jol Canadian Landform Examples - 40 Red River, Red River Valley, Manitoba
C.R. BROOKS Geological Survey of Canada, 601 Booth Street, Ottawa, ON, K I A OE8 (e-mail:
[email protected])
E. NIELSEN Manitoba Geological Survey, 360-1395 Ellice Avenue, Winnipeg, MB, R3G 3P2 (e-mail:
[email protected])
Introduction Extreme floods of the Red River inundate a broad area of the Red River Valley, Manitoba, threatening the City of Winnipeg, numerous small towns, and dispersed farmsteads. The flood characteristics are unique in Canada and are strongly influenced by the landscape of the Red River Valley. The purpose of this paper is to review the geomorphology of the Red River and the Red River Valley to provide a context for the flood hazard.
Red River The Red River is 880 km in length, from the confluence of the Bois de Sioux and Otter Tail rivers in southern North Dakota/Minnesota to Lake Winnipeg, Manitoba, in the north (Clark 1950; Figure 1). The area of the watershed is 290 000 km2, including the Assiniboine River basin (163 000 km‘) which joins the Red River at the ’Forks’in Winnipeg. Only about 16 percent of the Red River basin, above the confluence with the Assiniboine basin, is located in Canada (Figure 1). At Emerson, Manitoba (Figure 11, the mean annual discharge of the Red River is 98 m3s-’and the lowest and highest mean monthly flows are 22 m3s-’ (February) and 362 m’s-’ (April), respectively (Water Survey of Canada data, 1912-1995). The annual hydrograph typically consists of a freshet flow in April/May arising from snowmelt runoff. Secondary peaks generated by rainfall can occur in June through October. The measured flood of record at Emerson
occurred on April 27/28 1997, peaking at 3740 m’s’ (data from Manitoba Water Resources). However, higher ungauged historic flows are known to have occurred in 1852 and 1826 with estimated peak discharges of 3770 and 5350 m3s’, respectively (data from Manitoba Water Resources). Within Manitoba, the Red River is a single-channeled, meandering river with a sinuosity of 1.1 to 2.3 and an average valley gradient of 0.0001. The suspended sediment load of the river consists of over 90 percent silt and clay, regardless of sediment concentration and river discharge (Glavic et al. 1988). The annual suspended sediment load at Emerson varies between 0.4 and 1.5 M tonnes of which on average 51 percent is transported over a 37 day period (i.e., 10% of the year) beginning in March or April (1978-1986 data; Glavic et a/. 1988). Modern sediments in the river banks and on the floodplain are composed predominately of silt and clay, reflecting the composition of the suspended sediment load.
Red River Valley The Red River flows northward along the very flat Red River Valley where natural topographic variations, aside from incised stream courses and gullies, are subtle (Figure 2). Upham (1895) described the valley as “being a vast plain. .. 40 to 50 mi (64 to 80 kml wide and more than 300 mi long [the distance is about 530 km], stretching from Lake Traverse to Lake Winnipeg” (p. 20). Within Manitoba, the Manitoba Escarpment forms the western edge of the valley, but to the east the margin is much less distinct.
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Figure 1 Map of the Red River and the central ponion of the Red River Valley, Manitoba, depicting the limits of the 1997 flood and places and features mentioned in the text. The inset map shows the Red and Assiniboine drainage basins.
Despite a name that suggests a direct link to the modern river, the Red River Valley predates the establishment of the river. The basic form and slope of the valley were shaped by the gradual erosion of Mesozoic bedrock during the Tertiary and Quaternary Periods (Teller and Bluemle 1983). This erosion exposed underlying and more resistant ~~~
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Figure 2 Oblique aerial photograph of the Red River Valley, between Emerson and Morris; the Red River is situated in the background flowing from right to left. Note the very flat topography o f the landscape. Both farmsteads in the foreground are situated within the flood-prone zone of the Red River Valley. (GSC Photograph 2000-040)
Paleozoic bedrock, creating a lowland surface between the Manitoba Escarpment to the west and the Precambrian shield to the east. The bedrock surface of the lowland has moderate relief and a gradual northward slope. Along the central Red River Valley, the bedrock is buried beneath late Pleistocene glacial sediments that, in turn, are capped with a clay-rich deposit of glaciolacustrine sediments that aggraded within glacial Lake Agassiz, a large lake that occupied m6st of Manitoba, the northern part of North Dakota and Minnesota, northwestern Ontario and east-central Saskatchewan during the late Pleistocene and early Holocene (Teller and Bluemle 1983; Teller and Clayton 1983). The surficial deposits vary in thick-
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Distance upstream of Lake Winnipeg (km) Figure 3 Longitudinal profile of the Red River between Emerson at the Canada/USA border and Lake Winnipeg. Shown are the profiles of the river at low flow and during the 1950 flood as well as the level of the prairie surface. The vertical exaggerationof the profile is 1 OOOX. (Data from Red River Basin Investigation (1 951) and Red River Basin Investigation (1 953))
ness from about 5 to 70 m along the route traversed by the river (Teller 1976; G. Matile, pers. comm., 2000) and create the flat, gently northward sloping plain of the present day Red River Valley (Figures 2 and 3). Hereafter, this plain is referred to as the 'prairie surface'. Subtle lineations, up to 150 m wide and 10 km long, commonly mark the prairie surface. Difficult to distinguish on the ground, but readily visible on aerial photographs, these features are interpreted to be scours from icebergs grounding on the bed of glacial Lake Agassiz (Teller eta/. 1996). Grassland soils of the chernozemic order developed on the prairie surface following the recession of glacial Lake Agassiz, imparting a characteristic black colour to the ground. Numerous wetlands were present prior to the introduction of European agriculture (Warkentin and Ruggles 1970) because of the combination of the flat topography of the prairie surface and the low permeability of the clay-rich glaciolacustrine deposits. Most of the wetland areas have been drained artificially beginning in the late lgthcentury and are now under cultivation.
Stream-Cut Valley In southern Manitoba, the Red River developed on the bed of glacial Lake Agassiz between 8.2 and 7.8 ka BP as the lake waned and receded northward (Teller et al. 1996). The river eroded a shallow valley into the prairie surface, up to 15 m deep and 2500 m wide. The shallowness of the stream-cut valley origiThe Canadian Geographer / Le Geographe canadien 44, no 3 (2000)
nates from the flat topography and low northward gradient of the prairie surface between Grand Forks, North Dakota, and Lake Winnipeg. A slight drop in elevation along the river profile occurs just north of Winnipeg at Lister Rapids, where the river flows across an outcrop of resistant carbonate bedrock that controls the baselevel of the river upstream (Figure 3; Baracos and Kingerski 1998).The decrease in bed elevation across Lister Rapids is only about 7 m over 15 km (between km 40 and 5 5 on Figure 31, but it is significant considering the gentle valley gradient of the river. Aerial photographs and maps reveal that lateral migration of the river meanders has been low to negligible over the iast 130 years. The meanders, however, have migrated laterally in the past as revealed by a ridge and swale topography present on the floodplain along the broader reaches of the streamcut valley. This topography typically is the product of a single phase of lateral channel migration. In contrast, a mosaic of ridge and swale patterns representing multiple phases of channel migration and abandonment is commonly present along rivers experiencing higher rates of channel migration.
Flooding Since European settlement in the early lgthcentury, Red River floods have impacted the numerous communities in the Red River Valley (Bumsted 1997; Rannie, 1998). Major Red River floods are unusual in comparison to other Canadian rivers because flows spread across the flat prairie surface forming a long, broad flood zone. During the flood of 1997, for example, an area of about 2000 km2and up to 40 km wide was inundated between the Canada/USA border and the southern boundary of Winnipeg (Figure 1; data from Manitoba Natural Resources). In contrast, the flood zones of most other rivers, even underfit rivers occupying large meltwater spillways, are comparatively narrow because the floodwaters remain confined within a valley. Although the inundated zone of the Red River resembles a shallow, ephemeral lake in plan (Figure 11, it is a flow that advances slowly along the gentlysloped plain of the prairie surface. During the 1997 flood, the flow peaked on April 27/28 at Emerson and 5 days later on May 3/4 at the Floodway inlet, 90 km downstream. Because of the slow movement of the flood wave, major Red River floods rise and fall slowly over a period of several weeks.
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Figure 4 A flood protection infrastructure in the Red River Valley; a ring-dike surrounding Morris creates an island within the flood zone during the Red River flood of 1997 (photograph taken May 9, 1997). The Red River is in the foreground flowing from left to right. (GSC Photograph 2000-039)
The flooding characteristics of the Red River arise because the stream-cut valley has insufficient capacity to contain extreme flows. This low capacity is the result of the shallowness of the valley and the low valley gradient. The problem is further compounded because the area adjacent to the river is a broad, flat plain, which allows overtopping floodwaters to spread laterally for many kilometres on either side of the river valley. The prairie surface thus functions as the hydraulic floodplain of the Red River although it is not a genetic floodplain formed by the river. The lateral extent of the flood zone is controlled by minor topographic variations in the prairie surface. For example, the 1997 flood zone is broadest between Morris and the West dike where a low area in the prairie surface extends towards the northwest following the course of the Morris River (Figure 1). Prior to the construction of flood protection infrastructure around and within Winnipeg (see below),
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the area of the prairie surface prone to flooding narrowed gradually northwards until the flood waters were fully confined within the stream-cut valley. This confinement was caused by a gentle rise in the general level of the prairie surface beginning just north (i.e., downstream) of the Assiniboine confluence, and the steepening and shallowing of the water surface in response to the increase in gradient across Lister Rapids (Figure 3). Both factors increase the capacity of the stream-cut valley by increasing the relative height of the valley sides above the water surface so that extreme flows do not overtop the valley. This change in channel capacity is readily apparent on maps and satellite images of the 1997 flood which show a broad flood zone south (upstream) of the Floodway inlet, but an area of very narrow flooding north of the Floodway outlet where the river again carries the full flood discharge (Figure 1).
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3 10 Canadian Landform Examples
Flood Protection Infrastructure Efforts to mitigate the flood hazard along the Red River Valley have resulted in the construction of an array of flood protection infrastructure that represent artificial landforms of varying scales in the landscape. By far the largest of these is the Red River Floodway, an excavated channel about 48 km long, 210-305 m wide, and an average of 9.1 m deep, that diverts up to 1700 m3s-' (design discharge) of flow around Winnipeg (Figure 1; Mudry et a/. 1981). Completed in 1968, the Floodway system regulates the flow of the Red River through Winnipeg during floods and is intended to limit river stage to a level 0.6 m (2 ft) below the top of the primary dikes in the city. Excess water is diverted into the Floodway channel by raising a pair of submerged gates at a control structure situated immediately downstream of the entrance. Since it became operational, the Floodway system has been an unqualified success at reducing flood impacts within the City of Winnipeg (Mudry et al. 1981). During the 1997 flood, the Floodway successfully carried up to 2110 m3s1 (data from Manitoba Water Resources), which is in excess of the design capacity, and thus prevented the flooding and evacuation of about a third of Winnipeg. The Portage Division, located at Portage La Prairie, 84 km west of Winnipeg, is an excavated channel, 29 km long and 53 to 366 m wide (average 183 m), that can divert up to 700 m3s-'of flow (design capacity) from the Assiniboine River into Lake Manitoba (Mudry et a/. 1981). It is intended to substantially reduce flow from the Assiniboine drainage basin into Winnipeg during periods of elevated Red River discharge. Assiniboine flow is regulated by a dam built across the river and an inlet control structure at the mouth of the Diversion Channel. During the 1997 flood, up to 538 m35' of Assiniboine flow was diverted through the Portage Diversion into Lake Manitoba (data from Manitoba Water Resources). There are extensive diking networks within the Red River Valley that form physical barriers to flood waters. These include about 120 km of primary dikes along the river within Winnipeg, as well as the 32 km long West dike that extends west and south from the Floodway inlet (Figure 1). Large ring dikes surround a number of towns in the Red River Valley south of Winnipeg (Figure 4) and small-scale ring dikes protect buildings dispersed within the flood prone area. Earthen mounds are also used to raise the level of ground upon which a building can be constructed. ~~~
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The 'design level' of dikes and mounds upstream of the Floodway (i.e., south of Winnipeg) is now the local peak elevation of the 1997 flood plus 0.6 m (2 ft) as a factor of safety. Near many small-scale ring dikes and mounds are dug-out pits that were excavation sites for earthen materials used in dike and mound construction. Dug-outs commonly contain water and represent small perennial ponds in the Red River Valley landscape. Elevated road and railroad beds, up to 1-2 m high, form a grid network across the Red River Valley. These raised, linear features represent significant rises in topography and can function as dikes that influence the overland flow of modern floods across the prairie surface. However, they can create separate zones of flow between which significant hydraulic heads can form where the capacity of ditch culverts is exceeded. Where such a head exists, water flowing across an elevated road or railway bed can cause washouts at locations that would not have experienced any erosion in the flat natural landscape.
Summary The flat topography of the Red River Valley reflects the morphology of underlying bedrock covered with late Quaternary glacial and glaciolacustrine sediments. The meandering Red River occupies a shallow stream-cut valley that became incised into the Red River Valley plain following the final recession of glacial Lake Agassiz. Extreme flows carried by the river overtop the sides of the stream-cut valley and spread laterally up to several tens of kilometres across the plain. An array of flood protection infrastructure has been constructed in the landscape to mitigate the flood hazard. Acknowledgements Comments by R. Kostaschuk, W.F. Rannie and H.L. Thorleifson are sincerely appreciated.Funding was provided in part from the International Red River Basin Task Force to the International Joint Commission and the Red River Flood Protection Program.
References and KINCWKI, D. 1998 'Geological and geotechnical engineering for urban development of Winnipeg, Manitoba', in Urban Geology of Canadian Cities ed. by P. F. Karrow, and O.L. White, Geological Association of Canada Special Paper 42, 171-190. BUMW J. M. 1997 Hoods of the Centuries: a History of flood Disasters in the Red River Valley 1776-1997(Winnipeg:Great Plains Publications) CLARKR. n. 1950Notes on Red River Hoods with Particular Reference to the BARACOS, A.,
Canadian Landform ExamDles 3 1 1 Flood of 1950 (Winnipeg: Province of Manitoba, Department of Mines and Natural Resources) CVWIC. H., DAY, T.J., and WNK. T. R. 1988 'Interpretation of suspended sediment characteristics, Red River at Emerson Manitoba, 1978-1986' Environment Canada: Sediment Survey Section, report IWD-HQ-WRBSS-88-3,1-66. MUORY, N., REYNOLDS,P.J.,and ROSENBEXG, H.B. 1981 'Post-project evaluation of the Red and Assiniboine River flood control projects in the Province of Manitoba, Canada' in Methods of Post-Project Evaluation: Achievements and Remedial Measures Special Session R. 9. (Grenoble, Switzerland: International Commission on Irrigation and Drainage) 147.178. RANNIE. W.F. 1998 'A survey of hydroclimate. flooding, and runoff in the Red River basin prior to 1870 Geological Survey of Canada Open File 3705 RED RIVER BASIN INVEmGAnON 1951 'Red River hydrographic survey, mile 0 to 153.9 Department of Resources and Development, 41 map sheets. _. 1953 Report on Investigations into Measures for the Reduction of the Flood Hazard in the Greater Winnipeg Area: Appendix B - History of
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Floods on the Red River (Winnipeg: Department of Resources and Development) TULER, J.T. 1976 'Total thickness of clay, silt, sand, gravel and till in southern Manitoba' Manitoba Mineral Resources Division, Surficial Map 761. TELLFR. J.T., and BLUEMLE, J.P. 1983 'Geological setting of the Lake Agassiz region' in Glacial Lake Agassiz Special Paper 26 ed. J.T.Teller and L. Clayton (GeologicalAssociation of Canada). 5-20, TELLFR, J.T., and CLAYTON, L. 1983 'Glacial Lake Agassiz' Geological Association of Canada Special Paper 26 TELLFR,J.T.,THORLUFSON, L.H., mnu.G., and BRISBIN, W.C. 1996 'Sedimentology, geomorphology and history of the central Lake Agassiz basin (Field Trip 82)' Geological Association of Canada/Mineralogical Association of Canada, Winnipeg '96, May 27-29 UPHAM. w. 1895 The Glacial Lake Agassiz' United States Geological Survey Monograph 25 WARKENTIN,J., and RUGGLES, R. L. 1970 Historical Atlas of Manitoba: a Selection of Facsimile Maps. Plans and Sketches from 1612 to 1969 (Winnipeg, The Historical and Scientific Society of Manitoba)
