INSTREAM FLOWS AND THE DECLINE OF RIPARIAN COTTONWOODS ALONG THE ST-MARY RIVER, ALBERTA

Completed in 1951, the St. Mary Dam enables water storage and diversio n for irrigation; river flows downstream are consequently dramatically reduced during summer months. To assess historical changes in the abu ndance of riparian cottonwoods (Populus balsamifera, Populus angustifo lia, and a few Populus deltoides), airphoto analyses were conducted fo r 40-km river reaches upstream and downstream from the dam and along a djacent dammed and undammed rivers. Cottonwoods along the lower St. Ma ry River are confined by steep-walled canyons to narrow bands and cons equently analyses of the lineal river distance associated with cottonw oods were conducted. These revealed a 68% decline from 1951 to 1985. T he decline was progressive, with 28.9, 27.6, 15.1, and 7.6% of the rea ch associated with cottonwoods in 1951, 1961, 1981, and 1985, respecti vely. Ground surveys from 1985 to 1994 indicated further decline after 1985 and an absence of cottonwood seedlings and saplings. Cottonwood stands upstream from the St. Mary Dam and along adjacent rivers are mo re extensive and analyses of the areal extent of stands were consequen tly appropriate. These indicated minor change along the upper St. Mary (- 0.5%), the upper (+ 1.9%) and lower Waterton (+ 3.5%), and the upp er Belly (- 9.1%) rivers, and an increase in forest abundance along th e lower Belly River (+ 52.2%), between 1951 and 1985. Thus, the declin e of cottonwoods along the lower St. Mary River was not symptomatic of a general pattern of decline in the region. Analyses of historical st ream flows indicated that the cottonwood mortality was drought induced as a result of insufficient flows during the hot, dry summer periods and abrupt flow reductions following the high-flow period in the late spring. The riparian water table was determined to be closely coordina ted with river stage, as changes in river elevation were followed by q uantitatively similar changes in water table depth. Along the St. Mary River, reduced sedimentation downstream from the dam was not consider ed to be responsible for the cottonwood decline. The historically spar se cottonwood abundance along the lower St. Mary River may have reflec ted environmental conditions that were naturally only marginally suita ble, and those groves may have been particularly vulnerable to the imp acts of river flow regulation.