Instream flows and the decline of riparian cottonwoods along the St. Mary River, Alberta

Abstract

Completed in 1951, the St. Mary Dam enables water storage and diversion for irrigation; river flows downstream are consequently dramatically reduced during summer months. To assess historical changes in the abundance of riparian cottonwoods (Populus balsamifera, Populus angustifolia, and a few Populus deltoides), airphoto analyses were conducted for 40-km river reaches upstream and downstream from the dam and along adjacent dammed and undammed rivers. Cottonwoods along the lower St. Mary River are confined by steep-walled canyons to narrow bands and consequently analyses of the lineal river distance associated with cottonwoods were conducted. These revealed a 68% decline from 1951 to 1985. The decline was progressive, with 28.9, 27.6, 15.1, and 7.6% of the reach associated with cottonwoods in 1951, 1961, 1981, and 1985, respectively. 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 more extensive and analyses of the areal extent of stands were consequently appropriate. These indicated minor change along the upper St. Mary (−0.5%), the upper (+1.9%) and lower Waterton (+3.5%), and the upper Belly (−9.1%) rivers, and an increase in forest abundance along the lower Belly River (+52.2%), between 1951 and 1985. Thus, the decline of cottonwoods along the lower St. Mary River was not symptomatic of a general pattern of decline in the region. Analyses of historical stream 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 coordinated with river stage, as changes in river elevation were followed by quantitatively similar changes in water table depth. Along the St. Mary River, reduced sedimentation downstream from the dam was not considered to be responsible for the cottonwood decline. The historically sparse cottonwood abundance along the lower St. Mary River may have reflected environmental conditions that were naturally only marginally suitable, and those groves may have been particularly vulnerable to the impacts of river flow regulation. Key words: Populus, cottonwoods, instream flows, mortality, riparian vegetation.

Managing River Flows to Restore Floodplain Forests

“…Specific values were determined for various river reaches and minimum flow standards were subsequently implemented (Annear et al 2004). An unfortunate consequence of this approach was that minimum flows sometimes became target flows (Rood et al 1995). The seasonal release of more water than the minimum flow was discouraged, even when the water supply was abundant.…”

“…There are many river reaches that satisfy these criteria and here we present four case studies that are discussed in order of increasing level of intervention. (Rood et al 1995). Water was diverted from the reservoir, resulting in partial dewatering downstream as summer flows were frequently held for weeks at about 1 m 3 /second, about 5% of the natural flow (Rood et al 1995).…”

“…(Rood et al 1995). Water was diverted from the reservoir, resulting in partial dewatering downstream as summer flows were frequently held for weeks at about 1 m 3 /second, about 5% of the natural flow (Rood et al 1995). As a result of insufficient summer flows and abrupt reductions in spring flows, the riparian woodlands collapsed, with 90% of the cottonwoods dying between 1951 and 2000 (Rood et al 1995).…”

Effects of River Regulation on River-Margin Vegetation: A Comparison of Eight Boreal Rivers

“…van der Valk 1981, Keddy 1983, Nilsson 1987, Day et al 1988, Hughes 1990 may not be applicable outside the range of natural flows (but see Stevens et al [1995]). The floristic response to changes in water-level regime also depends on how sediment and litter dynamics (Hupp and Osterkamp 1985, Salo et al 1986, Nilsson and Grelsson 1990, Kalliola et al 1991, plant establishment (Fenner et al 1985, Rood and Mahoney 1990, Hughes 1994, and riparian water tables (Rood et al 1995) are affected. Studies are needed in which the effects of these processes are separated.…”

Effects of the “Run-of-River” Hydro Scheme on Macroinvertebrate Communities and Habitat Conditions in a Mountain River of Northeastern China

“…In the "depleted" reach, the surface flow is intermittent, groundwater levels along the riverbank show strong declines, and these hydraulic conditions are less available to the riparian vegetation. Flow reductions are usually associated with decreases in the riparian water table at higher elevations and losing riparian vegetation due to drought stress [30,31]. Riverbank collapse occurs when the driving forces exceed the resisting forces.…”