Wetlands occur where an excess of water saturates soils and collects at shallow depths on the land surface long enough to sustain waterlogged conditions. Geological characteristics determine how the water reaches the land surface and where it is distributed in watersheds. Where and how water is situated within wetlands, especially peatlands, is controlled by physical characteristics of the peat. Origins, composition and stratigraphy of wetland deposits depends on dissolved chemicals and materials derived from erosion and deposited from their surroundings. The rate and mode by which the sediment and peat has accumulated and decomposed through time, and how this carbon-rich material is stored are addressed using common geological and paleoecological tools. Further, these tools can be applied to elucidate more precisely what and how the geological connections are with wetlands, and what if any, the relationships are between the vast array of different wetland types. Wetlands are intimately connected to the surroundings of their individual basins and to their watersheds. The geology of wetlands focuses on understanding these relationships (Moore, 2001).

The importance of geology in wetland science seems to have escaped the attention of most geologists, despite covering about 15% and being a distinctive part of the Canadian landscape. Part of the problem is that wetlands have long been an enigma. Are they landforms, soil units, water features or vegetation communities? The answer is yes, to all of the above. Wetland science can be thought of as an amalgamation of all of these disciplines. Up until a decade or two ago, it was not possible to provide clear answers to these questions. Today, wetland science has become more refined and widely accepted as a distinct and unique discipline in the environmental sciences. A wetland classification system, begun in the 1970s, was an important development for defining and recognizing wetlands in Canada (Warner and Rubec, 1998). Though much remains to be learned about the intricacies of wetland hydrology, biogeochemistry, and biology, and its linkages with other environmental phenomena such as climate, natural and human-induced disturbances, and global sea levels, little advancement can be made in wetland science without regard for the important role of geology.

Canadian geologists were once far more involved with wetlands than today, and indeed, played a leading role in wetland studies in the early 1900s. The early interests revolved around geological surveys and economic evaluations of peat resources in the country. There is as much need today as there was in the early 1900s for Canadian geologists to be involved with wetlands and to re-assume their leading role, especially in view of current environmental and economic issues, such as climate change, landscape rehabilitation, sustainable wise use of economic resources, surface water and groundwater conservation and protection, and public and occupational health concerns. Perhaps the wane in interest in wetland geology in recent decades may be due to the narrow historical perception that geologists need only play a role with regard to peat as an economic resource and not with much broader issues such as understanding the origin, character, development, and management of the overall wetland resource in a country like Canada, whose wetlands are vast and varied.

Wetlands have historically been regarded as obstacles to development. Their widespread occurrence in this country has made it nearly impossible to avoid them. Technical and engineering skill, ingenuity, and luck have been instrumental in shaping Canada as it has come to be today. Engineers are moving away from their early and more traditional approaches that focussed on draining wetlands. Plans and designs now ensure that wetlands remain intact, and indeed, engineers are restoring and creating wetlands. This new outlook indicates an acceptance of the challenges of working with wetlands in the landscape and inventing new, more nature-sensitive technologies and codes of professional practice.

This paper examines what wetland geology is and points out how geology is related to wetland landforms in the Canadian context. There is discussion of some of the engineering and environmental applications of wetland geology. Wetland geology is far more relevant than is typically assumed. More geology needs to be included in high school and university courses that deal with wetlands and more wetland science needs to be included in courses on Quaternary and environmental geology and courses on related subjects. Governments at all levels need to accept a greater responsibility for ensuring care of Canada's valuable wetland resources for future generations. Much more needs to be done to increase awareness among both professionals and average Canadians of the relevance of linking geology and wetlands as biological landforms.

Much of the early work on wetlands in Canada focused on peatlands. Initial interests were primarily from entrepreneurs and engineers who investigated ways to mine and process peat for fuel (Warner and Buteau, 2000). The first commercial peat mining operation was establishing in 1864 on a bog near Victoriaville, east of Montreal, Quebec. Individual operations were started in Ontario, Newfoundland, and elsewhere in Quebec, but all of these operations ceased by 1877.

Interest in the use of peat for fuel was renewed in the 1890s but this interest was temporary. The fledgling peat industry at the turn of the century lobbied the federal government for assistance. This action prompted the Geological Survey of Canada to investigate the peat industry in Europe (Nyström 1908). The government purchased a small peat mining plant from Sweden for $50,000 and set it up in 1907 on the Alfred bog near Ottawa. Scandinavian geologists were hired by the Geological Survey of Canada to undertake national surveys of Canada's peatland resources. A series of reports were published over the next 25 yearsor so (e.g. Anrep, 1914, 1915, 1927; Auer, 1930; Haanel, 1923, 1925; Moore, 1908; Nyström, 1908; Nyström and Anrep, 1909). There was so much interest in peatlands during this period that a Canadian Peat Society was formed in 1911. This early society had no connection with the current Canadian Society for Peat and Peatlands, which began in the 1970s.

There was little interest in peatland geology during this early period outside of the Geological Survey of Canada. Ganong (1897) in New Brunswick and Lewis and Dowding (1926) in Alberta studied the stratigraphy and fossils preserved in peatlands. The advent of pollen analysis during this period in Europe, quickly led to collection of peat cores and publication of the first pollen diagrams in Canada (Auer, 1930; Bowman, 1931; Janson and Halfert, 1937; Hansen, 1940).

The general pattern and geographic extent of wetlands became much better understood once a scientific definition and a wetland classification framework were developed for Canada (Canadian National Wetland Working Group, 1988; Warner and Rubec, 1998). Inventories reveal that about 14% of the Canadian land surface and 125 X 10⁶ha is covered by wetlands, about 12% and 110 X 10⁶ ha of which are peatlands, making Canada the most peatland-rich country in the world (Fig. 1; Tarnocai et al., 2000, 2001). About 18% of the world's wetlands occur in Canada (Tarnocai et al., 2001). These figures for the most part do not include freshwater wetlands along the shores of lakes, banks of rivers, in estuaries and along the marine coasts.

There is a striking match between the peatland map of Canada (Fig. 1) and a map showing areas covered by proglacial lakes and inland seas (Fig. 2). The flat till plains and clay, silt and sand glaciolacustrine and glaciomarine plains laid down in large inland lakes and seas created poorly drained areas that have contributed to the widespread wetlands in Canada today. A notable example is in the southern parts of the Hudson and James Bay basins, where proglacial lake and seabed deposits have contributed to development of one of largest peatland complexes in the world.

There are obvious anomalies, notably in the south-central Prairies, southern Ontario, and the Ottawa-St. Lawrence valleys, where there are till plains and glaciolacustrine and glacio-marine clay, silt and sand deposits but few peatlands today. These regions, in fact, did support widespread wetlands and/or peatlands in the historical past, which subsequently became filled in, ploughed over, and drained. These regions, however, are especially good for wetland restoration and creation because the underlying glaciolacustrine and glaciomarine deposits provide good foundation material on which to reestablish wetlands. Other anomalies exist on the hyper-oceanic north coast of British Columbia and in Newfoundland and the east coast of Labrador where there are no significant glaciolacustrine and glaciomarine deposits but peatlands are common. Peatlands have arisen on these coasts because a hypermaritime climate sustains abundant precipitation and low evaporation, allowing peat to form directly over the impervious bedrock.

Table 1 shows how geological attributes can be used as criteria for a simple wetland classification in one part of the country. An extension of this kind of geological classification is to include sediment composition, stratigraphic, and paleoecological characteristics into what in Europe has been termed geo-hydrological or biogenetical classifications (e.g. Succow and Lange, 1984).

All landforms comprise solid material, gases and water. The most obvious difference between wetland landforms and all others is a predominance of water and organic or biological materials, most of which is peat, rather than the usual mineral or nonbiological materials (Warner 2003b). Some wetland landforms contain more organic material than others, and some are wetter than others, but constitute landforms nonetheless.

Wetland landforms have to be treated differently from the typical mineral landforms because biological processes dominate them. Wetland landforms have two primary structural layers. There is an outer or surface layer of soil with living plants, microbes and other biota, and a deeper layer comprising the dead remains of the biological inhabitants that once were part of the surface layer. Oxygen content is many orders of magnitude greater in the surface layer than in the deeper layer. The lower boundary of the surface layer is usually indicated by the lowest position of the water table in any given year. The upper boundary may be a water surface in the case of the wettest wetlands or the top of the living vegetation canopy in the case of the driest peatlands. The presence of living biota in the surface layer has led to wetlands being referred to as living landforms, and as such, they are dynamic and constantly changing.

Two broad types of wetland landforms have been distinguished in Canada. One group is organic wetlands, typically referred to as peatlands (Fig. 3a). Bogs and some fens and swamps are peatlands. Peatlands have a well-developed surface layer that has been described as having as many as six sub-units within it (Clymo, 1983; van Dierendonck, 1992). The surface layer in permanently frozen peatlands is more complicated, but probably can be thought of as having at least two distinct subunits, because the depth to permafrost table and depth to water table may be the same during dry periods and the water table can lie above the permafrost table during wet periods (Vardy et al., 2000). Water-logging or not above the permafrost table, therefore, affects oxygen conditions and the opportunity for peat material to survive decomposition and longer term accumulation. Much more work is needed to understand peat accumulation processes in permanently frozen peatlands. The term acrotelm has been used to refer to this surface layer in bogs (Ingram, 1978).

There is no maximum thickness for the deeper layer. Depths of 12 m have been recorded in Canada, the thickest known from peatlands in the Mackenzie River Valley (Tarnocai et al., 2000) and from a kettle hole near Cambridge, Ontario (Warner et al., 2004). Catotelm has been used to refer to the deeper layer in bogs (Ingram, 1978). The deeper layer determines the overall size, shape, composition and storage capacity of the peat landform, while the surface regulates the growth and formation of peat materials and accumulation processes by selecting and regulating which dead biota will reach and become stored as peat in the deeper layer below it.

The second type of wetland landform is mineral wetlands, which have a surface layer of living biota and most often are inundated, by definition, with no more than 2 m of water. They have either a thin and poorly developed or a complete lack of a deeper layer of accumulated organic material (Fig. 3b). In Canada, a minimum measurement of 40 cm for thickness of the two layers is used to separate peatland from mineral wetland (Warner and Rubec, 1998). Marshes, shallow open water, and some fens and swamps are mineral wetlands.

The water in both peatlands and mineral wetlands can be either permanently frozen, as is the case in permafrost regions, intermittently frozen for some years and not others, or only seasonally frozen. Both mineral wetlands and peatlands can be either permanently or intermittently covered by water for all or part of the year.

Most wetlands in Canada originate as mineral wetlands. Geological setting ensures that a steady supply of water remains on the land surface, enough to support wetland biota, thereby determining where wetlands occur in the landscape. Climate is also important because it can both supply and remove water, and so too, determines how much water is stored on the land surface. If either geological setting or climate does not allow water to stay on the land surface, a wetland will not form, or if either of these factors changes once a wetland has formed, the wetland will cease to exist.

Most peatlands develop out of mineral wetlands. Mineral wetlands occur in high-energy settings and often exchange water with adjacent landscape units (i.e., lakes, rivers or oceans). Peatlands occur in low-energy settings and are more independent of adjacent landscape units. Site-specific features of the hydroperiod and geochemistry of the waters influencing the site will determine the specific type of peatland that will develop from the mineral wetland, be it swamp, fen or bog. Eventually, a deeper layer of peat will become well developed, and organic materials will continue to accumulate and be added to the lower layer during the lifetime of the peatland.

Peatlands may also develop out of lakes. The term terrestrialization is used to denote the process of basin-infilling whereby a lake transforms into open water wetland and thereafter a peatland. A key attribute in the terrestrialization process is the position of the water table in the basin relative to the surface of the sediment. The water table is highest relative to the surface of the organics during the lake stage and early stages of wetland development. As organic matter continues to accumulate it reaches the water table, at which point the wetland develops into marsh, fen or swamp. The precise time at which lake becomes wetland, the timing of change from one wetland phase to another, and the nature of the final stage in the terrestrialization sequence is dependent upon the size and depth of the basin and the hydrological setting, assuming climatic factors or some other external force (e.g. human disturbances) does not interrupt the terrestrialization processes.

The ongoing accumulation of organic materials in the deeper layer may eventually cause a peatland to outgrow the physical confines of its basin. As the deeper layer thickens and moves out of the basin, it will essentially convert into peatland what was originally terrestrial land around the basin. This process is referred to as paludification. It is possible that overflowing peat from two adjacent basins may coalesce, thus producing a single peat landform out of the original two smaller separate peatlands. The large peatland landscapes and peat landforms in Canada represent mature landform complexes that grew out of many smaller independent wetland basins and depressions.

Raised bogs and some plateau bogs are good examples of peatlands that have outgrown their original basins (e.g. Warner et al., 1991). The net result is that the top of these landforms is higher in the center than in the periphery. A steady supply of precipitation is required to sustain the biota and peat accumulation processes. The dependence on precipitation as the only source for nutrients has led to specialized biota adapted to survival in nutrient-poor growing conditions.

In regions influenced by permafrost, water migrates along the thermal gradient from warm to cold, thereby feeding ice in the frozen core. The combined annual addition of peat along with an expanding ice core may cause these peatlands to grow beyond the confines of its basin and the surface to grow higher than its edges. Palsas and high-centred polygonal peatlands are examples of permanently frozen peatlands.

Terrestrialization and paludification processes have been going on throughout all of postglacial time (e.g. Bunting and Warner, 1998). These processes can be quite predictable provided climate, human activities or some other external factor has not altered the geological-hydrological-biological balance in the wetland system. However, the stratigraphic records of many peatlands reveal that there have been many interruptions to this balance, which has steered off course the "natural" direction and rate of the terrestrialization or paludification pathway. Peatlands are important repositories of paleoenvironmental information (Warner and Bunting, 1996). As such, they can be long-term sensors and archives of climatic shifts (e.g. natural and human-induced), geomorphic changes (e.g. sea levels, isostatic adjustments), natural catastrophes (e.g. storms, hurricanes, volcanic eruptions), and human activities (e.g. early cultures, land uses, pollution). The preserved disarticulated paleobiological indicators in the peat and organic sediment or geochemical markers in the peat and deep interstitial waters have been the primary tools for reconstructing these past events. The widespread abundance of peatlands makes them valuable archives of past events for many periods in pre- and postglacial time and at differential spatial scales for most of Canada.

It would be very useful if mineral wetlands and peatlands were differentiated on geological maps. At least 100 different wetland types occur in Canada (Warner and Rubec, 1998), Further recognition of terrestrialized and paludified wetlands would greatly refine wetland mapping and interpretation of landscape origins.

Wetlands represent landforms of various ages. Their organic composition presents an added benefit in that they can be used to age themselves, their internal archival records, and by inference, adjacent landscape units. The high-energy depositional environment and lack of a steady accumulation of organic materials makes it difficult to determine the age of mineral wetlands. This may also be a problem for peatlands whose early histories included a mineral wetland phase. For the most part, peatlands provide good dating control and continuous chronologies for large time spans, if not for their entire lifetime.

It is common in geology to regard the age of the bottommost organic sediments in wetlands as minimal estimates for the time of deglaciation or withdrawal of proglacial water bodies. This may be reasonable in situations where wetland inception occurred early, but not where wetland inception was delayed because climatic, geological, hydrological or biological factors caused a lag in wetland formation. Such lags are probably more common in Canada than might be expected because paludification, a major process responsible for peatlands throughout central Canada, did not become widespread until Middle to Late Holocene time in the central Boreal Ecozone (Vitt et al., 2000) and likely in other regions of the country too (e.g. Warner et al., 1991).

Not all peatlands will continue to grow indefinitely so it is equally important to determine the time at which peatland growth ceases. The age of the surface of permanently frozen peatlands throughout the Arctic and Subarctic Ecozones has been shown to be many thousands of years (Zoltai and Tarnocai, 1975; Vardy and Warner, unpublished). As such, some peatlands are relict landforms, no longer actively accumulating peat, while other peatlands may even be slowly degrading, some even back to their mineral wetland origins. Another example is swamps in southern Ontario where the near-surface peat is old. The surface layer has been "running on the spot" with no net addition of new materials reaching the deeper accumulation layer (Warner, unpublished).

It is especially important to understand the role of humans during the historical past on wetland developmental processes. Human activities, both direct and indirect, have been important in interfering with and even halting developmental and peatland growth processes. Alternatively, human activities may be responsible for setting up conditions conducive to wetland formation or accelerating developmental processes (e.g. Warner et al., 1989). Geologists have an important role to play in assessing human impacts on wetlands, because not all intact wetlands are as natural or pristine as we might assume.

Clymo's (1984) studies on peat accumulation and peatland growth started a whole new way of thinking of peat accumulation processes. His models were applied for the first time in North America on a site in New Brunswick, the most comprehensively dated peatland in North America (Warner et al., 1993). Subsequently, a number of studies on both the short-term and long-term peat accumulation processes have been undertaken in Canada (e.g. Belyea and Warner, 1996; Frolking et al., 2001) but there remain considerable gaps in our knowledge about peat accumulation processes in permanently frozen peatlands (Zoltai and Tarnocai, 1975; Vardy et al., 1996).

Peat is about 50% carbon (as dry matter). To understand peat accumulation is to understand carbon accumulation and storage in peatlands. Northern wetlands, especially peatlands, play a major role in global carbon cycles and climate change because they fix carbon from atmosphere in the biomass of the living vegetation. When the vegetation dies, the dead biomass eventually becomes peat in the surface layer where it is subjected to decomposition. That fraction remaining accumulates as peat in the deeper layer where it is stored for millenia. Canadian peatlands contain approximately 262 Gt of organic carbon, which is about 59% of the total carbon stored in Canadian soils (Tarnocai 1998). Canada contains about 18% of the worlds wetlands (Tarnocai et al., 2001) which means that Canada contains a significant proportion of the global terrestrial carbon pool. Geologists havea major role to play in assessing peatland carbon stocks, characterizing the dynamics of carbon cycling, and developing predictive models of carbon and climate change in Canada (Gorham, 1991; Frolking et al., 2001; Blodau, 2002). It would seem there is great value in differentiating "dead" and "living" wetland landforms and giving special conservation status to those systems most active in removing atmospheric carbon and sequestering the carbon as peat. Other important questions revolve around changes between carbon cycling processes, degree of climate changes, and occurrence and distribution of permafrost. How accurate are our inventories or complete is our knowledge of the extent and dynamics of perennially frozen wetlands, estimated to occupy approximately 42%? How might climate warming affect the carbon balance in these systems (Tarnocai, 1999)?

Wetland resources are important to the development and economic prosperity of Canada and play pivotal roles in supporting a prosperous and vital wetlands industry today (Warner, 2003a). It is difficult to avoid wetlands completely because they are so widespread and as such present major environmental and engineering challenges. The general tendency historically has been to drain wetlands and convert them to dry land if they got in the way. In recent years, this attitude has changed with realization of the positive values of both natural and of created and restored wetlands.

Canada contains more wetland than any other country in the world, and Canadian geologists and engineers have an opportunity and responsibility to take a lead in advancing wetland knowledge and technological development. Geologists have a role to play in our national carbon inventory because the net carbon stored in our wetlands greatly exceeds the carbon stored in our forests and agricultural soils. How or if wetland carbon is priced in economic terms for sale and trade on the world market has yet to be determined, but could prove to be important in assisting Canada to meet its international obligations to off-setting carbon emissions now that Canada has ratified the Kyoto Protocol.

There are a number of major wetland issues in the world that geologists need to be aware of because it is probably only a matter of time before Canadians may be confronted with similar problems. For example, there is a large oxygen depleted zone at the mouth of the Mississippi River in the north-central Gulf of Mexico that has caused major economic losses to fisheries. The "dead-zone" is attributed to agricultural nutrients and high sediment input brought about by the loss of wetlands throughout the entire Mississippi basin (Rabalais et al., 2002). A major program has been initiated to restore wetlands throughout the whole drainage basin, especially in riparian zones along stream and river courses to control erosion and nutrients reaching the Gulf of Mexico (Mitsch et al., 2001). It is not that long ago that Lake Erie was considered "dead". How can we best use wetlands in Canadian agricultural landscapes to help with controlling runoff and attenuating nutrients? Might more wetlands have helped to protect surface and drinking water supplies from deadly agricultural-derived pathogens in the Walkerton disaster?

Mobilization of natural mercury and its entry into the human food chain by hydroelectric reservoirs flooding of peatland terrain is another way that wetlands relate directly to human health issues. The natural source of the mercury has not been confirmed in all cases, though it is suspected to be in the parent soils and bedrock and undergoes geochemical transformation when soils become anaerobic by flooding (St. Louis et al., 1994).

When they are set ablaze, wetlands pose serious natural health hazards by contributing smoke andhaze. The burning peatlands of south Florida, USA, Kalimantan, Indonesia, and around Moscow, Russia are recent examples of global significance illustrating the magnitude of threats to human health. Many of the forest fires that occur annually in northern and central Canada include peatlands. Fires, large and small, are important for the ecological integrity of many natural ecosystems including wetlands, but the magnitude and intensity of fires, exacerbated by drought conditions in recent years, will continue to require watchful care in the near future. Coastal wetlands along the eastern seaboard of the USA are being eroded at extremely high rates for reasons not yet clearly understood (Hartig et al., 2002). The problem has recently attracted the media attention with feature presentations by CNN Television. Is it possible that parts of the Canadian coast and its wetlands farther north are disappearing at similar rates? High peat cliffs on the coasts of New Brunswick and Prince Edward Island are clear examples of coastal wetland loss by transgressing sea levels (Fig. 10).

Wetlands are central to terrestrial water cycles. Damming of rivers continues to be a problem because it changes river flow volumes, increases time taken to drain to the oceans which causes the water to age and change ecosystems, and there are alterations to natural biota and biodiversity. River diversions and channelization may connect flows across drainage divides. In many parts of the world, water control structures have fallen into disrepair and require replacement or renovation. The alternative is to abandon the structures, which presents the need to restore rivers and clean old reservoirs. Wetlands should be considered in the context of both cause for change and consequence of such changes in construction and reconstruction plans.

Widespread spring flooding, which seems to be common in Canada, is a direct result of the way humans have modified the natural land surface. Would more wetlands and natural vegetation cover have been able to store more spring meltwater, reduce surface flows, and save the enormous property loss in such recent floods as in the Saguenay area of Quebec or around Winnipeg and the southern Prairies?

Every Canadian probably comes in contact with wetlands, either directly or indirectly, on a daily basis. Wetlands are distinctive features of the Canadian landscape and are part of our identity. We have always had to deal with wetlands, whether we wanted to or not, and we will continue to need to do so. Wetland geology is probably more important than ever because we now know that wetlands are unique and distinctive parts of the landscape and specialized knowledge and skill is required to deal with the many new environmental and economic issues that will be facing Canadians in years to come.