Hydrometeorological conditions for Atlantic salmon rivers in the Maritime provinces

This study provides historical information on hydrometeorological conditions and trends for Atlantic salmon (Salmo salar) rivers within the Maritime provinces. In total, 8 rivers were studied and at each river, discharge characteristics (high flow, low flow and streamflow timing) were presented as well as long-term trends in the discharge time series. Long-term mean air temperatures and precipitations were presented at 8 meteorological stations close to the studied rivers as well as annual and summer air temperature trends. A spatial variability in mean annual air temperature exists within the Maritime provinces and air temperatures varied between 4.8 °C (Bathurst) and 6.6 °C (Halifax). This gradient in mean annual air temperature has an influence on river discharge and river temperature. Northern rivers experienced lower winter flows followed by a pronounced snowmelt runoff in the spring. The southern rivers experienced more mixed snow/rainfall dominated winters with correspondingly higher winter flows. The spring high flow period was also different among rivers where northern rivers tended to reach their high flow later (May) than southern rivers (April). The summer low flow period generally extended between the end of July to early September. Significant floods and low flow events have been observed within the study area over the past 20 years. For instance, many rivers experienced close to a 100-year flood event (Nashwaak River 2010; St. Marys River 2003; LaHave River 2003; Northeast Margaree River 2010). Similarly, 100-year low flow events were observed in Miramichi (2002), Nashwaak (2001) and LaHave (2016), rivers. When looking at low flows across the region, Nova Scotia (NS) rivers are experiencing the most severe summer low flows (100-year events). In fact, the St. Marys and LaHave rivers can reach less than 1% of the mean annual flow. Timing of high flow events differed among provinces as well; the majority of high flow events in New Brunswick (NB) rivers occurred near the spring freshet whereas high flow events in NS rivers were spread out throughout the year. High flow events at the Prince Edward Island (PEI) river (Wilmot River) were generally grouped, similar to NB rivers, but tended to occur earlier in the year. As in previous studies, river temperature across the region show some variability (both at the spatial and temporal scales). The Nashwaak and Little Southwest Miramichi rivers showed a significant number of days where daily Tmax was greater than 23 °C during the summer period (29 and 32 days on average per year, respectively). In contrast the Restigouche River (Butters Island) showed less that 10 days on average per year. When looking at the mean annual air temperature trends, most stations showed a significant increase in the Maritime provinces. The increase in mean annual air temperature was between 1.2 °C (Charlottetown) and 2.0 °C (Bathurst) over the past 100 years. Increases in summer air temperatures were also significant at most sites over the past 100 years, but slightly lower (1- 1.7 °C). Results also showed a significant increase in precipitation at 50% of the sites, with increases between 10-31 mm per decade. The high flow timing was significantly earlier at 75% (6/8) of the rivers representing a 2-3 days change per decade. Most trends in the magnitude of high flow were not significant. In fact, only one station showed a significant decrease in flow (Nashwaak River). The timing of summer low flow only showed one significant decrease in the timing (Wilmot River; 9 days per decade). The magnitude of low flow did not show any significant trends. Some rivers showed a significant increase in summer (July and August) water temperatures over the past 25 years (Little Southwest Miramichi River, Kedgwick River and Restigouche River at Butters Island). The increases were in the order of 0.8 °C to 2.2 °C per decade.

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