Paleoenvironmental Perspectives on Drought in Western Canada

Canadian Water Resources Journal Vol. 31(4): 197–204 (2006) © 2006 Canadian Water Resources AssociationRevue canadienne des ressources hydriques

Scott St. George1,2 and David Sauchyn3

1 GSC Northern Canada, Geological Survey of Canada, Ottawa, ON K1A 0E82 Laboratory of Tree-Ring Research and Department of Geosciences, University of Arizona, Tucson, AZ 857213 Prairie Adaptation Research Collaborative, Regina, SK S4S 7J7

Submitted September 2006; accepted October 2006. Written comments on this paper will be accepted until June 2007.

Paleoenvironmental Perspectives on Drought

in Western Canada — Introduction

Scott St. George and David Sauchyn

Abstract: Paleohydrology uses indirect evidence to describe the behaviour of hydrological or hydroclimatic systems prior to the initiation of direct monitoring. This evidence can be derived from either human sources or natural archives. The articles in this issue show how paleoenvironmental data may be used to: i) place recent observations within a context of the past several hundred years; ii) evaluate the impact of human modification of hydrological systems; and iii) examine the reliability of hypothesized connections between regional hydroclimate and remote climate forcings. As a group, these papers illustrate the dynamic and changing nature of the hydrology of western Canada, and provide a long-term perspective that can be crucial for good stewardship of water resources.

Résumé : La paléohydrologie fait appel à des preuves indirectes pour décrire le comportement des systèmes hydrologiques ou hydroclimatiques avant que la surveillance directe n’intervienne. Ces preuves peuvent être tirées de sources humaines ou d’archives naturelles. Les articles du présent numéro montrent comment on peut se servir des données paléoenvironnementales pour (i) situer les observations récentes dans le contexte des quelques derniers siècles; (ii) évaluer l’impact de la modification anthropique des systèmes hydrologiques et (iii) examiner la fiabilité des rapports hypothétiques entre le forçage hydroclimatique régional et le forçage climatique à distance. Regroupés, ces articles illustrent bien la nature dynamique et changeante de l’hydrologie dans l’Ouest du Canada et offrent une perspective à long terme pouvant s’avérer cruciale pour la bonne intendance des ressources hydriques.

Introduction

Hydrology is an observational science that relies heavily on the assembly and analysis of direct measurements of streamflow, lake fluctuations, drought severity and many other variables (Maidment, 1993). Such

measurements are used in numerous water management applications including projecting hydroelectric power production, estimating future drought and flood risks, and setting maximum allowable withdrawals from rivers and aquifers. Many of these applications are forecast problems, whereby the future behaviour of a

198 Canadian Water Resources Journal/Revue canadienne des ressources hydriques

© 2006 Canadian Water Resources Association

hydrological system is estimated from the dynamics it has been observed to exhibit in the past. This approach generally assumes that the hydrological and climate system does not change over time (at least not at timescales that are relevant to the future under consideration), and that the period of direct observation provides a good baseline of hydrological variability within a watershed.

The longest instrumental records of river flow in Canada span just over 100 years (Ashmore and Church, 2001). In many parts of the country, but particularly in northern Canada, continuous streamflow monitoring did not begin until the 1970s. Lake and groundwater measurements are similarly often restricted to the latter half of the 20th century. By comparison, the present network of rivers, lakes and aquifers across Canada has been in place since the most recent retreat of the Laurentide ice sheet (between ten and five thousand years ago, depending on the proximity of the retreating ice sheet to the watershed in question; Dyke, in press). Thus, although the current network of observations is clearly the foundation of most hydrological studies in Canada, the period covered by direct measurements makes up only a tiny fraction (one-half to two percent) of the total interval that the system has been in operation. It is therefore an open question as to whether observations of discharge, lake levels or groundwater fluctuations are adequate to describe the dynamics of hydrological systems or to provide a good indication of future behaviour. Short hydrological records pose a particular problem for estimating the risk of extreme events such as floods or droughts, which are by their nature infrequent.

Paleohydrology uses indirect evidence to obtain detailed information about the behaviour of hydrological or hydroclimatic systems prior to the initiation of instrumental monitoring. This evidence may be derived from human sources, such as personal journals, expedition reports or commercial correspondence, or from natural archives, such as tree rings, lake sediments, sand dunes or glacial ice. Paleohydrology expands our perspective to include hydrological changes that occurred during the last several centuries or several thousand years. This view allows us to place observations within a much longer context, either to evaluate the significance of recent trends towards lower or higher streamflows or to determine how often extreme floods or droughts have occurred in the past. Understanding how and why hydrological systems

have changed in the past is also necessary for accurate prediction of their future behaviour, as processes linked to natural climate variability will continue to operate in the future, regardless of any changes wrought to the climate system by human activity.

Probably the best-known application of paleohydrology to water resources management is that by Stockton and Jacoby (1976), who used tree rings to estimate past discharge in the Colorado River back to AD 1512. This study showed that average flows in the Colorado River were, over the long-term, much lower than the total amount of water allocated to American states and Mexico. Subsequent research, using new statistical methods and updated tree-ring and hydrological data (Hidalgo et al., 2000; Woodhouse et al., 2006), have verified the main results of this earlier work and confirmed that severe, sustained droughts are a defining feature of the Colorado River Basin (Figure 1). Stockton and Jacoby demonstrated that paleohydrological science could make a vital contribution to water resources management and opened the door for related studies in other parts of the United States (Stine, 1994; Cleaveland, 2000; Graumlich et al., 2003; Meko et al., 1995; 2001; Woodhouse, 2001).

Paleoenvironmental Perspectives on Drought in Western Canada

The articles that appear in this issue originated from a special session in the June 2005 biennial meeting of the Canadian Quaternary Association in Winnipeg, Manitoba. Sponsored by Manitoba Hydro, the Geological Survey of Canada and the International Geoscience Programme (Project 500—Dryland Change: Past, Present and Future), this session brought together researchers from Canada and the United States to define the state-of-the-art with respect to paleoenvironmental insights into drought in western Canada. Presentations addressed a diversity of topics ranging from the long-term reliability of water supplies, fire in the boreal forest, the impact of hydroelectric projects on hydrological and ecological systems, and perceptions and responses of Aboriginal peoples and early Euro-Canadians to past drought. Although the special issue has a strong focus on paleohydrological inferences from tree rings (and, to a lesser degree, historical records), it is important to emphasize that

St. George and Sauchyn 199


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studies of past hydrological change in western Canada have not been restricted to only these two archives. Lake sediment cores have been used to address a wide variety of ecological and environmental questions (Leavitt et al., 1999; Lewis et al., 2001; Cumming et al., 2002; Last and Vance, 2002; Laird et al., 2003; Moos et al., 2005; Wolfe et al., 2005), while records of past sand dune activity and quiescence have provided significant insight into the complex relationship between climate variability and semi-arid landscapes (Running et al., 2002; Wolfe, 1997; Wolfe et al., 2001; 2002).

The articles in this volume are presented in a rough west-to-east arrangement, with the exception of the final contribution, which discusses paleohydrological work in the United States. The first paper, by Emma Watson and Brian Luckman, briefly reviews the basis of dendrohydrology and its applications in western Canada and then describes a suite of results from an intensive research program in the Canadian Cordillera. A network of moisture-sensitive tree-ring sites in southern British Columbia and Alberta indicate that the period from 1917 to 1941 was the most prolonged and spatially extensive dry event in the region during the past 350 years, but also that droughts of equal or greater magnitude have occurred as often as once or twice a century. Extended precipitation records for this region are also used to demonstrate that droughts commonly coincide with El Niño events and anomalously cool conditions in the northeastern Pacific Ocean. Finally, the authors observe that the long perspective afforded by tree rings can be critical to understand the behaviour of river and glacial systems, particularly in situations where it is not possible to distinguish true secular trends from longer-term fluctuations (such as those associated with the Pacific Decadal Oscillation) using short instrumental records alone.

The next article by David Meko uses tree-ring data to examine long-term ecological and hydroclimatic variations in the Peace-Athabasca Delta (PAD). The construction of the W.A.C. Bennett Dam on the Peace River in 1968 has led to questions about the relative, possibly competing, influences of flow regulation and climatic variability on the health of the PAD, which is the largest boreal freshwater ecosystem in the world. This study describes the development of a new network of tree-ring sites within the delta and uses these data to increase the length and robustness of previous water level reconstructions. These new estimates extend back to AD 1801 and suggest that summer water levels

were at their lowest point near 1890, primarily due to low flows on the Peace River. Furthermore, although the post-dam period is characterized by relatively low water levels and high variability, the author reports that there is no clear statistical evidence that the Bennett Dam has affected summer water levels in the delta.

Similarly, Antoine Beriault and Dave Sauchyn use tree-ring records to evaluate hydrological changes in the Churchill River Basin in northern Saskatchewan, which contributes roughly 19 percent of the total energy supply of Manitoba Hydro. The authors describe a new network of tree-ring chronologies for this watershed that extend back to the early or mid-19th century; the overall length of these records is limited by the lifespan of trees growing in fire-dominated boreal forest environments. The tree-ring data are significantly correlated with mean annual and summer streamflow across the watershed, and those records that were highly (above 0.6) correlated with streamflow were chosen as potential predictors in linear regression models. Robust models (accounting for between 40 to 53 percent of total variance) were developed for three gauges in the basin and used to estimate annual streamflow at these locations since ca. AD 1840. Periods of above average mean annual flow include 1853 to 1882, 1894 to 1904, 1932 to 1936, and 1946 to 1979. Annual flows were consistently below average during 1840 to 1852, 1883 to 1893, 1905 to 1921, 1937 to 1945 and 1980 to 1997. Although the proxy records in this paper are relatively short by tree-ring standards, they indicate that some of the most severe and sustained low flows in the last two centuries have occurred in recent decades.

The next paper, by Bill Rannie, delves into the climatic context of the famous Palliser and Hind expeditions of 1858 and 1859. The author points out that the long-held view that John Palliser visited the Canadian Prairies during a drought is, at best, an incomplete rendering of the weather that affected the region in those years. At the same time that Palliser was reporting dry conditions in Alberta (especially in southern Alberta), Henry Hind was dealing with severe thunderstorms and frequent heavy rainfall in his travels across southern Manitoba. This paper highlights the difficulty in mapping the spatial extent and severity of droughts from sparse networks of proxy observations, and warns against making broad inferences about the entire Canadian Prairies from data that are limited to only the eastern or western sectors.



Severe drought is often a precursor to extensive forest fires. Martin Girardin and his colleagues describe the variability of weather conditions that promote fire in the boreal forests of central and eastern Canada. Changes in fire frequency can have an important effect on water quality, as fires often cause enhanced erosion and soil leaching, which leads to increased particle and nutrient loads in streams. The authors have assembled a large dataset of tree-ring sites in Manitoba, Ontario and Quebec, and use them to develop regional estimates of summer drought severity as represented by the Canadian Drought Code. These reconstructions indicate that large-scale droughts became more frequent in the western part of the network (Manitoba and northwestern Ontario) after around AD 1850. No trends were apparent for the central and eastern boreal forests. Based on the synoptic characteristics of recent droughts, the authors interpret this change as a result of an increasing frequency of meridional (north-south) upper-air circulation over Canada during the summer.

In the final paper, Connie Woodhouse and Jeff Lukas describe their efforts to integrate paleohydrologic data into the management of water resources in Colorado. In their view, the recent severe drought in the western United States led water managers to look more closely at tree rings as a potential source of insight into the relative magnitude and frequency of the current drought. Although the authors report that most water agencies in Colorado were interested in the same basic questions (especially relating to the range of past hydrological variability), each institution required data products and communication approaches that were tailored to its specific needs. For example, while smaller agencies were satisfied to use tree-ring data to obtain a qualitative assessment of recent variability in streamflow within the context of the last several centuries, larger water providers requested extended time series of key hydrological parameters that could be used to drive numerical supply models. Woodhouse and Lukas stress the importance of ongoing and interactive engagement between scientists and water managers and suggest that evidence of more persistence and severe droughts in the tree-ring record has begun to change the way in which water managers regard the character of current and future drought in Colorado.

Future Directions

Paleohydrology is still a relatively young science in western Canada, but one that has grown dramatically during the past ten to 15 years. Recent collections have steadily filled the geographic gaps in the network of proxy evidence in the region and have demonstrated that past hydrological conditions can be outside the bounds determined by direct observation. Collaborations with American scientists have led to a more cohesive outlook on the hydroclimate of northern North America, one that is not limited by arbitrary limitations set by political boundaries (Laird et al., 2003; Cook et al., 2004; Figure 2). Furthermore, although most of the research presented in this volume is focused on precipitation, drought indices, river discharge or lake levels, proxy evidence has also been used to document other hydrological parameters in western Canada, including groundwater levels (Ferguson and St. George, 2003), glacier mass balance (Watson and Luckman, 2004; Larocque and Smith, 2005) extreme floods (Gottesfeld and Gottesfeld, 1990; St. George and Nielsen, 2003) and ice jam events (Smith and Reynolds, 1983).

More and more, paleoenvironmental scientists are being called on to provide a long-term perspective that can be crucial for good stewardship of water resources. Experience in American jurisdictions has shown the importance of ongoing collaboration, as researchers refine their scientific products to meet the needs of water managers, and water managers learn more about the nature of proxy climate information and explore how these data can be incorporated into current planning tools and methods. We hope that this special issue contributes to an improved understanding of the dynamic and changing nature of the hydrology of western Canada, and leads to closer cooperation between paleoenvironmental scientists and water resource professionals.

Acknowledgements

We thank Editor Don Burn for allowing us to act as guest editors for this issue, and for providing the opportunity to present a paleoenvironmental perspective on drought to the audience of the Canadian Water Resources Journal. Support for the publication of this issue was provided by the International Geoscience Programme (Project 500



– Dryland Change: Past, Present and Future). We would like to express our thanks to our colleagues who contributed presentations to the drought session at the 2005 Canadian Quaternary Association meeting in Winnipeg. We also are grateful to Jim Teller and the rest of the CANQUA organizers for encouraging us to prepare the session. Finally, we appreciate sincerely the work done by the referees of the papers in this issue, whose detailed comments and thoughtful suggestions greatly improved the quality of the final product.

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Paleoenvironmental Perspectives on Drought in Western Canada