Water temperature, river discharge, and adult Sockeye salmon migration observations for the Tahltan watershed, 1959-2012 / by H.W. Stiff, K.D. Hyatt, M.M. Stockwell, P.M. Etherton, and W.D. Waugh.: Fs97-4/3018E-PDF

"Daily mean water temperature and adult Sockeye migration data were assembled for the Tahltan watershed, British Columbia. Regional air temperature data collected at Dease Lake were statistically related to intermittent water temperature time-series to hind-cast daily water temperature in Tahltan Lake/River for 1954–2012. Discharge data from the adjacent Tuya River watershed were used as an index of Tahltan flow conditions 1962–2011. Peak-over-threshold analyses were applied to reconstructed time-series to review long-term trends in temperature and flow by site. The climatology remains cool in this northern watershed, with little evidence of high temperature or low flow impacts on adult Sockeye migration. However, high flow impacts may exist. Although approximately 73% of Tahltan Sockeye migration dates occur between discharges of 20–40 cms, highest daily migration rates (>3% per day) occur at 40–80 cms. Moderate to high migration rates were distributed across the entire water temperature range during the normal migration interval (mid-Jul to late-Aug), with highest migration rates at 16–19°C. Low flow dates (defined as daily average flow <18 cms) averaged <20 per year but spiked to 36 in the 1990s, though almost half were in September, when few Tahltan Sockeye were migrating. Since then, a regional reversion to cooler and “wetter” summers in the 2000s is evident in the frequency of high flow dates (daily average >80 cms) in July and August, from an average of 4 per year from the 1960s–1990s 8 per year in the 2000s. The length in days of continuous high flow periods currently remains within the long-term average but the number of high flow periods escalated in the 2000s from a previous average of 8 to 24 per year"--Abstract, page viii.

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