Assiniboine River Research Articles

Two new floods, climate change, and ethics

Two new floods, climate change, and ethics

Observed trends and future projections of precipitation and air temperature in the Lake Winnipeg watershed

While the cause of eutrophication in Lake Winnipeg is known to be excess nutrient loading from watershed sources, few comprehensive studies have attempted to quantify the effects of changes in temperature and precipitation on the hydrological and nutrient transport regimes in the region. In this paper, results of spatial and temporal analyses of observed and regional climate model (RCM) projections of precipitation and air-temperature regimes within the Lake Winnipeg watershed (LWW) are presented. The Mann–Kendall trend analysis of observed data over a recent time period of 1961 to 2003 shows that, while there has been a consistent increasing trend in the mean annual air temperature in the region, there is no significant trend in annual precipitation. Seasonally, precipitation exhibited a significantly decreasing trend in the Saskatchewan and Assiniboine river basins during the winter and an increasing trend in part of the Red and Assiniboine river basins during summer season. Air temperatures showed significant increases in winter, spring and summer throughout the LWW. Future climate projections from three RCMs driven by the SRES's A2 emission scenario indicate that by the 2050s (2041–2070), total annual precipitation (air temperature) increases by 5.5–7.7% (2.1–2.8°C) compared to the 1980s (1971–2000). These observed and projected changes in the climate regimes of the LWW will have important implications for the hydrology and nutrient transport regimes of this region, which is the subject of additional investigation in a companion paper of this special issue (Shrestha et al., 2012).

A Reverse Auction for Wetland Restoration in the Assiniboine River Watershed, Saskatchewan

Wetland habitat continues to be lost in many watersheds across Canada and new program tools are needed to help restore drained wetlands. We used a reverse auction to restore drained wetlands in the Assiniboine River Watershed (ARW) of east‐central Saskatchewan which is an important target area for wetland restoration in the North American Waterfowl Management Plan (NAWMP). The reverse auction format was discriminative, with sealed bids and two rounds of bidding. Bidders could submit bids for 12‐year term agreements and/or perpetual conservation easements, and bids were submitted by quarter section (160 acres). Bids could be either in cultivated cropland or perennial forage, and were evaluated using an environmental benefits index based on the incremental increase in predicted hatched waterfowl nests relative to bid price. Potential bidders were solicited via contacts with existing conservation project cooperators, and a public media campaign. In the first round, 20 bidders submitted 118 bids to restore 713 wetlands totaling 670 acres at a price of $837,000. All bids were for 12‐year term agreements. Bid prices to restore drained wetlands within cultivated land were higher than for perennial forage. In the second round, 30 bids from seven bidders were approved to restore 211 wetlands totaling 211 acres in perennial forage at a price of $182,000. The price of successful bids varied from $20.83 to $391.22 per acre per year (average $118.52). The reverse auction provided information on cost variability and funding required for achieving NAWMP wetland restoration objectives in the ARW. La perte d'habitats humides dans de nombreux bassins hydrographiques du Canada se poursuit, d'où la nécessité d'élaborer de nouveaux programmes et outils pour favoriser la restauration des milieux humides qui ont été drainés. Nous avons procédéà une enchère inversée en ligne pour restaurer les milieux humides drainés dans le bassin hydrographique de la rivière Assiniboine dans la partie du centre‐est de la Saskatchewan, qui constitue une zone cible importante de la restauration des milieux humides dans le Plan nord‐américain de la gestion de la sauvagine (PNAGS). L'enchère inversée en ligne était discriminatoire et comprenait des offres secrètes et deux tours d'enchère. Les enchérisseurs pouvaient soumettre des offres soit pour des ententes d'une durée de douze ans, soit pour des servitudes de conservation perpétuelles, pour une superficie de un quart de section (160 acres) dans les deux cas. Les offres, qui pouvaient comprendre des terres cultivées ou des terres couvertes de plantes fourragères vivaces, ont étéévaluées à l'aide d'un indice des avantages environnementaux fondé sur l'augmentation progressive du nombre prévu d'œufs éclos dans les nids de sauvagine par rapport au prix offert. Nous avons joint des enchérisseurs potentiels par l'intermédiaire des coordinateurs de projets de conservation existants et de campagnes de publicité‐médias. Au premier tour, 20 enchérisseurs ont soumis 118 offres pour restaurer 713 milieux humides totalisant 670 acres, au prix de 837 000$. Toutes les offres visaient des ententes de douze ans. Le prix des offres pour restaurer les milieux humides drainés à l'intérieur des terres cultivées était plus élevé que le prix des offres pour restaurer les milieux humides drainés à l'intérieur des terres couvertes de plantes fourragères vivaces. Au deuxième tour, trente offres soumises par sept enchérisseurs ont été retenues pour restaurer 211 milieux humides totalisant 211 acres de terres couvertes de plantes fourragères vivaces, au prix de 182 000$. Le prix des offres retenues variait de 20,83$à 391,22$ l'acre par année (moyenne de 118,52$). L'enchère inversée en ligne a permis d'obtenir de l'information sur la variabilité des coûts et le financement nécessaire pour atteindre les objectifs de restauration des milieux humides du PNAGS dans le bassin hydrographique de la rivière Assiniboine.

Natural and unnatural complexities: flood control along Manitoba's Assiniboine River

Flooding and flood control long have been topics of concern for geographers and historians of North America, from Gilbert White's foundational work to attempts to understand the recent disaster in New Orleans. This paper considers the problem of flood control along the Assiniboine River in the province of Manitoba, Canada. The first section addresses the local landscape and the larger context, explaining changes in the relevant human and physical geography. The second section examines efforts to mitigate flooding, focusing on how the risk of inundation was shifted in location (not eliminated altogether) and compounded through time (not progressively resolved). The third focuses on a further source of complexity: the contested character of government jurisdiction. This final section also addresses major mid twentieth century changes in mitigation efforts: in the light of flooding on an unprecedented scale, governments became more open to compromise and more willing to undertake substantial projects. Gilbert White asserted that while floods are natural occurrences, flood damages are the responsibility of humanity. The question of responsibility might seem relatively clear in relation to flood mitigation, given that dykes are human constructions. The history of flood mitigation along the Assiniboine would suggest, however, that assigning responsibility for the failures and successes of flood mitigation is not so straightforward.

Intercomparison of the total storage deficit index (TSDI) over two Canadian Prairie catchments

Summary Retrieval of the terrestrial moisture storage dataset from the Gravity Recovery and Climate Experiment (GRACE) satellite remote sensing system is possible when the catchment of interest is of large spatial scale. These dataset are of paramount importance for the estimation of the total storage deficit index (TSDI), which enables the characterization of a particular drought event from the perspective of the terrestrial moisture storage over that catchment. Incidentally, the GRACE gravity signal over the 13,000 km 2 Upper Assiniboine River Basin on the drought-prone Canadian Prairie is so poor therefore making the computation of the total storage deficit index for this basin infeasible. Consequently, the estimation of the terrestrial moisture storage from other reliable sources becomes imperative in order to enable the computation of the TSDI over this basin. This study explores the utilization of the Variable Infiltration Capacity (VIC) model, a physically based, spatially distributed hydrologic model to simulate the total moisture storage over the Upper Assiniboine River Basin which was then employed in the estimation of the TSDI over this basin for subsequent characterization of the recent Prairie-wide drought. Interestingly, the temporal patterns in the computed TSDI from the VIC model reveal a strong resemblance with the same drought characterization undertaken over the larger adjacent Saskatchewan River Basin, which was accomplished utilizing terrestrial moisture storage from the GRACE-based approach. Additionally, these independent techniques employed in the characterization of the last Prairie drought over the two adjacently situated basins resulted in similar drought severity classification from the standpoint of the total moisture storage deficits over these basins. This study has therefore shown that in the computation of the total storage deficit index over small-scale catchments during anomalous climatic conditions that propagate extreme dryness through the terrestrial hydrologic systems, simulations of the total water storage from a structurally sound model such as the VIC model could be resourceful for the computation of the monthly total storage deficit index if no constraint is placed on the availability of accurate meteorological forcing.

Variation in CanadianGomphus fraternus(Odonata: Gomphidae) in relation to the recognition of subspeciesmanitobanus

AbstractSeven characters were evaluated in 146 specimens ofGomphus fraternus(Say), including both subspecies,G. f. fraternus(Say) andG. f. manitobanusWalker, to clarify their circumscription and geographic occurrence in Canada. Specimens corresponding to subspeciesmanitobanuswere all from the Assiniboine and Red rivers and formed a discrete group in a principal components analysis, supporting their taxonomic recognition. Their distribution in Canada suggests a separation of subspecies near the boundary of the prairie ecozone in central southern Manitoba. In subspeciesfraternusthe extensor surfaces of the tibiae are largely black, with pale areas often lacking and rarely up to 50% of the segment length. Similarly the dorsal surface of abdominal segment 9 is largely black. If there is a pale area, it is <1 mm long. In subspeciesmanitobanusthe extensor surfaces of the tibiae have pale markings on ≥50% of their lengths and the dorsal surface of abdominal segment 9 has a dorsal yellow spot ≥1.4 mm long. Widespread dragonflies in western North America are paler in drier climates and the increased pale colouration in the western subspeciesmanitobanuscorresponds to this trend. Currently known only from Manitoba, subspeciesmanitobanusmay occur in prairie habitats of the Midwestern United States, based on reports of pale individuals in that region.

Fluvial Sedimentology and Paleoecology of Holocene Alluvial Deposits, Red River, Manitoba

Stratigraphie and paleoecological analyses at five sections, together with age determinations based on 19 previously published and 21 new radiocarbon dates, provide a detailed late Holocene history of the Red River, Manitoba. Ecological information, such as age frequency analysis, relative abundance, diversity and association of species was drawn from 19 mollusc species. These data indicate that the Red and Assiniboine rivers cut the valleys they occupy today within a thousand years of the regression of Lake Agassiz. In the south, up to 14 m of alluvium has accumulated during the last 7000 years. A decrease in the sedimentation rate at 1400 BP is coincident with the shift in the position of the Assiniboine from the valley of the La Salle River to its present position. Overbank sedimentation did not start in the northern part of the area until ca. 5200 BP. Initial rapid sedimentation rates in this area are attributed to increased precipitation and a brief eastward excursion of the Assiniboine River into the Red. In spite of increased precipitation, flood frequencies remained low in the north until 1400 BP. Increased overbank sedimentation after 1400 BP is attributed to the northward shift in the position ot the Assiniboine.

Population Demographics of Hiodon tergisus (Mooneye) in the Lower Tallapoosa River

We describe age structure, growth, and fecundity of Hiodon tergisus (Mooneye) from the lower Tallapoosa River, AL. Mooneye (N = 49, 214–316 mm total length, 79–284 g) were aged using otoliths, and a von Bertalanffy growth model was derived for the species (L∞ = 316, K = 0.285, to = −0.7). Growth rates of Mooneye differed between the Tallapoosa River population and a previously studied population from the northern extent of the species' range (Assiniboine River, MB, Canada). In addition, fecundity of Mooneye from the Tallapoosa River was similar to the northern population, ranging from 5321 to 7432 eggs per female. Because the species is declining throughout its range in Alabama, we recommend that managers use our findings in conservation efforts. Future studies should investigate how hydrology influences the spawning success and early growth and development of Mooneye in regulated systems. More information about this species is needed regarding their early life history, including early growth, survival, and habitat use.

Paleolimnology of Lake Manitoba: the Lithostratigraphic Evidence

Lake Manitoba, the largest lake in the prairie region of North America, is one of the most intensively studied lacustrine basins in western Canada. New AMS14C dating, together with mineralogical, geochemical, and lithostratigraphic analyses of the 14-m-thick, offshore sediment sequence, document a complex Holocene history in which water levels and limnological conditions were controlled by the interplay of changing climate, variable river and groundwater inflow, and differential isostatic rebound. Varves, ice-rafted debris, and clast-rich laminated sediment record deposition in the lake when the basin was part of proglacial Lake Agassiz. As Agassiz retreated northward, Lake Manitoba became isolated by about 850014C yrs BP, and for the next 800 years was characterized by mainly shallow water to dry conditions. Deeper and more stable water conditions returned to the Lake Manitoba basin by 770014C yrs BP probably due to the damming effect of differential isostatic rebound and decreasing aridity. For the next 3000 years, relatively stable lake levels and water compositions were maintained, reflecting a delicate balance between differential isostatic rebound, climate, inflow of the Assiniboine River, and groundwater contribution. At ~450014C yrs BP the Assiniboine River was rerouted to its present easterly path, by-passing Lake Manitoba, and resulting in loss of a significant component of the lake’s hydrologic budget. Water levels dropped and the offshore sediments were once again subaerially exposed. By 370014C yrs BP, a cooler and wetter climate, together with continued southward transgression of water, compensated for the loss of fluvial input, resulting in reflooding of the basin. By about 2000 years ago the lake had evolved from a shallow, saline, and alkaline pool to its present depth and extent.

Flood Ring Evidence and its Application to Paleoflood Hydrology of the Red River and Assiniboine River in Manitoba*

Although paleoflood records developed from tree rings are much shorter than those developed from geological evidence, their brevity is offset by their exceptional utility for dating floods to a specific year. Inundation at the beginning of the growing season disturbs cambial processes in riparian Quercus spp., causing unusual anatomical features to develop within the annual ring, including small earlywood vessels, disrupted flame parenchyma and less wood fiber. These features are most strongly developed near the tree base, and may be caused by disruptions of auxin flow. Anatomical flood signatures can be used to determine the frequency, magnitude and hydrological causes of past floods, and to identify the influence of potential forcing mechanisms. In the lower Red River basin, Canada, flood-ring evidence has been used to identify several large floods during the mid 1700s, the early to mid 1800s and the latter half of the 20th century. Records for the Assiniboine River and the American portion of the Red River are developed from fewer trees, but suggest that severe floods in the Red and Assiniboine basins have coincided, albeit infrequently, during the past 500 years.

Sensitivity of the Red River Basin Flood Protection System to Climate Variability and Change

An original modeling framework for assessment of climate variation and change impacts on the performance of complex flood protection system has been implemented in the evaluation of the impact of climate variability and change on the reliability, vulnerability and resiliency of the Red River Basin flood protection system (Manitoba, Canada). The modeling framework allows for an evaluation of different climate change scenarios generated by the global climate models. Temperature and precipitation are used as the main factors affecting flood flow generation. System dynamics modeling approach proved to be of great value in the development of system performance assessment model. The most important impact of climate variability and change on hydrologic processes is reflected in the change of flood patterns: flood starting time, peak value and timing. The results show increase in the annual precipitation and the annual streamflow volume in the Red River basin under the future climate change scenarios. Most of the floods generated using three different climate models had an earlier starting time and peak time. The assessment of the performance of Red River flood protection system is based on the flood flows, the capacity of flood control structures and failure flow levels at different locations in the basin. In the Assiniboine River Basin, higher reliabilities at downstream locations are obtained indicating that Shellmouth reservoir plays an important role in reducing downstream flooding. However, a different trend was identified in the Red River Basin. The study results show that flood protection capacity of the Red River infrastructure is sufficient under low reliability criteria but may not be sufficient under high reliability criteria.

Impacts of land use on riparian forest along an urban – rural gradient in southern Manitoba

Extensive landscape modification by humans has led to the fragmentation of riparian forests across North America. We compared the vegetation of extant riparian forest along an urban-rural disturbance gradient. In 1999, twenty-five sites along Assiniboine River in Manitoba, Canada were categorized according to land use: urban, suburban, high intensity rural, low intensity rural, and relatively high quality reference forest. Differences in herbaceous, shrub, and tree species composition and diversity were related to the proportion of surrounding land use, forest patch size, connectivity, and area:perimeter ratio. Urban riparian forests were more disturbed and isolated. They were smaller and characterized by drier, more alkaline soils. Moreover, they had significantly lower native and overall understorey species diversity, and had a higher proportion of exotics including Solanum dulcamara and Hesperis matronalis. Suburban forests were less disturbed, faced greater development pressure, and had sandier soils. Although suburban understorey diversity was similar to that of rural forests, suburban sites had a higher proportion of exotic species, especially escaped horticultural and invasive species including Caragana arborescens and Rhamnus cathartica. Reference sites were relatively large and exhibited greater connectivity, but there was little difference in species composition and diversity among high intensity rural, low intensity rural, and reference sites. These site types were less disturbed than either urban or suburban forests, and reference sites were characterized by hydrophilic species including Scirpus fluviatilis and Carex aquatilis. Our results suggest that landscape measures of disturbance, and related changes in environment, may be confidently used to assess impacts of land use on vegetation along urban-rural gradients.

Spinitectus macrospinosus n. sp. (Nematoda: Cystidicolidae) from the channel catfish Ictalurus punctatus in southern Manitoba and its distribution in other Ictalurus spp.

Spinitectus macrospinosus n. sp., a parasite of the channel catfish Ictalurus punctatus, from the Red and Assiniboine rivers in southern Manitoba, Canada, is characterized by spines reaching a maximum length of 21.8-26.8 microm (means for males and females, respectively) in the anterior esophageal region, arranged in 4 sectors in the anterior region of the body. Spines in this region of the body increase in size and decrease in number (from a maximum of 6 to 3 per sector). The combination of spine length, number, and arrangement and the position of the excretory pore (between spine rows 6 and 7) readily distinguishes it from all its North American congeners, including the species it is most similar to, S. carolini. It is different from S. gracilis and S. acipenseri in having a posteriorly directed vagina, an excretory pore between spine rows 6 and 7, a longer stoma, a right spicule with a terminal ventral barb, and a heart-shaped caudal mucron. Spinitectus macrospinosus is similar to other North American species of Spinitectus, e.g., S. carolini, S. micracanthus, S. mexicanus, S. osorioi, and S. humbertoi, in having the anterior rows of spines arranged in 4 sectors. It is also similar to S. micracanthus and S. carolini in possessing a relatively long stoma, a posteriorly directed vagina, a right spicule with a terminal ventral barb, and a heart-shaped caudal mucron. Furthermore, it is similar to S. carolini in possessing an "area rugosa" with 2 rows of precloacal cuticular cleats. In southern Manitoba, S. macrospinosus appears to mature only in the channel catfish. Reexamination of museum specimens revealed that the nematode also is found in I. furcatus in Kentucky Lake (Kentucky-Tennessee) and in I. lacustris in Lake Texoma (Oklahoma-Texas).

The Causes, Progression and Magnitude of the 1826 Red River Flood in Manitoba

In this paper we review historical and paleoenvironmental evidence related to the 1826 Red River flood in Manitoba. In 1825, significant spring flooding and persistently high water levels in early autumn filled natural storage in the Red and Assiniboine river basins to capacity. Although daily historical records do not suggest that the winter of 1825–26 was exceptionally cold or snowy, other comments note an unusually deep snow pack near the Red River Settlement and throughout the southern basin. Historical accounts and paleoclimatic data document a cold, snowy April and an exceptionally late spring throughout much of central North America. The 1826 flood was exacerbated by abundant rainfall during the rising phase and was, according to historical and dendrochronological evidence, the largest event since at least C.E. 1648. The estimated peak flow for 1826 is approximately 6,370 m3/s (40% greater than the 1997 flood), which exceeds the current design capacity of flood protection for Winnipeg. Historical accounts in western Manitoba and anatomical signatures in alluvial logs suggest that the lower Assiniboine (downstream of Brandon) produced an exceptional flow in 1826, equivalent to perhaps 20% of the Red River’s upstream discharge. Reports of extremely strong, persistent winds from the south coincident with peak stage suggest that wind set-up may be an important factor that should be considered in future hydraulic studies.

System dynamics model for predicting floods from snowmelt in North American prairie watersheds

AbstractThis study uses a system dynamics approach to explore hydrological processes in the geographic locations where the main contribution to flooding is coming from the snowmelt. Temperature is identified as a critical factor that affects watershed hydrological processes. Based on the dynamic processes of the hydrologic cycle occurring in a watershed, the feedback relationships linking the watershed structure, as well as the climate factors, to the streamflow generation were identified prior to the development of a system dynamics model. The model is used to simulate flood patterns generated by snowmelt under temperature change in the spring. Model structure captures a vertical water balance using five tanks representing snow, interception, surface, subsurface and groundwater storage. Calibration and verification results show that temperature change and snowmelt play a key role in flood generation. Results indicate that simulated values match observed data very well. The goodness‐of‐fit between simulated and observed peak flow data is measured using coefficient of efficiency, coefficient of determination and square of the residual mass curve coefficient. For the Assiniboine River all three measures were in the interval between 0·92 and 0·96 and for the Red River between 0·89 and 0·97. The model is capable of capturing the essential dynamics of streamflow formation. Model input requires a set of initial values for all state variables and the time series of daily temperature and precipitation information. Data from the Red River Basin, shared by Canada and the USA, are used in the model development and testing. Copyright © 2002 John Wiley & Sons, Ltd.

Comment on "A Surface Area-Volume Relationship for Prairie Wetlands in the Upper Assiniboine River Basin, Saskatchewan" by L.H. Wiens, Canadian Water Resources Journal 26(4): 503–514

(2002). Comment on 'A Surface Area-Volume Relationship for Prairie Wetlands in the Upper Assiniboine River Basin, Saskatchewan' by L.H. Wiens, Canadian Water Resources Journal 26(4): 503–514. Canadian Water Resources Journal / Revue canadienne des ressources hydriques: Vol. 27, No. 2, pp. 229-231.

A Surface Area-Volume Relationship for Prairie Wetlands in the Upper Assiniboine River Basin, Saskatchewan

The proliferation of geographical information system applications in hydrology and spatially distributed hydrologic models, all using areal input data, has created a need for a simple technique for estimating the storage volumes of wetland areas. Prompted by a hydrologic basin study in support of a Federal/Provincial study of the upper Assiniboine River basin in Saskatchewan, surface area-volume data sets for 177 wetland sites were analysed using regression analysis techniques. A composite relationship was derived, made up of two regression formulae, one for use with wetland areas under 70 hectares and a second for wetland areas between 70 and 500 hectares. A comparison was made between the composite area-volume relationship resulting from this study and similar area-volume relationships derived for two Iowa river basins in 1967. Although the two Iowa regression formulae compare reasonably well with the results of this study for wetland areas under six hectares, volumes derived by the three relationships diverge rapidly for areas greater than six hectares. Further analyses of wetland area-volume relationships using data from other areas of the prairie region could lead to the development of a suite of formulae which could eventually be consolidated and distributed into regional zones of application for use by the water resource community.

System Dynamics Modeling of Reservoir Operations for Flood Management

There exists a strong need to explore simulation techniques that not only represent complex dynamic systems in a realistic way but also allow the involvement of end users in model development to increase their confidence in the modeling process. System dynamics, a feedback-based object-oriented simulation approach, is presented for modeling reservoir operations. The increased speed of model development, the trust developed in the model due to user participation, the possibility of group model development, and the effective communication of model results are main strengths of this approach. The ease of model modification in response to changes in the system and the ability to perform sensitivity analysis make this approach more attractive compared with systems analysis techniques for modeling reservoir operations. The proposed approach is applied to the Shellmouth reservoir on the Assiniboine River in Canada. Operating rules are developed for high flow/flood years to minimize flooding. Alternative operating rules are explored by changing reservoir storage allocation and reservoir outflows. Impacts on the flood management capacity of the reservoir are investigated by simulating a gated spillway in addition to an existing unregulated spillway. Sensitivity analysis is performed on the reservoir levels at the start of the flood season and the outflow from the reservoir.

A variable lacustrine environment in Lake Winnipeg, Manitoba: Evidence from modern thecamoebian distribution

Thecamoebians (testate protozoan) were examined in 18 surface sediment samples from the North and South basins and the Narrows of Lake Winnipeg, Manitoba. Significantly higher numbers of thecamoebians and tintinnids in the North Basin compared to the Narrows and South Basin are attributed to the effects of urban development around the South Basin of Lake Winnipeg. Human population growth in this area has led to increased nutrient concentration in runoff, causing eutrophication of the southern lake, which in turn allows for increased algal productivity. Cucurbitella tricuspis is found in large abundances in the South Basin, particularly close to the inlet of the Red and Assiniboine rivers. High abundances of this species have been attributed to eutrophic conditions, which this species appears to withstand more successfully than other species. Increases in domestic waste output, that have led to elevated heavy metal concentrations in lake bottom sediments of the South Basin, may have resulted in lower abundances of thecamoebians, further reducing competition. Strong currents in the Narrows cause a slightly coarser substrate and sweep away food sources such as phytoplankton. This results in a lower faunal abundance and slightly lower species richness of thecamoebians. Robust species such as the coarse-grained Difflugia viscidula and species which feed on bacteria such as Centropyxis aculeata show increased abundances. Modern thecamoebian assemblages are comparable to Late Holocene faunal associations in terms of species composition. Individual species abundances, however, have changed. For example, in the North Basin the Late Holocene dominance of Difflugia manicata is replaced by various strains of Difflugia oblonga during recent times. A common species of the South Basin from Late Holocene to recent times is Difflugia globulus. It would appear that faunal differences between basins are the result of differences in algal food sources.

Siliceous microfossils and mineral magnetic characteristics in a sediment core from Lake Manitoba, Canada: a remnant of glacial Lake Agassiz

A 14.2 m long core was recovered from the southern Lake Manitoba basin. The sediment, consisting mainly of silty clay, was studied for siliceous microfossil content and mineral magnetics; 14 new accelerator mass spectrometry (AMS) radiocarbon dates provide chronological control of the paleoenvironmental history of the basin. The basal 5 m contains ice-rafted clasts and is largely barren of siliceous microfossils; these sediments were deposited when the lake was part of glacial Lake Agassiz. Sediments immediately overlying the barren part of the sequence contain AMS dates of 7700-7400 BP and reflect a dramatic change in conditions in the basin. Diatom abundances rise abruptly. Magnetic characteristics change substantially. The presence of freshwater taxa such as Stephanodiscus niagarae, together with brackish water diatoms, indicate that shallow, turbid, high-nutrient conditions with variable salinity occurred during the early part of the middle Holocene. Although climatic conditions throughout the northern Great Plains are known to have become drier and warmer during the mid-Holocene, there is a distinct change in diatom taxa in the Lake Manitoba sequence toward less saline conditions at this time. The presence of the riverine diatom Aulacoseira granulata in this interval supports previous conclusions that these freshwater conditions resulted from the northward diversion of the Assiniboine River into the basin. Following this, diatoms indicate an abrupt increase in salinity to >1500 mg·L-1 total dissolved solids between 4000 and 2600 BP, reflecting the diversion of the fresh waters of the Assiniboine River away from Lake Manitoba. Increasingly cooler and wetter conditions during the late Holocene, combined with differential isostatic rebound, caused a freshening of the lake during the late Holocene.