Flood Processes in Canada: Regional and Special Aspects

Canadian Water Resources Journal / Revue canadienne des ressources hydriques

ISSN: 0701-1784 (Print) 1918-1817 (Online) Journal homepage: http://www.tandfonline.com/loi/tcwr20

Flood processes in Canada: Regional and special aspects

James M. Buttle, Diana M. Allen, Daniel Caissie, Bruce Davison, Masaki Hayashi, Daniel L. Peters, John W. Pomeroy, Slobodan Simonovic, André St- Hilaire & Paul H. Whitfield

To cite this article: James M. Buttle, Diana M. Allen, Daniel Caissie, Bruce Davison, Masaki Hayashi, Daniel L. Peters, John W. Pomeroy, Slobodan Simonovic, André St-Hilaire & Paul H. Whitfield (2016): Flood processes in Canada: Regional and special aspects, Canadian Water Resources Journal / Revue canadienne des ressources hydriques, DOI: 10.1080/07011784.2015.1131629

To link to this article: http://dx.doi.org/10.1080/07011784.2015.1131629

Published online: 29 Jan 2016.

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Flood processes in Canada: Regional and special aspects James M. Buttlea*, Diana M. Allenb, Daniel Caissiec, Bruce Davisond, Masaki Hayashie, Daniel L. Petersf, John W. Pomeroyg, Slobodan Simonovich, André St-Hilairei and Paul H. Whitﬁeldb,j aTrent University, Peterborough, Canada; bDepartment of Earth Sciences, Simon Fraser University, Burnaby, Canada; cDepartment of Fisheries and Ocean, Moncton, Canada; dEnvironment Canada, Saskatoon, Canada; eUniversity of Calgary, Calgary, Canada; fEnvironment Canada, Water & Climate Impacts Research Centre, University of Victoria, Victoria, Canada; gCentre for Hydrology, University of Saskatchewan, Saskatoon, Canada; hDepartment of Civil and Environmental Engineering, Institute for Catastrophic Loss Reduction, Western University, London, Canada; iINRS-ete, Québec, Canada; jEnvironment Canada, Vancouver, Canada (Received 13 April 2015; accepted 8 December 2015)

This paper provides an overview of the key processes that generate floods in Canada, and a context for the other papers in this special issue – papers that provide detailed examinations of specific floods and flood-generating processes. The historical context of flooding in Canada is outlined, followed by a summary of regional aspects of floods in Canada and descriptions of the processes that generate floods in these regions, including floods generated by snowmelt, rain-on-snow and rainfall. Some flood processes that are particularly relevant, or which have been less well studied in Canada, are described: groundwater, storm surges, ice-jams and urban flooding. The issue of climate change-related trends in floods in Canada is examined, and suggested research needs regarding flood-generating processes are identified.

Cet article dresse un portrait des principaux processus essentiels à la génération des crues au Canada et conséquemment, donne le ton pour les autres articles inclus dans ce numéro spécial, dans lesquels on traite d’événements spécifiques et des processus qui en font la genèse. Le contexte historique des crues au Canada est résumé sous forme régionale, avec une description des processus spécifiques à chaque région, qui incluent entre autre les crues nivales, celles causées par des précipitations liquides sur couvert de neige et les crues pluviales. Certains processus jugés particulièrement pertinents ou qui ont été moins étudiés au Canada sont décrits : eau souterraine, surcotes associées aux tempêtes, embâcles de glace et les crues en milieu urbain. La problématique des changements climatiques au Canada est aussi examinée et des pistes de recherche liée aux processus causant les crues sont identifiées.

Introduction The spatial ubiquity of flood-generating processes Flooding, the inundation of normally dry areas with also varies for the particular process and size of drainage water, is the most common and costliest natural disaster basin being considered. Much of Canada is seasonally for Canadians (Sandink et al. 2010), and can be gener- snow covered, and these regions experience snowmelt- fl ated by a range of processes. These include snowmelt generated oods often supplemented by rain-on-snow fl runoff, “flash flooding” due to intense rainfall, ice jams events. Such oods are often the maxima in large drai- that develop during ice formation or breakup, failure of nage basins, when the entire basin may contribute water fl natural dams, and coastal flooding from storm surges, to the outlet. Similarly, oods can be generated across hurricanes and tsunamis (Figure 1). Flooding may also most of the country by rainstorms with large depths and/

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 be induced by human activity, including flooding caused or intensities (Figure 1). Thus, convective and frontal by urban development and by failure or abnormal opera- systems can generate large short-duration rainfall intensi- tion of engineered flood-management structures such as ties (Alila 2000) which can occur in all regions (Table 1). fi fl dams and levees. Nevertheless, the signi cance of such storms to ood Flooding can occur at almost any time of the year generation varies across the country, with the greatest somewhere in Canada; however, the relative significance depths and intensities for short-duration events in south- of a specific flood-generating process may vary markedly ern parts of Canada and the smallest in the Arctic. These fl throughout the year. Thus, snowmelt-driven floods are short-duration events are often responsible for ood gen- more frequent in spring and early summer and ice jams eration in relatively small drainage basins, given the are associated with spring breakup of river ice cover, greater chance of high-intensity rainfall occurring over fl while flash floods generated by intense rainfalls happen the entire basin (Watt et al. 1989). Rainfall-driven oods in summer when atmospheric convection is more in larger basins are usually associated with long-duration common.

*Corresponding author. Email: [email protected]

which have received relatively little attention to date in Canada are discussed. Climate-related trends in ﬂoods are summarized, and the paper concludes with suggested research needs regarding ﬂood-generating processes in Canada.

Historical context of flooding in Canada Flooding is a costly natural disaster for Canadians (Sandink et al. 2010), claiming the lives of more than 200 people and causing over CAD $2 billion in damage during the twentieth century (Jakob and Church 2011). This value is conservative, given the ~CAD $1 billion in Figure 1. Flood disasters in Canada by type between 1990 fl and 2013. damage from the 1996 Saguenay ood alone (Leclerc and Secretan 2016). Figure 2 combines flooding information from the Canadian Disaster Database (CDD; Public storms which tend to have greater areal coverage Safety Canada 2014) and Brooks et al. (2001) to summa- (Dingman 2002). Such events occur across southern rize the number of events, deaths and evacuations, and Canada (Table 1), although the generating mechanisms damages resulting from floods that caused reportable may differ. In eastern Canada (east of 83° longitude; damage and/or loss of life from 1900 to 2010 with esti- fl Watt et al. 1989) these oods may be linked to hurricane mates for the years following. Damage estimates from remnants (Milrad et al. 2009; Watt and Marsalek 2013). the CDD (based on federal, provincial, insurance and Long-duration rainfalls in western Canada may be asso- non-governmental organization payments, and municipal ciated with bands of concentrated near-surface water and other government department costs) were used; dam- fi vapour over the Paci c Ocean (atmospheric rivers ages are expressed in 2010 Canadian dollars and were “ ” referred to popularly as the Pineapple Express , PE) adjusted by the Consumer Price Index (CPI; as reported which can generate intense storms of orographically in Public Safety Canada 2014). enhanced precipitation (P) in coastal mountain regions There are several deficiencies in using these sources (Roberge et al. 2009; Dettinger 2011; Spry et al. 2014), to construct a flood record for Canada. Not all events in or mesoscale systems in inland regions such as the Brooks et al. (2001) appear in the CDD and vice versa, Mackenzie River basin (Smirnov and Moore 2001)or suggesting that neither provides a comprehensive listing over Alberta (Milrad et al. 2015). of major floods in Canada. Both sources sometimes list fl While snowmelt- and rainfall-driven oods can occur multiple floods as single events, and some major floods fl across Canada (Figure 1), other particular forms of ood- have been categorized as another hydroclimatic event ing are more geographically restricted. These include type (e.g. the CDD categorized the Hurricane Hazel fl geomorphically generated oods in high-relief areas of flood as a hurricane). The CDD provides little or no western Canada, and storm surges on the Atlantic, Paci- description of some floods, and no indication of informa- fi c and Arctic coasts as well as on major inland water tion sources used to populate the database. This compli- fl bodies such as the Great Lakes. Similarly, ooding asso- cates attribution of the flood mechanism to particular ciated with the overwhelming of storm sewer networks flood events, such as is attempted in Figure 1. Events in is confined to urban areas, while flooding induced by ris-

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 the CDD were only considered to 2010 due to the ing groundwater tables may manifest itself in permeable absence of a formal updating process (Lara Deacon, Pub- fl alluvial oodplains along large streams and rivers. lic Safety Canada, pers. comm. 2014) that would have This paper provides an overview of the key processes included floods such as the 2011 Assiniboine River flood fl that generate oods in Canada. An exhaustive review of (Blais, Clark et al. 2016, this issue; Blais, Greshuk et al. fl fi ood-generation processes and their relative signi cance 2016, this issue) and the 2013 Bow River Flood across the country is beyond the scope of this paper; (Pomeroy et al. 2016, this issue) in the CDD. instead, the intent is to provide a broader context for the Despite these caveats, several observations can be fi fl fl detailed examinations of speci c oods and ood-gener- made. The first is that the number of significant floods, ating processes that are given in the individual papers in where damages were high enough to be reported in the this special issue. The paper begins with an outline of CDD, was low before 1950 and has been relatively fl the historical context of ooding in Canada, followed by stable in recent decades, averaging five or six events per fl a summary of regional aspects of oods in Canada and a year since the 1970s. Deaths associated with floods were fl description of the processes that drive oods in these greatest in 1951–1960, reflecting the impact of Hurricane regions. Flood processes that are particularly relevant or Hazel on southern Ontario (ON); fatalities have been Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016

Table 1. Mean annual extreme rainfall depth and hourly intensity for various durations for locations in regions of Canada, as denoted by the maximum isohyet shown in Hogg and Carr (1985). Depths and intensities are given for locations with secondary maxima within a given region if present.

Region Mean annual extreme isohyet (mm; mm/hour) 24-hour 12-hour 6-hour 1-hour 30-minute 5-minute BC Maximum 160; 6.7 – sw Vancouver 100; 8.3 – sw Vancouver Island 60; 10.0 – w 18; 18.0 – w 12; 24.0 – w 6; 72.0 – w

Island Vancouver Island Vancouver Island Vancouver central BC hydriques ressources des canadienne Revue / Journal Resources Water Canadian Island Secondary 150; 6.3 – Lower 80; 6.7 – Lower Mainland 50; 8.3 – Lower 16; 16.0 – Lower 10; 20.0 – 4; 48.0 – w Mainland Mainland Mainland Haida Gwaii Vancouver Island; se BC NT, NU, Maximum 30; 1.3 – sw NT; se YK 30; 1.3 – sw NT; se YT 20; 3.3 – sw NT; se 12; 12.0 – sw NT; se 10; 20.0 – sw 4; 48.0 – sw NT YT YT YT NT AB, SK, Maximum 70; 2.9 – sw AB 50; 4.2 – se MB, sw AB 40; 6.7 – se MB 30; 30.0 – se MB 24; 48.0 – se 10; 120.0 – sMB MB MB Secondary 55; 2.3 – se MB; s 40; 3.3 – central AB 30; 5.0 – central 20; 20.0 – central 18; 36.0 – 8; 96.0 – central central MB and sw AB; central AB; central SA central AB AB SA ON Maximum 60; 2.5 – w of L Ontario 55; 4.6 – w of Lake Ontario 45; 7.5 – e of Lake 30; 30.0 – sw ON; 22; 44.0 – sw 11; 132.0 – sw Huron; sw ON nw ON ON; nw ON ON; nw ON Secondary 55; 2.3 – sw ON; n 50; 4.2 – sw ON; central ON; nw ON 40; 6.7 – n central 28; 28.0 – e of Lake 20; 40.0 – n ON 10; 120.0 – nw central ON; nw ON ON; nw ON Huron, Georgian Bay ON QC Maximum 60; 2.5 – e Gaspé, North 50; 4.2 – e Gaspé, North Shore; 45; 7.5 – Eastern 30; 30.0 – Eastern 24; 48.0 – 11; 132.0 – sw Shore; Eastern Townships Eastern Townships; Montreal; Quebec Townships Townships Eastern QC City Townships Secondary 40; 6.7 – w QC 22; 22.0 – w QC 18; 36.0 – s 10; 120.0 – Gaspé Eastern Townships NS, PEI, Maximum 80; 3.3 – s NB, s NL 65; 5.4 – s NB, s NL, e NS 55; 9.2 – s NL, e 24; 24.0 – sw NS, 18; 36.0 – nw 8; 96.0 – sw NB, NB, NS Sable Island NB, sw NS, s nw NB NL NL Secondary 70; 2.9 – e NS 50; 8.3 – s NB 22; 22.0 – s NL 6; 72.0 – sw and e NS, se NL

BC: British Columbia, NT: Northwest Territories, NU: Nunavut, YT: Yukon Territory, AB: Alberta, SK: Saskatchewan, MB: Manitoba, ON: Ontario; QC: Quebec; NS: Nova Scotia, PEI: Prince Edward Island; NB: New Brunswick; NL: Newfoundland and Labrador. n: north, e: east; se: southeast; s: south; sw: southwest; w: west; nw: northwest. 3 4 J.M. Buttle et al.

Breton NS) are largely granite and metamorphic rock, while sedimentary rocks underlie lowland areas (e.g. NB’s eastern lowlands, all of PEI and the NS lowland from Annapolis to Sydney). Most drainage basins have extensive forest cover: boreal forest occurs mainly in Cape Breton and northern NB whereas the Acadian For- est occupies much of the southern part of the Maritime Provinces. Newfoundland and Labrador (NL) can be subdivided into the Island and Labrador. Labrador’s bedrock geology is dominated by metamorphic rocks of the Precam- brian Shield, while Newfoundland consists of sedimentary, metamorphic and volcanic rocks associated with the Appalachian orogen plus intrusive rocks (lar- fi Figure 2. Floods in Canada identi ed by Brooks et al. gely granite). Vegetation ranges from tundra in northern (2001) and the Canadian Disaster Database, CDD (Public Safety Canada 2014). The CDD does not contain all floods but Labrador to mixed deciduous/coniferous forests in south- only those considered significant because of financial transfers. western Newfoundland, with extensive peatlands man- Adjusted costs are based on 2010 Canadian dollars and were tling interior plateaus and coastal lowlands of the Island. adjusted for the Canadian Consumer Price Index. The region’s climate is relatively wet and cool, especially along the coast. Nevertheless, summer temperature generally fewer since the 1950s. The number of people can exceed 30°C, particularly in central NB, while mean requiring evacuation is quite varied, the maximum being annual air temperature varies between 3°C in northern in the 1941–1950 period associated with floods in Mani- NB to 7°C in southwestern NS. Annual P is relatively toba (MB) and British Columbia (BC). Costs and homogenous throughout NB and PEI at ~1200 mm, with adjusted costs of flood events exceeded CAD $1 billion slightly higher amounts (~1400 mm) in southern NB on in the 1990s (all dollar values correspond to the year in the Bay of Fundy. NS typically receives slightly higher which they were reported) and continue to rise, consis- annual P than NB and PEI (~1400–1500 mm), with the tent with an upward trend since the 1960s. Floods in greatest amount in Cape Breton. The regional air temper- 2011–2013 continue the pattern of increasing costs. One ature gradient means southern areas (e.g. southwest NS, interpretation would be that Canadian society has taken 16% of annual P as snow) typically have less snow than steps to reduce the number of lives lost during flood northern NB does (35% of annual P as snow). Snowfall events but is accepting growing financial cost in amount and accumulation have significant impacts on response to the rising frequency of events. flood magnitude and timing, and on flood-generation Many authors have provided classifications of flood processes in the Maritime Provinces. Northern Labrador processes (Watt et al. 1989; Caissie and El-Jabi 1993; has an arctic climate with a mean annual temperature of Pietroniro et al. 2004; Peters et al. 2006; Whitfield −1°C and mean annual P of ~750 mm (about 50% as 2012). Table 2 provides a typology of flood-generating snow). Mean annual temperature decreases to −3°C in processes, including meteorological, hydrological, geo- western Labrador, accompanied by an increase in mean morphic and human induced. Specific generating mecha- annual P to ~950 mm. Mean annual temperature in New- nisms are summarized, and reference made to case foundland increases from 1°C on the Northern Peninsula fl

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 studies in this special issue. The different ood-inunda- to 5°C on the Avalon and Burin Peninsulas, while mean tion mechanisms are summarized in Table 3, which lists annual P on the Island ranges from 1000 to almost 2000 conditions that modify the magnitude of floods as well mm, with the greatest amounts along the south coast and as risk modifiers. Risk modifiers are aspects of the physi- the Avalon Peninsula, and over the Long Range Moun- cal or social setting that either increase or decrease expo- tains on the west coast. The greatest snowfall in New- sure to flood damage; flood preparations such as dykes foundland occurs on the western mountains and the east or levees reduce exposure while development within the coast. floodplain increases risk. Flood magnitude and timing can differ considerably across the region (Figure 3). Most NS rivers experience peak flows throughout autumn and winter; however, Floods within the Maritime Provinces, Newfoundland flood timing is earlier in mainland NS (Roseway River; and Labrador end of March) than for Cape Breton rivers (Northeast The Maritime Provinces comprise New Brunswick (NB), Margaree River; mid-May). NB rivers generally experi- Nova Scotia (NS) and Prince Edward Island (PEI). ence peak flows between March and May (typically early Highland areas (e.g. Central Highlands of NB and Cape May); however, floods occur later for northern NB rivers Canadian Water Resources Journal / Revue canadienne des ressources hydriques 5

Table 2. A typology of floods, based upon two levels of flood generating processes, after Whitfield (2012). The final column indi- cates event case studies detailing events that have occurred since 1995.

Class/type of flood Flood generating process Region or zone Case study in this issue Meteorological Brief Convective cells, thunderstorms; may be Prairies, Great Lakes 1996 Saguenay (Leclerc and Secretan 2016, torrential isolated or coupled to monsoon or other large this issue) rain system; may produce flash floods Heavy rain Synoptic or mesoscale systems Coastal areas, 2005 Alberta (Shook 2016, this issue) 2013 mountain fronts Alberta (Pomeroy et al. 2016, this issue) 2014 Assiniboine River (Ahmair et al. 2016, this issue; Blais, Clark et al. 2016, this issue) Torrential The amount of rain is particularly abundant, rain had a fast onset and/or lasts for a long period of time Extra- Extensive low-pressure system which may Mid-latitudes, coastal tropical move large quantities of water (e.g. Pineapple areas storm Express) Tropical Extensive low-pressure system Tropics, coastal areas storm Tidal/storm Coastal areas surge Hydrological Rain-on- Rate of snowmelt is enhanced by rain and Mountainous areas, snow warmer temperature, leading to more rapid cold regions, northern melt latitudes/high elevations Snowmelt Large accumulations of water in snowpack, Northern latitudes/high 1997 Red River (Rannie 2016, this issue) and high rate of melt elevations, 2009 Red River (Wazney and Clark 2016, mountainous cold this issue) 2011 Richelieu River (Saad et al. regions 2016, this issue) 2008 St. John River (Newton and Burrell 2016, this issue) Ice jam/ Rising water levels break surface ice on rivers Cold regions, 2011 Red River (Stadnyk et al. 2016, this breakup and lakes which forms a jam impounding mountains, northern issue) water, increasing water levels upstream during latitudes and downstream following failure Groundwater Groundwater levels rise above the soil surface Valley bottoms Geomorphic Avalanche Snow pack stability failure creates ponding of Mountainous areas related surface water, increasing water levels upstream during and downstream following failure Landslide Landform stability failure creates ponding of Mountainous areas surface water, increasing water levels upstream during and downstream following failure Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 Outburst Glacial dam fails releasing impounded water, Glacierized basins flood increasing water levels downstream following failure Tsunami Seismic activity generates low waves that move rapidly and become high in shallow and coastal areas Human induced Dam/levee Structural failure break Designed Management decision releases

(e.g. Upsalquitch River). PEI rivers experience flood tim- similar to northern NB rivers (e.g. Upsalquitch and ing similar to mainland NS rivers (e.g. Roseway and Northeast Margaree Rivers). The lowest flood flows in Wilmot Rivers), while flood timing in Cape Breton is NL per unit area are in Labrador (e.g. Ugjoktok River), 6 J.M. Buttle et al.

Table 3. Flood-generating processes, inundation types and modifying conditions and risks. The ﬂood-generating processes are described in more detail in Table 1. Many of these factors exhibit seasonal variation.

Flood-generating processes Inundation types Modifying conditions Risk modifiers Meteorological River floods Precipitation intensity Flood plain elevation Brief torrential rain Flash floods Precipitation volume Human encroachment Extra-tropical storm Ice-jam floods Precipitation timing Flood preparation Heavy rain Glacial lake outburst floods Precipitation phase (rain or snow) Torrential rain Urban floods Antecedent river conditions Tropical cyclone Sewer floods Antecedent watershed conditions Tropical storm Antecedent urban conditions Tidal surge Status (frozen or not frozen) Hydrological Rain-on-snow Snowmelt Ice jam/breakup Groundwater Geomorphic Avalanche related Landslide Outburst flood Human induced Dam/levee break Managed release

usually between April and July in response to snowmelt. average floods for a given basin area in NB tended to be Floods on the Island of Newfoundland are the greatest in in the Bay of Fundy area, with correspondingly higher eastern and southwestern regions (e.g. Isle aux Morts annual P. Major floods in the St. John River basin in River) relative to the central and Northern Peninsula southwestern NB, such as that in 2008, can be generated regions, largely reflecting greater P in the former areas. by snowmelt and rain-on-snow, augmented by ice jam- Floods in these regions are possible throughout the year. ming (Beltaos and Burrell 2015, Newton and Burrell Despite these differences in runoff and flood timing 2016). Similarly, Cape Breton rivers had above-average in the Maritime Provinces and NL (Figure 3), flood mag- floods for a given basin area in NS, and experience the nitude is consistent across the region (Figure 4), with highest P in NS. The largest floods in NL occur along similar relationships between the 50-year flood and basin Newfoundland’s south coast in response to the large P area. PEI rivers showed slightly lower flood magnitudes the region receives. for given basin areas, which partly reflects their limited basin size (less than 150 km2) and low relief. NS basins showed more variability (Caissie and Robichaud 2009), Floods in Québec especially for mid-range basins (100–400 km2). Above- Canada’s largest province is dominated by the rock outcrops, thin soils and abundant lake coverage of the Pre- cambrian Shield. This region is bounded by the extensive wetlands of the James Bay lowlands in the northwest, and the St. Lawrence lowlands to the south. Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 The latter comprise a narrow plain with relatively thick soil cover lining the St. Lawrence River. The Eastern Townships and Gaspé Peninsula consist of the low Appalachian Mountains (mean elevation of 300 m) with thin soils separated by thicker soiled valleys. Vegetation on the Precambrian Shield (Gouvernement du Québec 2003) grades from herbaceous and shrub arctic tundra in the extreme north to forest-tundra (spruce) to the boreal forest zone with spruce in the northern portion and balsam fir–white birch to the south. The balsam fir–white birch domain also covers the highest portions of the Gaspé. Balsam fir–yellow birch dominate the lower Figure 3. Daily runoff for selected rivers within the Maritime elevations of the Gaspé, the Saguenay–Lac St. Jean Provinces and Newfoundland and Labrador. region and the southern edge of the boreal forest. The Canadian Water Resources Journal / Revue canadienne des ressources hydriques 7

rapid mechanical breakup of the ice cover. Rivers often impacted by ice jam flooding include the Chaudière River near Québec City (Roy et al. 2003), the steep Mat- apedia River in the Gaspé region (Beltaos and Burrell 2010), and the Ouelle River in the southwest St. Lawr- ence region (MacNider-Taylor et al. 2009). Heavy summer rainfall can also generate floods, and short-duration (5-minute to 1-hour) maximum rainfall intensities equal or exceed those in other parts of Canada (Table 1). Intense rainfall (> 250 mm in 72 hours on the Chicou- timi and Ha! Ha! River basins) was the main cause of the infamous 1996 Saguenay flood. Flood control structures were insufficient and inefficient in controlling flows, as some dikes toppled and some sluice gates underperformed (Leclerc and Secretan 2016, this issue). Dams on many rivers in QC shift flood seasonality Figure 4. Flood flows (50-year flood) for rivers within the from peak events in the spring to a phase difference Maritime Provinces and Newfoundland and Labrador (data (spring floods upstream of dams and winter floods down- from Department of Environment and Lands 1992; Caissie and stream), and may increase flood duration below the dam Robichaud 2006). (Fortier et al. 2011). In spite of regulation of the St. Lawrence River for hydroelectric production, flood con- southernmost portion of the Precambrian Shield and the trol and navigation safety, some relatively large and shal- St. Lawrence lowlands are covered by hardwoods such low reaches (e.g. Lake St-Pierre between Sorel and as sugar maple, yellow birch, basswood and butternut Trois-Rivière) are subject to major water-level fluctua- hickory. tions because of the flow variability of tributaries (e.g. Southeastern QC’s maritime climate is influenced by Ottawa, Richelieu, Yamaska and St.François Rivers) the Gulf of St. Lawrence, while most of the St. Lawr- entering downstream of the Beauharnois dam. High ence valley has a humid continental climate which water levels in 1976 (1 m above the summer average) changes to a cold polar climate in the extreme north. were observed in Lake St-Pierre, and define the upper The result is strong north–south gradients in both air limit of sensitive wetland ecosystems (Hudon 1997). temperature and P. Average air temperature ranges from There is a relatively consistent relationship between 20°C in the south to 3°C in the north (summer), and maximum discharge (Qmax) and basin area for various −8°C in the south to −25°C in the north (winter) sub-regions (Figure 5) compared to other regions in (Ouranos 2010). Summer mean P increases to the north- Canada (e.g. the Prairies and the Cordillera discussed west from nearly 450 mm in the south to 120 mm in the below) despite QC’s great areal extent. The exception north. Some southern mountainous regions receive 350 appears to be basins in the far north, where Qmax is mm of winter P with only 50 mm of snow water equiva- much more variable for a given basin area. This may lent (SWE) in the far north (Ouranos 2010). Snow accu- reflect the role of lakes as key storage elements in the mulation typically begins in October in northern QC and landscape, reducing flood peaks and detaining flood November in southern QC. Precipitation generally waves in basins where their size and number exert a key – fl Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 remains in the form of snow until March April in the control on the ow regime. The Qmax from many rivers south, while northern QC typically does not receive sig- in the Gaspé often plots above those for other sub-re- nificant rainfall (> 0.2 mm) before May. gions at a given basin size, consistent with the relatively Most important floods result from spring snowmelt steep gradients of streams in the Appalachian Mountains (Javelle et al. 2003), and St-Laurent et al. (2009) noted of this sub-region. that 59% of major floods from 1865–2005 in the St- François River basin occurred between March and May. Such floods may be augmented by rain-on-snow events Floods in southern Ontario (e.g. 1913 St-François River flood [Castonguay 2007]; Southern ON (Figure 6a) is bounded by the Great Lakes 2011 Richelieu River flood [Saad 2014]). In the case of to the south, west and northwest and the Ottawa and St. the latter, heavy snowfalls (> 30% above normal) pre- Lawrence Rivers, and has modest relief (Sangal and ceded spring snowmelt, coinciding with above-average Kallio 1977). All river basins drain to the Great Lakes or rainfall amount and intensity, producing peak flows with the Ottawa River. an estimated return period of 90 years (Saad et al. 2016). The region’s climate is strongly modified by the Other major causes of floods include ice jams following Great Lakes. Mean annual P ranges from 660 to 1010 8 J.M. Buttle et al.

Figure 5. Maximum daily mean discharge vs. basin area for unregulated Water Survey of Canada (WSC) gauging stations in sub-regions in Québec. Maximum daily mean discharge was obtained for all naturally ﬂowing stations in the region with 10 years or more of discharge record with a known drainage area, and includes all active and discontinued sites (data from WSC 2014).

mm (Brown et al. 1968), with the highest values on Figure 6. (a) Physiographic regions of southern Ontario iden- slopes east of Lake Huron and Georgian Bay (Moin and tified by Moin and Shaw (1985). (b) Maximum recorded flood Shaw 1985). Mean annual snowfall ranges from < 100 (daily mean discharge) vs. basin area recorded at unregulated cm along the northwest shore of Lake Erie to > 300 cm or minimally regulated Water Survey of Canada (WSC) gauging stations in each region with at least 10 years of record to around Georgian Bay (Ontario Ministry of Natural 2012. Resources 1984). The portion of annual P falling as snow increases from south to north (Moin and Shaw The presence of a temporary river ice cover means 1985), ranging from less than 20% to more than 30% that some flooding in southern ON results from ice-jams (Ontario Ministry of Natural Resources 1984). Distribu- that augment snowmelt-generated flooding, usually fol- tion of P throughout the year is relatively uniform. lowing an early spring thaw when the ice cover is still Historically, snowmelt and rain-on-snow have been strong. The Moira River in Belleville is subject to this the most frequent flood-generating processes in southern type of flooding, as are lower sections of the Thames

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 ON (Irvine and Drake 1987; Watt et al. 1989; Gingras and Sydenham Rivers in southwestern ON (Gerard and et al. 1994), and heavy spring rainfalls following snow- Davar 1995). Finally, coastal flooding occurs in some melt are also common flood generators, such as in the areas along shorelines of the Great Lakes (Watt et al. Cambridge area of the Grand River basin in May 1974 1989). High lake levels can lead to flooding of property, (Moin and Shaw 1985). The significance of summer and wave erosion. In some areas (e.g. the downwind end storms and high-intensity rainfalls in flood generation of Lake Erie) this is exacerbated by wind set-up raising (e.g. Buttle and Lafleur 2007) has increased in recent water levels as much as 2 m above the static level. years. Snowmelt generally starts in March, although Changes in wind velocity and air pressure can also set mid-winter melt events are common. The proportion of up oscillations in lake levels (seiches), which have led to snowmelt-generated peaks increases northward and east- flooding at the eastern end of Lake Erie (Trebitz 2006). ward across the region, with the mean duration of snow- Coastal flooding may occur along all the Great Lakes’ melt peaks also increasing from south (< 2 days) to shorelines bounding southern ON, but is mitigated north (> 8 days) (Irvine and Drake 1987). Another cause slightly along the Lake Ontario shoreline due to partial of winter floods is rain on frozen ground (Watt et al. control of that lake’s water levels (McRae and Watt 1989; Buttle 2011), particularly in agricultural areas. 2006). Canadian Water Resources Journal / Revue canadienne des ressources hydriques 9

The general increase in Qmax with basin area in eastern QC (Fisheries and Environment Canada 1978). southern ON (Figure 6b) displays considerable scatter This general west-to-east increase is interrupted by and overlap between some regions (e.g. Regions 4 and increased P in the lee of Lake Superior in ON. There is 5). Nevertheless, Qmax values from Regions 1 and 4 also a slight increase along a north–south gradient in ON diverge, with greater floods for Region 4 beyond a basin and QC. Annual evapotranspiration (ET) usually exceeds size of ~100 km2. This may reflect water storage in the annual P in the northwestern boreal, while P – ET numerous lakes and wetlands in Region 1. increases moving eastward. Rainfall intensities are gener- Southern ON is one of the most urbanized regions in ally lower in the boreal forest than in the Prairies and Canada, containing such major urban centres as Toronto, southern ON and QC for a given duration and return Ottawa, Hamilton, London and Kitchener/Waterloo, and period (Table 1). Mean annual snowfall increases from a later section on floods in urban areas addresses flood ~160 cm in northeastern BC to > 400 cm in eastern QC, generation in these and other Canadian cities and towns. interrupted by an intermediate peak snowfall in the lee Agricultural land in much of ON needs drainage of Lake Superior. Mean January daily air temperatures improvement via subsurface tile drainage systems to are < −22.5°C in northeastern BC and −12°C in eastern allow farmers earlier access to the land following spring QC, declining to < −27.5°C in northern Saskatchewan snowmelt (Irwin and Whiteley 1983). While there is no (SK) and MB. Mean July daily air temperatures are consensus as to the impact of land drainage on floods, 17.5°C along the southern edge of the boreal forest in enhanced soil storage capacity towards the point of soil MB, ON and QC, dropping to < 12.5°C in northern QC. saturation should decrease peak flows at the field scale, On average, rivers freeze over on 1 December in the after which tile drainage likely increases peak flow southern boreal in ON and QC, and as early as 1 slightly by adding subsurface outflows to overland runoff November in northern SK. The mean date when rivers (Irwin and Whiteley 1983). The influence of tile drainage become ice-free is 15 April along the boreal forest’s on flooding at the basin scale is similarly equivocal. southern edge, and 1 June along its northern edge (Fish- Drainage of surface depressions previously unconnected eries and Environment Canada 1978). Permafrost > 10 m to the flow network will likely increase flood peaks; in depth may be present in upland areas of northern por- however, the effect on flooding at a specific location on tions of the boreal forest, but is generally absent under the stream network depends in part on the degree of syn- major lakes and rivers due to their thermal influence chronization of flow contributions to the channel (Irwin (Newbury et al. 1984). and Whiteley 1983). The west-to-east shift in annual P – ET from negative to positive values across the boreal forest has significant implications for flood potential in the region. Floods in the boreal forest Negative P – ET in the western Boreal Plains often leads The boreal forest region extends from the Yukon (YT) to to disconnection between uplands and stream networks. NL and combines Canada’s Boreal Plains and Boreal This reduces the frequency of significant runoff at the Shield ecozones; floods in the Boreal Cordillera zone are basin scale, and means that flood potential in response to addressed later in the section on western Cordilleran spring and summer rainfalls in a given year is maxi- floods. The Boreal Shield comprises almost 20% of mized when large P inputs in the preceding summer and Canada’s landmass and contains 22% of Canada’s fresh- fall are combined with large winter snow accumulation water area (Urquizo et al. 2000). It is a rolling landscape (Devito et al. 2005). Interannual variability in flood of igneous and metamorphic bedrock outcrops, generally potential in the boreal forest decreases moving eastward –

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 thin soils, numerous lakes and wetlands. It contains the in response to increases in annual P ET. Granger and headwaters of several major river systems (e.g. Nelson Pomeroy (1997) showed that mature boreal forest stands and Churchill Rivers in MB, St. Lawrence in ON, East- had greater ET losses relative to regenerating stands or main and Rupert in QC), and is largely forested. North- recent clearcuts, leading to greater summer and fall soil ern areas are dominated by white and black spruce, moisture in forest clearings and young regenerating balsam fir and tamarack, while white birch, trembling stands (Elliott et al. 1998). Higher soil moisture in aspen, balsam poplar, and white, red and jack pine are clearcuts combined with compaction of clearcut soils common in southern regions. The Boreal Plains extend promoted reduced infiltration capacity. As a result, all from the Peace River district in BC to southeastern MB. sub-canopy rainfall infiltrated mature forest soils during They have fewer lakes and bedrock outcrops relative to a severe summer storm in the central SK boreal forest (> the Boreal Shield, and forest cover consists of jack and 150 mm), whilst over half of the rainfall formed runoff lodgepole pines, white and black spruces, tamarack and in recent clearcuts (Elliott et al. 1998). aspen. Most annual maximum floods in the boreal forest are Mean annual P in the boreal forest ranges from < driven by snowmelt (Woo and Waylen 1984; Alberta 400 mm in extreme northeastern BC to > 1200 mm in Transportation 2004), and boreal forest cover affects 10 J.M. Buttle et al.

snowmelt energetics and rates, meltwater infiltration into into the Burntwood–Nelson system at Southern Indian frozen soils, and runoff generation (Prevost et al. 1991). Lake in northern MB (Newbury et al. 1984). Wet soils in clearcuts in the fall from reduced summer Our knowledge of flood characteristics of boreal for- ET result in saturated frozen soils in the spring with est rivers is less than for other regions (e.g. southern ON restricted infiltration capacity (Pomeroy, Granger et al. and QC), partly due to its sparse population. Neverthe- 1997). Sublimation losses from intercepted snow less, variability in the Qmax vs. basin area relationship (Pomeroy et al. 1998) lead to maximum spring snow- for the boreal forest (Figure 7) is greatest for rivers packs under evergreen forest canopies that are one third draining to Great Slave Lake, which also have many of to one half of those in clearings, burned or deciduous the smallest Qmax values for a given basin size. The lat- stands (Pomeroy et al. 2002). Melt rates are three times ter likely reflect the frequent disconnection between higher in clearings than under forest stands (Pomeroy uplands and stream networks noted earlier for this sub- and Granger 1997), generating vastly greater snowmelt region, often resulting in minor flood peaks relative to runoff formation in recent clearcuts than in mature stands wetter parts of the boreal forest. (Pomeroy, Granger et al. 1997). Spring and snowmelt floods are often linked to river ice breakup, which cre- fl ates water levels far higher than those for equivalent Prairie ooding open-water discharges (Pietroniro et al. 1996). Neverthe- The plains of the southern portions of Alberta (AB), SK less, short-duration intense thunderstorms can produce and MB consist of gently rolling hills separated by deep severe flooding in small- to medium-sized basins river valleys, with a general west–east slope. The sedi- (Alberta Transportation 2004). Flood hydrology in parts mentary bedrock is covered by glacial deposits of vari- of the boreal forest is complicated by river impoundment able thickness. The region’s natural vegetation is for hydroelectric generation and the associated flow grasslands, and trees are largely confined to river valleys; diversion, such as the diversion from the Churchill River however, it has been heavily altered by agricultural activity. Southwestern AB and southeastern SK have a semi-arid cold climate, becoming more humid and colder to the north and east. Maximum January and July mean daily air temperatures are generally warmer in this semi- arid sub-region (> −10°C and > 20°C, respectively). Annual P ranges from < 300 mm per year in semi-arid grassland to > 700 mm in central MB. Mid-winter melts (frequent in the southwest and infrequent in the northeast) punctuate the region’s protracted winter (usually 4– 5 months). High surface runoff occurs during spring snowmelt due to relatively rapid water release from snowpacks to frozen soils (Gray et al. 1985). Most rainfall in spring and early summer is from large frontal systems, and intense rainfall in summer is from convective storms over small areas (Gray 1970; Shook and Pomeroy 2012; Table 1). High ET with low rainfall and soil moisture from mid-summer to fall result in little runoff (Gran-

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 ger and Gray 1989). This is exacerbated by poorly drained stream networks such that large areas are internally drained and do not contribute to major river systems except during flooding (Martin 2001; Shook et al. 2013). Generally low P makes the accumulation, redistribu- Figure 7. Maximum daily mean discharge vs. basin area for tion and ablation of snowcover of critical importance to unregulated Water Survey of Canada (WSC) gauging stations in major drainages across the boreal forest in the Yukon Terri- the hydrology of the Prairie region. Snowpacks typically tory, British Columbia, Northwest Territory, Alberta, Saskatche- form in November and begin ablation in March and wan, Manitoba, Ontario, Québec and Labrador. Drainage basin April. Over-winter wind redistribution and sublimation numbers correspond to the major drainage basins of Canada reduces peak SWE and increases variability in spatial dis- used by WSC (see Table 4).Maximum daily mean discharge tribution with large snowdrifts near water courses, den- was obtained for all naturally flowing stations in the region with 10 years or more of discharge record with a known drai- sely vegetated sites and topographic depressions, and nage area, and includes all active and discontinued sites (data wind-scoured zones in summer fallow fields and on hill- from WSC 2014). tops (Fang and Pomeroy 2009). Mid-winter melt and Canadian Water Resources Journal / Revue canadienne des ressources hydriques 11

Table 4. Major drainage basins in Canada.

Code Major drainage Selected examples 01 Maritime Provinces drainage St. John, St. Croix 02 St. Lawrence River drainage Ottawa, Richelieu, Saguenay 03 Northern Quebec drainage Churchill, Grande 04 Southwest Hudson Bay drainage Attawapiskat, Hayes, Severn 05 Nelson River drainage Assiniboine, Bow, Oldman, Red, Red Deer 06 Western Hudson Bay drainage Churchill, Kazan, Thelon 07 Great Slave Lake Athabasca, Peace, Slave, Smoky 08 Paciﬁc drainage Columbia, Fraser, Lillooet, Skeena, Stikine 09 Yukon River drainage Porcupine, White 10 Arctic drainage Liard, South Nahanni, Mackenzie 11 Mississippi River drainage Frenchman, Milk, Poplar

sublimation in the southwestern Prairies from adiabati- cally warmed winds in the lee of the Rocky Mountains (chinooks) reduce SWE substantially. Rain-on-snow contributions are infrequent, although flooding in 2011 showed a rain-on-snow contribution in SK and MB (Stadnyk et al. 2016). Snowmelt runoff is also strongly controlled by infiltration to frozen mineral soils, which itself is sensitive to the influence of cultivation on macropores and fall soil moisture status. Restricted infiltration through frozen soils may develop with any of mid-winter melting, early spring rainfall and high fall moisture content, and can generate flooding (Granger et al. 1984). The more common limited infiltration con- dition can reduce flood potential even when SWE is above normal, but the combination of high snowpacks and restricted infiltration is most strongly associated with flood development (Gray et al. 1985, 1986). In the poorly drained central, northern and eastern Prairies, snowmelt runoff flooding alone does not generate stream flooding as the landscape is characterized by Figure 8. Maximum daily mean discharge vs. basin area for numerous small post-glacial depressions known locally unregulated Water Survey of Canada (WSC) gauging stations as “sloughs,”“wetlands” or “potholes” and here termed in sub-regions of the Prairies. Drainage basin numbers corre- “ponds.” Most ponds are internally drained, forming spond to the major drainage basins of Canada used by WSC closed basins (LaBaugh et al. 1998; Hayashi et al. (see Table 4). Maximum daily mean discharge was obtained for fl 2003), which are non-contributing areas in dry to normal all naturally owing stations in the region with 10 years or more of discharge record with a known drainage area, and Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 conditions (Godwin and Martin 1975). However, ponds includes all active and discontinued sites (data from WSC connect to one another during floods through the “fill 2014). and spill” mechanism (van der Kamp and Hayashi 2009; Spence 2010). Peak streamflows are influenced by snow- generation potential (van der Kamp et al. 2003; Fang melt rates and volumes near the ponds, incident P, and and Pomeroy 2008). Modelling the impact of wetland antecedent soil and surface storage status (Fang and drainage on peak flow generation suggests that peak Pomeroy 2008; van der Kamp and Hayashi 2009; Fang flows can be increased by approximately one third when et al. 2010). The storage potential of ponds makes them pond drainage approaches 50% in a prairie pothole-dom- important hydrological elements (Hayashi et al. 2003) inated drainage (Pomeroy et al. 2014), as a result of the which can regulate flood peaks by retaining slope runoff enhanced efficiency of the drainage network in convey- that might otherwise reach the stream. Land-use altering slope runoff to the basin outlet. The relationship ation in surrounding upland areas can produce noticeable between Qmax and basin area for Prairie rivers (Figure 8) impacts on snowpack trapped by pond vegetation, sur- shows greater scatter than that in other regions that have face runoff to ponds, pond levels and subsequent flood been described. This may partly reflect the internal 12 J.M. Buttle et al.

drainage of many prairie basins, combined with spatial Lower Mainland were greater for PE storms than for variations in the intensity of the aforementioned drainage non-PE storms. Frontal boundary floods along the east of prairie wetlands. slope of the Rockies in AB have occurred on many occasions (e.g. Ford 1924; Hoover 1929; Pomeroy et al. 2016). The largest are generated when mesoscale con- Floods in the western Cordillera and Intermontane vective systems stall over the foothills when moving region upslope towards the mountains. Streams receiving sub- The western Cordillera extends from southern BC north- stantial snowmelt inputs generally have annual peak wards to the Beaufort Sea, and includes areas of YT and flows in May and June, and event magnitude is deter- the western portion of the Northwest Territories (NT). mined by SWE and weather conditions during the melt. The region is dominated by three mountain systems: Rain-on-snow floods are common in autumn and winter eastern fold mountains (e.g. Rocky Mountains); the sedi- in coastal areas, and also occur into late spring in interior mentary, metamorphic and igneous rocks of the interior areas (McCabe et al. 2007). They are often the largest plateaus and mountains; and the western system of the floods in larger basins (e.g. Pomeroy et al. 2016), and metamorphic and intrusive igneous rocks of the Coast the severity of rain-on-snow events depends on P Range and western-most mountains of Haida Gwaii and amount, elevation of the freezing level and the amount Vancouver Island. Much of the region has considerable and spatial distribution of snow (McCabe et al. 2007). forest cover, and slope aspect plays a key role in local Warm wet Pacific storms (often PE events) cause rapid variations in vegetation cover. Windward slopes on melting of the existing snowpack (where sensible- and southern mountain ranges have evergreens such as Dou- latent-heat exchanges supply 60–90% of the energy for glas fir, western hemlock and red cedar. Trees decrease snowmelt), augmenting the event’s discharge. in size with increasing elevation, and transition to a tun- Floods in the Cordillera can also be generated by ice dra landscape above the tree line. Leeward slopes may jams as in other regions of Canada, particularly on more be covered by vegetation typical of semi-arid landscapes, northerly or high-elevation streams. In addition, the particularly in southern BC. region experiences flood mechanisms that are relatively The Cordillera’s location between the Pacific Ocean unique to the Western Cordillera. These include debris and Canada’s interior, its great latitudinal extent and its flows (rapid flow of saturated debris in a steep channel), rugged terrain result in a great variety of climates. debris floods (rapid flow of debris-laden water in a steep Annual P ranges from > 4000 mm on the west coast of Vancouver Island and Haida Gwaii to < 300 mm in the southern BC interior and parts of the YT. January mean daily temperature ranges from ~2.5°C in the BC lower mainland to < −30°C in the YT, while July mean daily temperature ranges from > 20°C in the southern BC interior to 5°C along the arctic coast. Relief and slope aspect play important roles in sub-regional climates. Valleys are warmer than mountain slopes, and the rain-shadow effect makes windward mountain slopes generally wetter than leeward slopes. Mountainous regions of Canada are characterized by fl

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 multiple ood-generating mechanisms (Watt et al. 1989; Woo and Liu 1994). Floods can be generated by rainfall, snowmelt and rain-on-snow, whether in coastal basins (Melone 1985; Loukas et al. 2000) or in the BC Interior (Eaton et al. 2002). The larger the basin, the more likely that floods are generated by different processes (Woo and Liu 1994). Except for the largest rivers, rainfall floods may be of the highest magnitude but are small in number. Summer floods produced by heavy rainfalls, although rare, are more significant in smaller basins. Det- Figure 9. Maximum daily mean discharge vs. basin area for tinger (2011) described PE which deliver large amounts unregulated Water Survey of Canada (WSC) gauging stations of warm moist air onto the west coast of North America, in sub-regions of the Cordillera in British Columbia. Maximum daily mean discharge was obtained for all naturally flowing sta- often resulting in high snowlines, large rainfalls and tions in the region with 10 years or more of discharge record extreme floods (Neiman et al. 2013). Spry et al. (2014) with a known drainage area, and includes all active and discon- showed that both P and streamflow rates in the BC tinued sites (data from WSC 2014). Canadian Water Resources Journal / Revue canadienne des ressources hydriques 13

channel) and snow avalanches (Desloges and Gardner area is usually less than the basin area due to redistribu- 1984; Jakob and Jordan 2001; Jakob et al. 2016), and tion of snow that results in a highly variable spring glacial outburst floods caused by the rapid drainage of snowpack (Essery et al. 1999), cold content in deep ice-dammed lakes (Desloges and Church 1992; Geert- snow drifts that must be overcome before meltwater can sema and Clague 2005). Wildfire-induced changes to soil be generated (Marsh and Pomeroy 1996), and spatially infiltrability may contribute to debris flow and debris variable melt energy to the patchy snowcover that per- flood initiation (Jordan and Covert 2009). sists for many weeks during the snowmelt period (Pohl Discharge maxima for a given basin area (Figure 9) and Marsh 2006). These spatially variable effects desyn- are generally greater for the wetter coastal area, while chronize meltwater production and are exacerbated in the lowest maxima are found in the drier south–central mountain tundra environments such as in the YT (Carey interior of BC. There is considerably greater scatter in and Woo 1999; Pomeroy et al. 2003). In tundra plains the Qmax vs. area relationship for relatively small (< 100 and uplands, absence of trees promotes substantial snow km2) basins in the Cordillera compared to eastern redistribution to valleys (Pomeroy, Marsh et al. 1997), Canada (Figures 4–6), which likely reflects the combined where spring meltwater can accumulate behind snow- influence of the highly variable climate across the Cor- and ice-choked channels and cause downstream flooding dillera and variations in basin characteristics. Maxima when temporary dams rupture and release potentially from basins draining the east and west slopes of the large volumes of impounded water (Woo and Sauriol Rocky Mountains do not show consistent differences; 1980;Woo1983; Church 1988). Drainage of ice- however, there is much less variability in the Qmax vs. dammed lakes has the potential to produce extreme area relationship for west slope relative to east slope floods (Cogley and McCann 1976; Church 1988). basins. The relatively low maxima for some of the latter Rainfall-generated floods in the Arctic are rare and may partly reflect basins with hydrometric stations seldom documented (Kane et al. 2003; Dugan et al. located far enough downstream that a greater portion of 2009); nevertheless, extreme summer flows may occur the basin is in the rain shadow of the Rocky Mountains. (e.g. Cogley and McCann 1976). Continuous permafrost underlying Arctic basins precludes typical groundwater systems and severely limits subsurface storage during the fl Regional aspects of oods in the Arctic summer, leading to inflated storm runoff response rela- Canada’s Arctic comprises over 40% of Canada’s land- tive to more southern basins (Kane et al. 2003). Church mass, covering all of Nunavut (NU) and the majority of (1988) suggested that the most extreme discharge could the NT (Arctic Islands), as well as the northern edges of ultimately arise from rainfall even in the High Arctic. YT, MB, ON, QC and NL. It is north of the boreal forest Although summer rainfalls are generally of low intensity of the subarctic region and is predominantly comprised and minimal hydrological significance in the polar desert of tundra and barren landscape underlain by permafrost environment compared to nival floods (Table 1; Dugan (see Prowse et al. 2009 for a more detailed discussion of et al. 2009), low-pressure systems over intermediate- the region’s physical geography and climate). sized basins allow the entire basin to contribute to storm The region has long, cold winters and short, cool flow and produce summer floods three to four times summers, leading to snow and ice cover for most of the greater than the maximum snowmelt flood (Woo et al. year with short periods of runoff. Spring runoff supplies 2008). the bulk of the annual flow, especially for small- to med- The High Arctic is characterized by a short-lived ium-size basins wholly within the Arctic. Many headwa- nival hydrological regime with the maximum discharge

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 ter systems are ephemeral as a result of freezing to the usually generated by spring snowmelt (Woo 1983; bed and/or flows confined to early spring. Open-water Church 1988). High-gradient headwater basins are domi- flow lasts several weeks in the northern Arctic to several nated by summer floods, while snowmelt drives flooding months in more southerly regions. Several large, north- in basins on low-gradient coastal plains (Kane et al. ward-flowing perennial river systems originate in the 2008). Snowmelt floods also occur in large basins (e.g. boreal, plains and mountain ecozones, such as the 1.8- Mackenzie River) that drain extensive southern areas. million-km2 Mackenzie River basin that is Canada’s lar- Nevertheless, river ice breakup and associated ice jams gest contributor of water to the Arctic Ocean. dominate the generation of annual high-water levels Whilst snowmelt drives Arctic flood discharge, river throughout the Arctic (von de Wall et al. 2009). This ice breakup and subsequent jamming have the potential flood-generating process is most pronounced in sub-re- to generate extreme flood levels (Watt et al. 1989). Other gions possessing the combined effects of low relief and important flood mechanisms are snow/ice blockage of a cold, dry arctic climate. channels, icings and glacier bursting. Snowmelt produces Restricted infiltration due to the presence of per- runoff over large areas but often at a slow rate due to mafrost in the Arctic promotes flooding of wetland areas the variable contributing area of meltwater runoff. This (Woo et al. 2008). Unless winter snowfall is limited, the 14 J.M. Buttle et al.

wetland streamflow regime is typified by the spring fre- This section describes in more detail several distinct shet (Woo 1983). Rapid snowmelt releases large water aspects of flooding in Canada. The first is the little-stud- quantities while the active layer is frozen, producing sur- ied issue of groundwater flooding. The second is ice-jam face runoff that floods wet meadows and infills ponds flooding, which although not unique to Canada is a com- and lakes. For instance, the maximum extent of wetland mon process in cold regions. The third type is storm- flooding usually occurs immediately following spring surge flooding, which is generally restricted to coastal snowmelt in the High Arctic polar desert (Woo and areas. Fourth, the concentration of Canada’s population Young 2006). in cities makes the issue of urban flooding of particular Wetland ecosystems are also found in deltaic envi- interest. ronments. The aquatic, semi-aquatic and terrestrial zones in deltas form highly diverse and variable ecosystems, fl and the Mackenzie River system contains three major Groundwater ooding deltas: the Peace–Athabasca Delta (PAD), Slave River Groundwater flood events are generated by four mecha- Delta and Mackenzie River Delta (MRD). The ecological nisms (Hughes et al. 2011): (1) natural water table rises integrity of deltaic floodplains depends on their natural and surface saturation caused by extreme high-intensity dynamic character, which is controlled by periodic flood- and/or long-duration rainfall (e.g. Robins and Finch water inputs, deposition of material, and flushing during 2012); (2) groundwater flow through alluvial deposits high river-stage events (Peters et al. 2016). bypassing river channel flood defences; (3) groundwater A prominent feature of these deltas is an abun- level rise due to cessation of groundwater abstraction; dance of shallow, productive wetland and lake basins and (4) underground structures creating barriers to (> 25,000 in the MRD) with varying degrees of sur- groundwater flow that result in water tables rising to face water connectivity to the main flow system cause flooding (e.g. Edwards 1997). An additional mech- (Lesack and Marsh 2010). Low-elevation wetland and anism is over-irrigation in semi-arid and arid areas, lake basins are potentially flooded annually and for which causes groundwater levels to rise along with relatively long durations, while the frequency and related salinization (Xiong et al. 1996). duration of flooding of basins at higher elevations are Reports of groundwater flood events in Canada are much less (Marsh and Lesack 1996). Wetland and lake uncommon, although numerous studies describe ground- basins in the MRD and PAD perched above the main water–surface water (GW–SW) interactions under variable flow system depend on high spring snowmelt runoff stream flow conditions (e.g. Cloutier et al. 2014), and and ice jamming to raise backwaters enough to unpublished accounts indicate basement flooding related recharge water levels back to the spill elevation (Peters to GW–SW interactions in communities built on alluvial et al. 2006; Lesack and Marsh 2010). Although open- aquifers. One such example comes from the relationship water floods can inundate low-lying areas of these del- during the major flood of 20–21 June 2013 between the tas, ice-jam flooding is often the only process capable groundwater level in Canmore, AB and the hydrograph of recharging wetland and lake ecosystems. These of the Bow River (from the Banff Water Survey of would otherwise desiccate over decadal timescales in Canada [WSC] gauging station) which flows through the semi-arid environments where ET generally exceeds P town (Figure 10). The Canmore groundwater observation (Marsh and Lesack 1996; Peters et al. 2006).

Some special aspects of floods in Canada Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 Many rivers in Canada flow northward, and flooding in these rivers (e.g. Richelieu, Red, Peace–Athabasca, Hud- son Bay rivers) is often connected to delayed melting in northern portions of the basin relative to earlier snowmelt in southern portions (Bruce 1939). Spring breakup can be the largest physical disruption in rivers that flow northward to cooler regions where spring thaw is later (Smith 1980; Rood et al. 2007). Such floods put cities such as Winnipeg at risk (Blais, Clark et al. 2016) while providing an essential ecological service to deltas such as the PAD (Peters et al. 2006). Several flood case studies in this special issue report on such flooding events Figure 10. Discharge for the Bow River at Banff, Alberta for (Rannie 2016; Riboust and Brissette 2016; Saad et al. the 20–21 June 2013 flood, and groundwater level in the allu- 2016; Wazney and Clark 2016). vial aquifer in Canmore, Alberta. Canadian Water Resources Journal / Revue canadienne des ressources hydriques 15

well is 24 km downstream of the Banff WSC station in a strong control on flood generation in many areas of thick (up to 100 m) sand and gravel aquifer (Toop and de Canada (Ashton 1986; Gray and Prowse 1993). The la Cruz 2002). Aquifer thickness at the well is 73 m, establishment of river ice cover decreases flow con- which is 730 m from the current river channel. Bow veyance by reducing channel cross-sectional area and River discharge in Banff rose sharply near midnight of 20 flow velocity, and increasing flow resistance through an June and peaked at 401 m3/s in the late afternoon of 21 enlarged wetted perimeter. The addition of an ice cover June (Figure 10), exceeding the highest instantaneous with a similar roughness to the river bed results in ~30% flow recorded at this station (399 m3/s on 14 June 1923). increase in water depth over open-water channel flow Flood peak travel time between Banff and Canmore is conditions (Gray and Prowse 1993). On a large river sys- likely on the order of a few hours. A kinematic wave tem, water abstraction to feed ice growth and hydraulic induced by a rapid increase in river stage (Jung et al. storage behind the accumulating ice can substantially 2004) likely led to the quick response of the groundwater decrease downstream flow (Moore et al. 2002; Prowse level, which reached the maximum rise of ~1.6 m by and Carter 2002). Backwater storage is released during midnight on 22 June (Figure 10). Much of Canmore is spring melt, contributing to the largest hydrologic event built on the alluvial aquifer (Toop and de la Cruz 2002) of the year for most ice-covered rivers at northern lati- and is vulnerable to flooding associated with the rising tudes; e.g. > 20% of the spring freshet volume in the water table, which can back up sewer systems and cause Mackenzie River was estimated to be over-winter water basement as well as surface flooding due to the reduced released from ice-induced hydraulic storage at the time sewer drainage capacity. The 20–21 June 2013 flood of river ice breakup (Prowse and Carter 2002). event was particularly severe due to the combination of Interaction of a large flood wave with an intact and extremely high river stage and intense local infiltration mechanically strong ice cover can result in dynamic (me- caused by heavy rain (Blair Birch, Town of Canmore, chanical) river ice breakup and ice jamming, with poten- pers. comm. 2014). Approximately 130 km downstream tial generation of extreme flood levels (Watt et al. 1989). of Canmore, the Bow River flows through Calgary, where Ice jams may present significant resistance and/or densely populated urban areas are also located on the obstruction to streamflow, leading to river stage several alluvial aquifer (Cantafio and Ryan 2014). Similar cases metres greater than for the same discharge under open- of groundwater-induced flooding were reported in Cal- water conditions (von de Wall et al. 2010; Figure 11) gary during the same flood (Osborn and Ryan 2014). and occasional over-banking of channel water (Gray and Prowse 1993).

Storm surges Storm surges are water-level oscillations in a coastal or inland water body with a period of a few minutes to a few days, resulting from atmospheric weather systems. These include extra-tropical cyclones, hurricane remnants and squall lines embedded in larger scale synoptic systems. Resulting water-level changes along the shoreline can be on the order of several metres. Storm surges are frequent in coastal areas of Canada, the deadliest of which was the 1869 Saxby Gale in the Bay of Fundy which killed more

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 than 100 people in the Maritimes. They have resulted in near-shore inundation on the Atlantic (Danard et al. 2003) and Paciﬁc (Murty et al. 1995; Crawford et al. 2000) coasts, in the Gulf of St. Lawrence, St. Lawrence Estuary, Bay of Fundy, Hudson Bay, James Bay, Northwest Pas- sage, Beaufort Sea (Harper et al. 1988; Marsh and Schmidt 1993), the Great Lakes (Watt et al. 1989; Trebitz 2006), and other large lakes such as Lake Winnipeg. With rising sea levels, storm-surge damage may increase in future (e.g. Lyle and Mills 2016).

Ice jam-related floods Figure 11. Open-water and ice-influenced peak water levels vs. discharge at the Mackenzie River at Arctic Red River Water The formation, growth and ablation of ice cover and the Survey of Canada (WSC) station for the years 1972 to 2006 associated development of ice jams exert a particularly (modified from von de Wall et al. 2010). 16 J.M. Buttle et al.

Floods in urban areas in damages – the most expensive natural disaster ever in A number of processes can produce flooding in cities; Toronto and ON. the focus here is on the role of urbanization itself. Flood- The wide range of urban stormwater best manage- ing in urban areas is a special concern since Canada’s ment practices (BMPs) include such end-of-pipe options fi urban population (people living in urban areas as defined as in ltration basins and stormwater management ponds by national statistical offices) was just above 27 million (Zimmer et al. 2007); the latter store runoff temporarily (81% of Canada’s total population) in the 2011 census and release it over a protracted period to downstream (Statistics Canada 2014). Urbanization involves the con- drainage systems (Marsalek and Schreier 2009). Increas- struction of buildings, roads and related infrastructure, ing emphasis has been placed on low-impact develop- with consequent reduction of the infiltration capacity of ment (LID) based on design features to reduce overland fl previously permeable surfaces and a general improve- ow and enhance groundwater recharge, such as the dis- ment in the urban landscape’s hydraulic efficiency connection of downspouts from storm sewers, use of per- through the creation of roadside gutters, sewer networks vious pavement, and inclusion of lot-level storage and fi and lined stream channels. The result is increased surface in ltration features such as rain gardens and street swales runoff, more rapid water transmission through the drai- (Zimmer et al. 2007). Such BMPs include green roofs, nage network, and increased flood risk in terms of both which may be particularly appropriate for reducing run- peak flow magnitude and frequency of occurrence of that off in downtown areas where extensive impervious sur- flood magnitude (Leopold 1968; Hall 1984; Leith and face coverage and high land prices in downtown areas fi Whitfield 2000). make creation of vegetated space for water in ltration The most common way of dealing with enhanced very expensive (Roehr and Kong 2010). runoff in urban areas has been to carry it away as quickly as possible via underground pipes and sewers (Hall 1984). Older (pre-1950s) communities frequently Observed and anticipated climate-related trends in flood have combined sewer systems, where rainfall drains into drivers and flooding in Canada sewers carrying waste water and both are transferred to Research into historical trends of flooding in Canada and sewage treatment works. Since 1960, many new develop- its drivers at the national and regional scales, as well as ments have separate sewer systems where water from anticipated changes in flooding associated with climate gutters and roads may be carried through pipes to the change, has been spurred in part by the expectation that nearest watercourse, but often simply joins a combined climate change may intensify the water cycle via sewer (Waller 1976). Most modern urban flood drainage increases in ET and P (Huntington 2006). A major theme systems were designed to cope with rainfall events that of this work is the attempt to link any changes in flood- occur with a one-in-30-year probability (the design ing to historical and forecast trends in climate variables storm); however, older parts of the system may be oper- such as air temperature and P (Mortsch et al. 2015). ating to a lower standard. It is inevitable that the capaci- There was no significant trend in mean annual tempera- ties of sewers, covered urban water courses and other tures over the latter half of the twentieth century for the piped systems will sometimes be exceeded. When the entire country; however, Zhang et al. (2000) reported piped system is overwhelmed or cannot drain effectively that annual maximum temperatures increased by 1.5– into an outfall, the excess travels down roads and other 2.0°C during this period in northern BC and western NT, paths of least resistance, and floods low-lying areas. and cooled by ~1.5°C in northeastern Canada, with a These areas can contain property and infrastructure similar pattern for annual minimum temperatures. Annual Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 where flooding can cause costly damage, distress and P increased by 5–30% during this period across Canada, sometimes loss of life. with statistically significant increases mostly in the Arc- As an example, Toronto, Canada’s largest city, expe- tic. Such climatic changes (particularly the increase in rienced major flooding due to extreme rainfalls in 2000, annual and in some cases seasonal P) may result in 2005 and 2013 (Kovacs et al. 2014). The 8 July 2013 increased flood risk in various parts of Canada, espe- flood in Toronto resulted from 126 mm of rain generated cially if there is a corresponding increase in P intensity by two sequential thunderstorm cells. Rainfall intensity (Morgan et al. 2004). Zweirs and Kharin (1998) pre- far exceeded storm sewer capacity, and caused runoff to dicted that doubling of atmospheric carbon dioxide travel along city streets to creeks and rivers. The storm (CO ) would increase rainfalls with a 20-year return per- caused major transit disruptions and delays, road clo- 2 iod in Canada by ~14% using a global climate model sures, power blackouts, flight cancellations and flooding (GCM). Mailhot et al. (2012) used regional climate sim- across Toronto and Mississauga, including flooding of ulations to forecast large relative increases in annual 3000 basements. The Insurance Bureau of Canada esti- maximum P for a range of durations and return periods mated the 8 July storm costs at close to CAD $1 billion Canadian Water Resources Journal / Revue canadienne des ressources hydriques 17

for 2014–2070 for southern ON, southern QC and the ON. They concluded that the frequency of rainfall-gener- Prairies, with the smallest increases in coastal areas. ated floods is different during ENSO periods, with Nevertheless, evidence of increases in extreme rain- important consequences for water resources managers falls in Canada from the historical record that might sup- particularly in regard to flood forecasting and reservoir port such projections is equivocal. Thus, Kunkel et al. management. (1999) found no long-term trend in an extreme P index Analyses of temporal trends for floods in the north- for Canada for 1951–1993. Mekis and Hogg (1999) western boreal forest indicate that spring runoff is start- observed an increased fraction of annual P falling in the ing earlier in the year apparently due to increasing largest 10% of daily events with measurable P for 1940– spring air temperatures (Burn et al. 2004). Warm Pacific 1995 over a large portion of Canada; however, this trend Decadal Oscillation (PDO) phases (positive values) are may have been unduly influenced by stations in northern associated with warmer and drier winters, while cold Canada with relatively short records (Zhang et al. 2001). PDO phases (negative values) are linked to cooler and Zhang et al. (2001) noted no identifiable trends in either wetter winters. During warm PDO phases, the timing of the frequency or the intensity of extreme rainfalls at the annual maximum and snowmelt-induced floods in the national scale. They found a significant linear trend in northwestern part of the boreal forest shifts towards the spring heavy rainfall events for 1900–1998 in eastern spring, and moves towards the summer for the cold Canada, but no trends in heavy rainfall events in other PDO. Such decadal-scale climate fluctuations also affect seasons or regions. They suggested that the failure of flood magnitudes in the Boreal Plains, where periods of their analyses to support GCM projections of increased above-average P can increase the effective contributing heavy rainfalls made by Zwiers and Kharin (1998) and area for flood generation by increasing hydrologic con- others may possibly reflect the relatively early stages of nectivity in a basin through rising water levels in ephem- global warming induced by greenhouse gases. Shook eral lakes in closed depressions and areas of internal and Pomeroy (2012) found no increase in rainfall fre- drainage (Alberta Transportation 2004). quency or depth in single-day events in the Prairies over Changes in P associated with such variations in tele- records extending to 100 years, but an increase in the connections may be superimposed on long-term trends. number and intensity of multiple-day rainfall events was Thus, St-Laurent et al. (2009) ascribed an increase in the evident over much of the region, and in some cases the frequency of occurrence of major floods since the begin- number of multiple-day events has doubled. A substan- ning of the twentieth century in the St-François basin to tial increase in early spring and late fall rainfall can be increased P, although periods of below-average P (1950– hydrologically significant since these rains may cap fro- 1960 and 1980–1990) were also observed. Cunderlik and zen soils with ice layers that restrict infiltration or induce Ouarda (2009) observed no significant trends in the tim- rain-on-snow flooding. ing of rainfall-driven floods, but significant negative Research into trends in flooding drivers has examined trends in snowmelt flood magnitude over the last three the role of teleconnections between two dominant modes decades in southern ON. This was attributed to earlier of atmospheric variability – the North Atlantic Oscilla- snowmelt as result of a warming climate. Cunderlik and tion (NAO) and the Pacific/North America teleconnec- Ouarda (2009) also noted that basins with mainly snow- tion pattern (PNA) – and P amount and intensity. Stone melt-driven floods have a unimodal flood seasonality, et al. (2000) found seasonally increasing trends in total while those experiencing snowmelt-driven plus rainfall- P resulting from increases in all levels of event intensity driven floods in late summer and fall have a bi-modal in southern areas of Canada during the twentieth century. seasonality. They suggest the potential for flood season-

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 They observed a rainfall response to the NAO in north- ality at a station to change from unimodal to bi-modal eastern Canada in summer, while the PNA strongly influ- with time, which may imply an increasing intensity of enced P variations in southern BC and the Prairies. The rainfall events in recent years. PNA only influenced the frequency of heavier rainfalls In the Prairies, Shook and Pomeroy (2012) noted an in the autumn in ON and southern QC, with a negative increased rainfall fraction of P in March and clustering PNA generally leading to more extreme P events. The of summer rainfall events with greater numbers of multi- potential link between the NAO and extreme P in QC ple-day rainfall events. Dumanski et al. (2015) found may extend to flood characteristics (magnitude, fre- that these trends are causing an increase in flooding in a quency and duration), and Fortier et al. (2011) concluded small basin in SK and greater contributions from rain- that low-frequency oscillations in this teleconnection pat- on-snow. The appearance of rainfall-runoff high flows in tern may exacerbate flood conditions. Gingras and Ada- 2012 and the flood of record in 2014 suggest a rapidly mowksi (1995) compared flood magnitudes during and changing prairie hydrology where snowmelt runoff over outside of El Niño–Southern Oscillation (ENSO) periods, frozen soils is no longer the exclusive mechanism of and found a significantly lower probability of floods dur- flood generation, and flow volume from snowmelt is ing warm El Niño periods for five basins in southern exceeded by that due to rainfall-runoff. 18 J.M. Buttle et al.

Future and present trends in floods in mountainous floods generated by specific processes at the regional and regions must consider multiple flood-generating mecha- national scale. The deficiencies in the Canadian Disaster nisms and resist the temptation to generalize, since reli- Database (CDD) that were highlighted earlier need to be able detection of trends in floods in mountainous regions addressed. Since the public costs of floods are increas- is complex. This is exacerbated by issues associated with ing, more should be done to ensure that the CDD is pooling data with different start times, durations and complete and consistent. basin characteristics (Viviroli et al. 2012). Using a com- Changes to the family of snowmelt-, rain-on-snow-, mon period of observations, Cunderlik and Ouarda ice jam- and rainfall-generated floods related to climate (2009) showed that flood magnitudes are decreasing in variation and change continue to warrant attention, as AB and BC, although there appear to be no significant these processes affect most areas of Canada. Progressive trends in the magnitude of rainfall floods in streamflow warming has meant a shift from snowmelt-dominated records across Canada (Burn and Whitfield 2016). This flooding to rain-on-snow or rainfall-runoff flooding in is echoed by Harder et al. (2015), who found no trend in some areas, and these changes have challenged local peak flows over 50 years in Marmot Creek, AB which authorities who manage infrastructure or predict flooding. drains part of the front ranges of the Rocky Mountains. Rising sea levels and changes in storm frequency suggest Byrne et al. (1999) argued that shifts in synoptic types that storm-surge flooding may increase in coastal areas. in future climates could either increase or decrease floods This prospect needs to be addressed. based on basin location, while Whitfield et al. (2003) Approaches to improve our understanding of flood- showed that the type of future changes in BC’s moun- generation processes will by necessity vary between tainous Georgia Basin depends upon the hydrological regions across the country, given differences in such fac- regime. In rainfall-driven streams, modelled flood events tors as hydroclimate, geology, physiography and land increase in number but not magnitude, whereas in hybrid cover. For example, in the Prairie landscape there is a streams the frequency of winter events increases while need to consider the role of various hydrological pro- that of summer snowmelt floods decreases. In snowmelt- cesses in altering the states and locations of water and driven streams, the magnitude and duration of summer the consequent effects on streamflow generation (Shook floods decrease. All of these changes reflect the increas- et al. 2015). These transformations include wind redistri- ing domination of rainfall-driven events in this region bution and ablation of winter snowfall, snowmelt and (Whitfield et al. 2003). runoff over frozen soils, and filling and spilling of The few data available for time series analysis of depressional storage as runoff contributes to streamflow. river ice breakup events in the Arctic suggest that a gen- Improvements in understanding the flood hydrology of eral (although not statistically significant) increase in the Arctic have to be made in the face of a scarcity of magnitude and decrease in timing of maximum breakup gauged streams (Woo et al. 2008; Dugan et al. 2009) water level has occurred since the 1970s (von de Wall which limits knowledge of spatial variations in flood 2011). Restricted by similar station limitations and time generation across the Arctic, and the lack of complemen- period, a time series analysis by Monk et al. (2011) tary data (e.g. P, air temperature and soil moisture) revealed mostly non-significant trends in peak annual which complicates building a consensus on runoff runoff magnitude. response for the region (Kane et al. 2008). Expansion of the Arctic hydrometric monitoring network occurred during the International Polar Year Program (2007–2008); Conclusions and areas of future research however, wide-scale coverage is constrained by the

Downloaded by [University of Saskatchewan Library] at 06:56 29 January 2016 Flooding in Canada can occur from a broad range of expense of installing and maintaining such remote sys- processes, described here and in accompanying papers in tems. Efforts must continue to integrate data from the this special issue. While many processes responsible for existing network, climate reanalysis products, remote flood generation occur in most if not all regions of sensing approaches for estimating discharge and areal Canada, there are large regional contrasts in the predomi- extent of inundation on large rivers, and hydraulic and nance of specific generating mechanisms. These are hydrologic models to improve our understanding of flood expected to differ with both climate variations and characteristics across the Arctic. change in the future, and these similarities and differ- Similarly, the research needs associated with flood ences have been summarized above. processes that have received little attention to date or are A major research need highlighted in this paper is particularly relevant to Canada are varied. Thus, the pro- the importance of constructing a reliable historical flood cess of groundwater flooding needs to be considered record for Canada. This would assist such efforts as more explicitly by flood monitoring and risk-assessment improving our understanding of the processes driving processes. Although groundwater flooding can cause sig- flooding at particular locations, and the assessment of nificant social and financial impact, particularly to base- temporal changes in the magnitude and frequency of ments of structures located close to rivers (Jacobs GIBB Canadian Water Resources Journal / Revue canadienne des ressources hydriques 19

Ltd. 2006), impacts from this form of flooding could be the winter when soil freezing may limit water infiltration overlooked in the planning process. Engineering solu- (Zimmer et al. 2007). tions to overcome the risk of fluvial flooding, such as Human actions or inactions play an important role in levees, would not protect such developments from flood- both the changes of flood magnitude and frequency and ing caused by groundwater underflow through permeable the risk to society. Changes in land use also play an materials driven by high river stages. important and understudied role in changes in floods. Flood risks associated with storm surges in Canada Whether or not future floods increase in frequency or are strongly linked to the potential for climate change to magnitude, Canadians need to recognize that continued intensify weather systems and associated wind fields, development in floodplains and areas of urban develop- thus resulting in bigger surges and an east–west and ment increase the risk and potential costs of flooding. north–south shift in the tracks of the weather systems The decisions that our society makes will determine if (Danard et al. 2003). Such changes in storm tracks could flooding is going to remain the most common and costli- subject new areas to storm surge activity. Future work est natural disaster for Canadians. needs to build on previous attempts to model storm- surge events (e.g. Bobanović et al. 2006), given the influence of rising sea levels and increasing development Acknowledgements in low-lying coastal areas on storm surges in future cli- The authors acknowledge the significant contributions of the mates. Concurrent needs include an improved capacity to many colleagues who have over the past decades provided the foundation for the study of floods in Canada. Carole Holt- map potential inundation zones for storm surges that Oduro (Alberta Environment and Parks) provided unpublished might occur in populated coastal areas (Danard et al. groundwater data, and Blair Birch (Town of Canmore) provided 2003; Webster et al. 2004, 2006) to reduce or avoid the information regarding groundwater levels. The Hydrological flood risk associated with these events. Expertise Center of the Québec Department of Environment, Climatic shifts in most of Canada to wetter winters Sustainable Development and Climate Change (https://www. cehq.gouv.qc.ca/, accessed 18 September 2014) was an impor- and heavier summer rainfall events (Kunkel et al. 1999; tant source of information regarding floods in Québec. We also Institute for Catastrophic Loss Reduction 2012) also may express our thanks to the reviewers and editors for their sug- affect flooding in urban areas. Design storms based on gestions. specified return periods, durations and intensities have long been used to plan urban drainage systems, despite the imprecise definition of the design storm concept in References Canada (Marsalek and Watt 1984; Watt and Marsalek Ahmair, H., E. Blais, and J. Grehuk. 2016. The 2014 flood 2013). Concerns about climate change and the need to event in the Assiniboine River Basin: Causes, assessment and damages. 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