Sb. Rood et al., INSTREAM FLOWS AND THE DECLINE OF RIPARIAN COTTONWOODS ALONG THE ST-MARY RIVER, ALBERTA, Canadian journal of botany, 73(8), 1995, pp. 1250-1260
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.