3.1 Watershed and meteorological conditions leading up to the spring freshet

3.1.1 Flow generating mechanisms in Ontario

The main causes of riverine and lake flooding are snow and ice melt, intense and/or long-lasting precipitation, rain on snow, riverine ice jams, or a combination of these causes. The risk of flooding is also influenced by conditions within the drainage basin, such as pre-flood water levels, presence of snow and ice, the soil moisture content, how early and deep the frost, and land use change, including more efficient rural drainage and urbanization. Some of the largest notable floods in Ontario have occurred as a result of major rainstorms. For this reason, riverine flood hazards limits are generally defined based on the flood resulting from the actual rainfall experienced during a major rain storm, such as the Hurricane Hazel storm (1954) or the Timmins storm (1961), transposed over a specific watershed or the 100-year flood, whichever is greater.

In Ontario, streamflow regimes are generally classified as snowmelt-dominated, whereby most of the winter precipitation falls as snow and melts during the spring. Temperature affects the type of precipitation (i.e. rain versus snow), the accumulation of a snowpack, and the timing and amount of ice and snowmelt runoff, while influencing the snow water equivalent of the snowpack. Snow water equivalent defines the amount of liquid water in the snowpack that would be formed if it were completely melted. Precipitation determines the potential magnitudes of flow generated during different times of the year. The timing of high spring flows in snowmelt regimes are also affected by geographic location, whereby snowmelt occurs later further north and at higher elevations, and is affected by the size of the drainage basin and the amount of storage within the drainage basin as affected by the size and number of lakes and wetlands. In more southerly areas of the province, streamflow regimes, while influenced by snowmelt, can be less defined by it.

The amount of snow on the ground, or the water that it contains, can often be a driver or indicator of the potential for flooding. The amount of this water that becomes runoff for the river depends on the timing and rate of snowmelt. For example, a very rapid melt of an average snowpack could lead to flooding. Conversely, a slow melt of a very heavy snow pack may not result in flooding. This becomes challenging for water managers as the impact of the snow on the river is heavily driven by the weather, which is difficult to predict. This provides a rationale as to why in some years with higher than average snow on the ground, flooding may not be experienced, whereas years with an average snowpack significant flooding may be observed.

3.1.2 Summary of general meteorological and watershed conditions

A series of noteworthy, atypical and synergistic factors contributed to the severity of the flooding in the spring of 2019. Environment and Climate Change Canada has characterized the 2018/19 winter as very long and extremely cold, including higher than normal snowfall across northern, central and eastern Ontario. Furthermore, this year’s winter did not experience the one or more significant thaws that has come to characterize winters in south-central Ontario. The result was a well above average snowpack on the ground in early April, with the snowpack experiencing a rapid melt by mid-April, and a considerable depth of snowpack remaining past mid-April in those areas experiencing flooding. This melt was intensified by a series of strong large-scale storms that resulted in significant rainfall amounts in the latter portions of April, whose occurrence was widespread across the portion of the province experiencing spring flooding.

While April’s temperatures in the northwest, the far north and southern Ontario were in the normal range, most of the province, including those areas that experienced the most notable flooding in the spring of 2019, also experienced temperatures 1 to 2 degrees Celsius below normal, with the greatest temperature anomalies—up to 3 degrees Celsius below normal—observed in northern Ontario. These trends towards lower than average temperatures contributed to the maintenance of widespread snowfall distributions later into the spring and extended the duration of conditions that would support frozen soils into the spring compared to an average year. The effect of these temperature and snowfall trends contributes to a greater snowpack later in the season, and frozen or partially frozen soils with a limited ability to absorb runoff water.

More specifically, across central and eastern Ontario, April experienced some extremely active weather, owing in part to a west to east jet stream pattern over southern Canadian latitudes that supported frequent weather disturbances, including above average precipitation. Above average precipitation was observed throughout the spring months of March, April and May across many of the areas affected by flooding in 2019, receiving 50 to 100 millimetres or more precipitation than on average across the three months with the latter half of April 2019 representing above average to 200% of normal values. A low-pressure system developed over the southern U.S. on April 25 and moved northeastward bringing heavy rain on April 26 to 27, focused on southwestern and central Ontario and the upper Ottawa River catchment region of Quebec, with amounts ranging from 32 to 43 millimetres. While a portion of the snowpack had or was beginning to melt by this time, the soil would have remained partly frozen and where thawed, would have exceeded its capacity to support meltwater infiltration. A significant fraction of the rain that fell during this period would have reached surface water features that were already at or exceeding their capacity to contain water.

This rainfall contributed greatly to the flooding in the affected portions of Ontario in addition to the high-water levels experienced on the Great Lakes, in particular Lake Erie and Lake Ontario and the downstream St. Lawrence River system. The Great Lakes in particular received significant inputs of water from both sides of the border, as the U.S. tributaries experienced the wettest 12-month period on record, from May 2018 to May 2019.

In summary, key drivers leading up to the flooding of spring 2019 included: a colder than average winter without a significant winter thaw, contributing to a larger than average snow pack and higher than average snow water equivalents. These conditions were enhanced by colder than usual spring temperatures that maintained the snowpack further into the spring period than usual, and the melt of this snowpack was accelerated and exacerbated by numerous rain-on-snow events that occurred as temperatures began to rise significantly in mid-April.

3.2 Climate change – a recent history

It is particularly difficult to distinguish natural variability from the effects of climate change. The abundance of water in Ontario’s numerous rivers and lakes, including within the Great Lakes, tends to oscillate in a cyclical fashion. Research has shown that the periods of these cyclical oscillations can be linked to changes in the global water cycle, including alignments with large scale cyclical patterns of water movement driven by oceanic (e.g. North Atlantic Multidecadal Cycle, Pacific Decadal Oscillation) and other related processes (El Nino/La Nina/Southern Oscillation). Regardless of the mechanisms, history has shown that flows and levels within Ontario’s streams, rivers and lakes, including within the Great Lakes, are well established to increase and decrease over a range of temporal and spatial scales.

Recent climate change reports have suggested that Canada’s climate is warming twice as fast as the global average. Temperature and precipitation are fundamental climate variables that directly affect natural and human systems. Changes in temperature can affect the timing and extent of snowpack development, soil freezing, snow and ice melt, and rainfall potential during colder seasons, as well as the timing, intensity, duration, amount and phase of precipitation events (rain vs. snow). A recently published MNRF study documenting results for the period 1980 to 2010 has confirmed a significant decreasing trend in maximum snow water equivalent of 6.4% per decade, or approximately minus 9 millimetres, across Ontario, representing a reduction of 5 to 10% of the annual precipitation in affected watersheds.

With climate change, alterations in the global water cycle are expected to bring a range of variability affected by increasing water content of the atmosphere, strengthening of climatological precipitation/evaporation patterns, a more pronounced spatial structure, and sharp gradients in precipitation change. While precipitation is generally projected to increase in the future, summer precipitation, particularly in parts of southern Canada, is projected to decrease. The most serious impacts of climate change are expected to be the changes in climate extremes. Extreme precipitation is projected to increase in the future; however, the observational record has not yet shown evidence of consistent changes in short duration, high intensity precipitation across the province.

For instance, although not backed by a wealth of empirical evidence, recent history in Ontario suggests that hydrological extremes may be increasingly affected by so-called “ninja” storms/rainfall events that are sporadically observed across the province. These rain events are not usually forecast, and are often defined by short duration, high intensity rainfall that can result in extreme rainfall amounts often over a relatively short period of time, generally distributed over a small area and not adequately captured by the rainfall gauge network. There is also growing evidence to suggest that the assumption of climate stationarity, which is fundamental to traditional flood frequency analysis, may no longer be assumed. In other words, under a changing climate, we should perhaps not be relying on hydrologic extremes that are based on an analysis of data from the past to predict the magnitude of future hydrological events, including design storms.

The changing frequency and intensity of precipitation can be expected to lead to a changing likelihood of extreme events, such as floods and droughts. Over the Great Lakes Basin, evidence suggests that climate change is leading towards an increase in precipitation. At the same time, climate change has also shown trends in increasing air temperatures and increasing lake temperatures across the basin, which naturally leads to higher rates of evaporation. These two processes act as competing forces on water levels. Higher precipitation and/or lower evaporation at times will lead to more water on the landscape and higher water levels, but at other times, higher evaporation and/or lower precipitation will lead to lower water levels. With these two competing dynamic forces at odds with each other, some have alluded to this as a “tug-of-war” between stronger climatic forces. The higher water levels in the Great Lakes in 2014 was a change from record low water levels in 2012 and 2013. It was believed to be initiated by a combination of increased precipitation, but more importantly a slowdown in the rate of evaporation triggered by the very cold winter of 2014, which was caused by a polar vortex that sent cold air southward and froze the Lakes. This reduced evaporation and increased precipitation resulted in water levels that have risen in the Great Lakes to the record extremes observed in 2017 and 2019. Climate experts believe that this swing of the pendulum, going from one extreme of low water to one of high water, is in fact a consequence of climate change across the Great Lakes Basin and at continental scales.

Although one cannot unequivocally say that climate change is causing the flooding that has been observed over the last number of years or the last decade in Ontario, we do, however, know that water levels and flows are affected by a combination of temperature, precipitation and solar inputs that affect changes in precipitation and evaporation. Changes that have been observed at a global scale and a continental scale in North America, including changes in temperature and precipitation, are affecting water levels and flows in a way that would make them different from what would have otherwise been in the past. The potential exists that flooding may become increasingly more prevalent and the swing from wet to dry more volatile, making the flooding hazard more pronounced under a changing climate and its associated swings in variability.