Twentieth-century decline in streamflows from the hydrographic apex of North America

The Rocky Mountain region near the Canada-United States border provides the North American hydrographic apex with headwater streams flowing to the Pacific, Arctic and Atlantic oceans. The area contains numerous national parks and protected areas with relatively pristine watersheds that permit analyses of historic streamflow patterns with minimal human impacts that would alter hydraulic linkages between precipitation and river discharge. Consequently, we analyzed patterns of mean annual discharge (Qa) from 31 river reaches that were generally free-flowing with hydrologic records typically commencing in the 1910s and extending to about 2002. To maximize the records of six rivers we undertook regression analyses to extrapolate Qa from sequential hydrometric gauges or from early, summer-only Q data. Spearman ρ and Kendall τ b non-parametric correlations and a parametric approach involving linear regressions combined with analyses of variance were highly consistent in detecting significant historic trends in Qa and the regression analyses estimated the trend magnitudes. These analyses revealed flow declines (exceeding 0.1%/year over the historic record) for 21 reaches (5 with p<0.1, 10 with p<0.05), while 10 rivers displayed little change (<0.1%/year and not significant). Flow declines were prominent for the Alberta rivers, which flow to Hudson Bay and the Arctic Ocean, and also observed for some Pacific and Atlantic drainages. Overall, the rivers displayed a mean Qa reduction of 0.22%/year (median=−0.17%/year) and four rivers had recent decline rates exceeding 0.5%/year. The progressive decline was superimposed on an approximately half-century oscillation in streamflow that was strongly associated with the Pacific Decadal Oscillation. Following from the observed river flow decline over the past century, it is likely that there will be continuing decline in future decades; this prediction contrasts with many current climate change forecasts. Historic and continuing reductions in these streamflows will impact aquatic and riparian ecosystems and diminish water supplies for irrigation, industrial and domestic use, and hydroelectric power generation, with effects extending from these mountain headwaters downstream through other ecoregions.