In western Canada growing demand for water resources has increased vulnerability to hydrological drought. The near full allocation of water supplies in the Oldman and Bow River subbasins of the South Saskatchewan River Basin has resulted in a moratorium on new surface water licenses. In this region, short instrumental records limit the detection of long-term hydrological variability. To extend the historical record, we collected 14 new moisture-sensitive tree ring chronologies and reconstructed the average October through September flow of the Oldman (1618-2004) and South Saskatchewan (SSR) (1400-2004) rivers. Our SSR proxy record updates a previously published reconstruction. While the 20th century is representative of drought frequency over the long term, droughts are of greater severity and duration in the preinstrumental proxy record. A spectral analysis of the reconstructed flows revealed quasiperiodic cycles at interannual to multidecadal scales. Copyright 2009 by the American Geophysical Union.