Streamflow
regime refers to the seasonal distribution of flow, influenced predominantly by
the prevailing climate in the region (e.g., Moore et al., 2017). Temperature
affects the type of precipitation (rain versus snow), the accumulation of a
snowpack, and the timing and amount of ice and snowmelt runoff. Precipitation
determines the potential magnitude of flow generated during different periods
of the year. In Canada, streamflow regimes are classified as nival
(snowmelt-dominated), glacial (glacier-dominated), pluvial (rainfall-dominated),
or mixed. Across much of the country, most of the winter precipitation falls as
snow and melts during spring and early summer. As a result, the vast majority
of rivers are nival. These regimes exhibit high flows in spring and early
summer (due to snowmelt), and the timing depends on geographic location (since
snowmelt is later farther north or at higher elevations) and on the size of the
catchment. Glacial regimes are confined to mountainous regions of western
Canada and the high Arctic islands, where glaciers and ice caps are present.
These regimes are associated with an initial snowmelt runoff, followed by
continued flow into late summer sustained from ice melt. Pluvial regimes are
driven by the seasonal distribution of rainfall. At lower elevations on the
west coast of Canada, this consists of high flows during winter and low flows
during summer and autumn (see Figure 6.7c). On the east coast, higher flows are
most common in spring and autumn. Combinations of these regimes (known as mixed
regimes) are also found in Canada (see Figure 6.7d). For instance, nivo-pluvial
regimes are influenced by both snow and rainfall, the exact proportion
depending on the location of the stream. In British Columbia, for example, the
seasonal flow patterns transition from pluvial (rain-dominated) in
coastal/low-elevations to nival (snow-dominated) toward the continental
interior of the province and higher elevations (Moore et al., Nival catchments
are predominantly found in northern and western Canada, while pluvial basins
are located on the east and west coasts, and mixed catchments are mainly in
southern Ontario and Quebec and Atlantic Canada. Glacial regimes were not
identified in this analysis (Burn et al., 2016). The characterization of
regimes is based on longer-term hydroclimatic averages, but, in most of Canada,
there is considerable year-to-year variability in these patterns.
