 from the Bois Brule River and Pikes Creek, two Wisconsin tributaries to western Lake Superior. These data were compared with limited historical data from the Bois Brule River and nearby streams along the lake's north shore in Minnesota. Within the Lake Superior basin, steelhead exhibited significant variation of fecundity among spawning runs in different rivers and among years but not between spring and fall (autumn) runs in the Bois Brule River in 1985. Steelhead that spawned in Wisconsin tributaries to Lake Superior during 1985–1987 were significantly more fecund at a given size than were steelhead collected from tributaries along the north shore in Minnesota 20–25 years ago. Steelhead from Wisconsin tributaries produced egg numbers (3,000–6,900/female) similar to numbers (1,200–6,900) produced by Pacific Ocean steelhead in the same size range....

Variations in Sediment Accumulation Rates and the Flux of Labile Organic Matter in Eastern Lake Superior Basins

Benthic environments in Lake Superior are variable and dynamic. The complex topography and morphology of eastern Lake Superior results in a variety of depositional environments, from shallow coastal bays to deep profundal basins. Direct observations of bottom types and collection of sediments across the gradient in environments reveal benthic systems that range in sediment accumulation rates from ∼1.7 mm per year to virtually zero net long-term accumulation. Diagenetic modeling of organic carbon and nitrogen under steady state conditions indicate that 15% to 40% of the organic carbon and nitrogen deposited is recycled into the overlying water. This “metabolizable” fraction has a residence time in the sediment column estimated to be 30 to 60 years for organic carbon and 15 to 30 years for nitrogen. Concentration gradients of total dissolved inorganic carbon, ammonium, and oxygen in sediment pore waters yield fluxes of remineralized carbon and nitrogen across the sediment-water interface consistent with diagenetically modeled fluxes. Spatial variability in sediment accumulation patterns, however, is great, particularly in the deep, steep relief basins in the open lake. Here, distances of a few kilometers can radically alter deposition patterns. Evidence of aperiodic bottom currents were observed in the deepest charted basin of the lake as extensive fields of sediment ripples over hard, compacted relict silt and clays. While no currents were observed in these summertime dives, it is assumed that storms during isothermal conditions generate suffficient bottom currents at depth to very effectively scour the bottom. Differences in sediment metabolism appear to be determined by the flux of labile organic matter to the bottom coupled to the physical processes of sedimentation, resuspension, and horizontal transport.

Paleomagnetic dating of late glacial and postglacial sediments in Lake Superior

The oldest unit of the late Quaternary sedimentary sequence of Lake Superior consists of glacial till that was deposited about 9900 years BP during the last glacial readvance into the Lake Superior basin. The till deposits are unconformably overlain by a thin sequence of bedded sands and silts, which are overlain by a thick sequence of varved sediments (> 10 m in thickness), which in turn are overlain by postglacial silty clays.Unfortunately, 14C has not been found to be a useful tool in dating the late glacial and postglacial sediments of Lake Superior. However, paleodeclination and paleoinclination logs provide a useful method of correlating and indirectly dating cores taken of the late glacial and postglacial sediments. It is concluded from paleomagnetic dating that rhythmic sedimentation ceased before 9200 years BP in the southeastern lake proper and at about 8700 years BP in the southeastern bay areas. However, rhythmic sedimentation continued until about 8200 years BP in the northern part of Lake Superior and until about 8000 years BP in the Nipigon Bay area.

History of sedimentation in the northwestern Lake Superior basin and its relation to Lake Agassiz overflow

Following the Marquette glacial advance, which blocked the eastern outlets of Lake Agassiz and reached northern Michigan about 10 000 years BP, the ice margin wasted back toward the northeast, eventually allowing Lake Agassiz to overflow into the Lake Superior basin through a series of channels. Sediments deposited east of Thunder Bay near the mouth of the Wolf, Wolfpup, Shillabeer, and Black Sturgeon channels reflect three phases in the history of Lake Superior and provide the basis for reconstructing the early postglacial history of the region.The lower part of the sedimentary sequence in the northwestern Superior Basin consists of a distinctive red, stoney, sandy till deposited during the Marquette glacial advance and is overlain by pink rhythmites deposited in Lake Superior when it was a deep proglacial lake at the Minong level. The nearly 300 rhythmites deposited at this time typically consist of 4 cm thick silt + clay couplets, which are punctuated by silt laminae and sandy turbidites that probably represent major thaw periods or storms. These are seasonal rhythmites, deposited prior to the reopening of the Lake Agassiz outlets into the Superior Basin, and they display a decrease in dropstones, grain size, and thickness upsection that reflects a receding ice margin.The first eastern outlets of Lake Agassiz were uncovered around 9500 years BP, and water began overflowing into the Superior Basin in a series of catastrophic floods. Subaqueous fans developed at the Wolf, Wolfpup, and Shillabeer confluence and at the mouth of the Black Sturgeon channel. Large sandy turbidites, 45–65 cm thick, were deposited in the proximal part of these fans, with scouring and large (1 m) trough cross-beds resulting from the largest Lake Agassiz floods. These sediments are transitional to distal, clayey silt rhythmites, 10–22 cm thick. A gradual decrease in flooding from Lake Agassiz is reflected in the upward decrease in rhythmite thickness to 1–3 cm by about 8200 years BP. The final sequence of sediments shows a transition to sandy units as water levels dropped in the Superior Basin and the influence of nearshore processes increased.

Paleogeographic reconstruction of shoreline archaeological sites around Thunder Bay, Ontario

AbstractPaleoindian peoples migrated into the cul‐de‐sac between Lake Agassiz, the Superior basin, and the receding ice margin about 9500 years ago. Around Thunder Bay there is a close but not exclusive association of habitation sites with Lake Minong and its subsequent declining stages. Reconstruction of shorelines by detailed morphological mapping provides explanation of known sites, reasonable longshore correlation, and a predictive tool for focusing new investigations. Sites near taconite sources, in sheltered embayments, at river mouths, and near lookout points are preferred, but some occupance continued after water level decline. the full sequence of Paleoindian to Shield Archaic periods is uniquely destroyed by the Nipissing transgression (8000‐5500 years BP), which buried and truncated earlier shorelines. This sequence is potentially preserved on the north shore of Superior, but limited archaeological resources exist in the region, despite healthy, interdisciplinary cooperation.

Near Real‐Time Forecasting of Large Lake Supplies

The Great Lakes Environmental Research Laboratory (GLERL) has developed conceptual model‐based techniques for making outlooks of basin moisture conditions, basin runoff, water supplies, and lake levels several months into the future for large lake basins. The techniques consider the water stored in the basins about the lakes and the uncertainty of future meteorologic conditions. We estimate a large lake basin's current moisture storage with GLERL's tank‐cascade runoff model, applied to each of the subbasins about the lake, and with near real‐time meteorology. We select historic meteorologic sequences, representing anticipated meteorology, based on the National Weather Service monthly and seasonal forecasts of precipitation and air temperature probabilities, for use with the runoff model to generate our near real‐time outlooks. Error analysis for the Lake Superior Basin identifies the outlook error components and their relative magnitudes, indicates the outlook is superior to climatology, and suggests that improvements await better weather forecasting and lake evaporation modeling.

Lake Superior Atmospheric Wet Deposition 1980-1983

Abstract Wet deposition estimates were computed from monthly wet precipitation samples collected in the Lake Superior Basin. Sulphate and nitrogen loadings from wet precipitation corresponded to 142 and 40 thousand tonnes per year. On a percentage basis, wet deposition of sulphate and nitrogen accounted for 21% and 54%, respectively, of the total load. Atmospheric sources for other major ions ranged from 1 to 10%. Atmospheric deposition at the eastern end of Lake Superior was higher than the western end of the Basin. The relative accuracy of these estimates were examined in relation to the changes in lake chemistry in Lake Superior from 1973 to 1983.

Sur l'origine oceanique des metabasites de la chaine du mayombe (Gabon, Congo, Zaïre, Angola)

Abstract The Precambrian Mayombian belt is actually considered to be the result of the closure of an oceanic basin located between a western Eburnean craton and an eastern Archaean craton. The cratonic collision, which took place about 1000 Ma ago, induced the tectonic emplacement of ophiolites along the axis of the belt. This collision favoured the formation of the Nyanga's and Niari's epicontinental basins, deformed later during the Panafrican episode. Geochemial studies of the metabasites of the suture confirm their oceanic origin: they are olivine tholeiites similar to abyssal tholeiites. Different from the tholeiites intruded in a lithospheric stretching model, they are almost identical to the tholeiites of the Red Sea. This evolution model for the Mayombian belt: Kibarian narrow ocean floor basins, which closure leads to the Precambrian superior epicontinental basins, is similar to the Burundian of Burundi and to the Kibarian of Zaire. It seems to be general for the post-Eburnean belts in Central Africa.

Paleomagnetism of the Powder Mill Group, Michigan and Wisconsin: A reassessment of the Logan Loop

The apex and the western arm of the 1.1‐Ga Logan Loop, a prominent feature of the North American Precambrian apparent polar wander path, are defined by a large body of data from Keweenawan rocks of the Lake Superior Basin. Until now the eastern arm of this paleomagnetic feature has been largely defined by two data points from the Powder Mill Group of Michigan. A reexamination of this sequence reveals a primary reversed magnetization which plots close to the apex of the Logan Loop. Data from the Mellen Complex in Wisconsin, together with our data from the Powder Mill Group, suggest that for steeply dipping volcanic units, the true direction of a prefolding reversed magnetization is not obtained by simple structural rotation about presently observed strike. Reversely magnetized sites from localities where the Powder Mill volcanics have shallow dips yield a pole (146°W, 46°N, A95 = 9.2°) on the apex of the loop. A well‐defined, stratigraphically continuous, normal to reversed polarity change near the base of the Powder Mill Group is not confirmed. Normally magnetized flows have been found at two localities, only one of which is at the base of the lava sequence. The higher metamorphic grade of normally magnetized sites suggests that normal magnetizations within the Powder Mill Group are secondary and should not be used in definition of the eastern arm of the Logan Loop.

Calcium carbonate cement (caliche) in Keweenawan sedimentary rocks (∼ 1.1 Ga), upper Peninsula of Michigan

Abstract A complex caliche profile, and numerous widespread zones of carbonate cement occur in a thick sequence of fluvial siliciclastic sandstones and conglomerates that were deposited in the Lake Superior basin during and after continental basaltic volcanism, as interflow conglomerates of the Portage Lake Volcanics and as the Oronto Group. In conglomerates the carbonate cement forms discontinuous zones, up to 60 cm thick and several meters in length, consisting of up to 25 vol.% carbonate. Whereas the caliche horizon at the Centennial Mine shows typical upward-grading features characteristic of pedogenic caliche, the widespread sparry carbonate cement is less diagnostic. It is interpreted also as caliche for the following reasons: (1) it predates regional epidote metamorphism; (2) a caliche profile has been described from the same strata; (3) textures characteristic of caliche such as carbonate-rimmed pebbles, pendulose calcite on pebbles, sand grains floating in a carbonate matrix, and exfoliated chlorite; (4) possible vadoids; (5) the carbonate-rich zones are widespread stratigraphically and geographically in basinward-inclined alluvial fans, and (6) the distribution of these zones, and the total quantity of contained carbonate requires other than a point source for the origin of the carbonate. Several lines of evidence suggest that the prevailing climate may have been arid, with a fluctuating, perhaps seasonal rainfall pattern required for caliche: (1) a paleolatitude about 30° north; (2) carbonate-filled shrinkage cracks denoting high evaporation; (3) intermittent, flashy, or ephemeral streams recorded locally in fluvial beds by numerous superimposed horizons of mud-chip conglomerate, enormous, previously cemented mudstone rip-ups; and desiccation-cracked mud drapes. Interpretation of the cement as vadose caliche is compatible with a sedimentation model for the region as a semi-arid intermontane basin, flanked by alluvial fans and with a central saline lake.

Tidal estimation in the Atlantic and Indian Oceans

Abstract An estimation technique has been developed to extrapolate tidal amplitudes and phases over entire ocean basins using existing gauge data and the altimetric measurements which are now beginning to be provided by satellite oceanography. The technique was tested previously in the Lake Superior basin by Sanchez et al. (1985). The method has now been developed and applied in the Atlantic‐Indian ocean basins using a 6° × 6° grid to test its essential features. The functions used in the interpolation are the eigenfunctions of the velocity potential (Proudman functions), which are computed numerically from a knowledge of the basin's bottom topography, the horizontal plan form, and the necessary boundary conditions. These functions are characteristic of the particular basin. The gravitational normal modes of the basin, computed as part of the investigation, are used to obtain the theoretical forced solutions for the tidal constituents. The latter provide the simulated data for testing of the method and se...

Correlation of late Wisconsin glacial phases in the western Great Lakes area

Late Wisconsin glacial phases of the Superior Lobe in Wisconsin and adjacent Minnesota are tentatively correlated across northern Wisconsin and adjacent Michigan to phases of the Green Bay and Lake Michigan Lobes. Spanning the period from about 26,000 yr B.P. to about 9500 yr B.P., 7 phases of the late Wisconsin Laurentide Ice Sheet are discussed, but only 2 of these are precisely dated. Ice advanced out of the basins of Lakes Superior and Michigan and stabilized during the St. Croix-Hancock phase, the most extensive late Wisconsin phase in most of the area, between about 18,000 and 15,000 yr ago. Following the St. Croix-Hancock phase, the general wasting of the ice was interrupted by stillstands or readvances of the ice margin during the intermediate and Mountain phases. The ice then wasted into the Superior basin before readvancing, over a landscape containing ice masses from earlier phases, during the Winegar-early Athelstane phase. This phase is believed to have occurred just prior to growth of the Two Creeks Forest in eastern Wisconsin. The ice then wasted back and advanced three more times during the Marenisco-late Athelstane, Porcupine (about 11,000 yr B.P.), and Marquette (about 9900 yr B.P.) phases.

Early Proterozoic Geology of the Great Lakes Region

Interest in the Precambrian geology of the Great Lakes Region has been growing as documented by a new generation of multi‐authored books. Two have been published recently: the volume under review and a companion volume, Geology and Tectonics of the Lake Superior Basin, Mem. 156. The Minnesota Geological Survey has published a regional Precambrian geological map at a scale of 1:1,000,000, and in 2 years' time newer data will appear in a DNAG (Decade of North American Geology) volume on the Precambrian of the conterminous United States. Thus, the papers in the volume under review provide a comprehensive view of the status of geological knowledge as it existed in the spring of 1981.

Mid-Continent Rift System--A Frontier Hydrocarbon Province: ABSTRACT

Geophysical evidence in the Mid-Continent has led to delineation of a rift system active during the Proterozoic Y Era. The Mid-Continent rift system can be traced by the Mid-Continent gravity high and corresponding aeromagnetic anomaly signature from the surface exposure of the Keweenawan Supergroup in the Lake Superior basin southwest in the subsurface through Wisconsin, Minnesota, Iowa, Nebraska, and Kansas. The Mid-Continent gravity high, which includes the highest gravity values within the continental United States, has been interpreted historically as a reflection of mafic extrusives, structurally high to the surrounding basement complexes. However, studies of analogous continental rifts and current rift theory call for existence of an anomalous mantle-derived body within the crust. The presence of this anomalous dense body is expressed by the narrow gravity high. The gravity high is imposed upon a broader gravity low which reflects the crust-mantle boundary's isostatic response to injection of mafic mantle material into the crust. Seismic refraction studies support this with findings of an unusually thick crust in the Lake Superior region. The surface response to the crustal attenuati n and subsidence due to mafic loading during the late Proterozoic was the development of a deep rift valley and sedimentary basin. The aeromagnetic anomaly signature of the rift trend discloses where these sediments have been preserved. Thick accumulations of upper Proterozoic sediments are indicated by both upward continuation of the aeromagnetic profiles across the rift trend and gravity models which incorporate: (1) a deep mafic body to create the narrow gravity high, (2) anomalously thick crust to account for the more regional gravity low, and (3) sedimentary accumulations on the Precambrian surface to explain the small-scale notches which occur within the narrow gravity high. Reflection seismic data are virtually unknown in the rift area; however, data recently acquired by COCORP across the southern end of the feature in Kansas provide evidence of thick stratified sequences in the rift valley. Studies of the East African rift have revealed that the tropical rift valley is an exceptionally fertile environment for deposition and preservation of kerogenous material. Although penetrations of the Keweenaw rift sediments are extremely scarce, the occurrence of indigenous mobile crude oil in the White Pine mine on the Keweenaw Peninsula, Michigan, and bituminous partings in a core at the periphery of the Twin Cities basin, Minnesota, strongly support the extrapolation of a rich algal-fungal community throughout the sediment-filled valley of the Mid-Continent rift. Rift valley basins areally represent only 5% of the world's basins, but they have been determined to contain 10% of the world's present reserves. The Sirte, Suez, Viking, Dnieper-Donetz, and Tsaidam basins are just a few of the rift basins currently classed as giant producers. The existence of a rift basin trend with thick accumulations of preserved sediments, demonstrably organic rich, introduces the northern Mid-Continent United States as a new frontier for hydrocarbon exploration. End_of_Article - Last_Page 1325------------

Origin, Distribution, and Postglacial Dispersal of a Swimbladder Nematode, Cystidicola stigmatura

New distributional records indicate that Cystidicola stigmatura occurs in lake trout (Salvelinus namaycush) and Arctic char (S. alpinus) in the Great Lakes and St. Lawrence River drainage basins, in lakes along an arc extending from northwestern Ontario to Great Bear Lake and into the Canadian Arctic Archipelago but not elsewhere. This distribution can be explained by the parasite's probable postglacial history. I hypothesize that the nematode survived in only one glacial refugium. During deglaciation the parasite probably dispersed with lake trout from its refugium in the upper Mississippi River region into the Lake Michigan basin. Confluence of glacial waters would have given the nematode access to the drainage basins of Lakes Huron, Erie, and Ontario. Invasion of the St. Lawrence River basin occurred during a postglacial connection with the Lake Ontario basin. Dispersal from the Lake Huron basin was via glacial Lake Algonquin and later via glacial Lake Barlow–Ojibway and gave rise to present populations in eastern Ontario and western Quebec. Northward dispersal was probably via the glacial waters of the Lake Michigan–Lake Superior basin through glacial Lakes Agassiz, McConnell, and Coppermine to the Arctic Ocean. Arctic char would have first acquired the parasite at this time. Extant parasite populations along this dispersal route occur in Minnesota, northwestern Ontario, Manitoba, Saskatchewan, and the Northwest Territories. Invasion of the Arctic Archipelago has been with anadromous fishes. The apparently restricted preglacial distribution of the nematode and recent history of its intermediate host (Mysis relicta) in freshwater suggests that C. stigmatura may have arisen as a species during the last glaciation.