River regulation and riparian woodlands: Cottonwood conservation with an environmental flow regime along the Waterton River,Alberta

Following water withdrawal, riparian cottonwoods have declined downstream from some dams in western North America. Analyses of aerial photographs and field observations in the 1980s suggested that the black and narrowleaf cottonwoods (Populus trichocarpa and Populus angustifolia) along the Waterton River, Alberta, were declining due to drought stress following the 1964 damming and diversion. This raised concern for the riverine ecosystems and in 1991, “functional flows” commenced with 2 changes: (a) the minimum flow was increased from 0.9 to 2.3 m³/s (mean discharge 21.9 m³/s) and (b) flow ramping provided gradual stage recession after the spring peak. This provided an environmental flow regime that was delivered for 2 decades and this study investigated the consequent river flow patterns and riparian woodlands upstream and downstream from the Waterton Dam. Analyses of aerial photographs from 1951 to 2009 assessed 4 flow management intervals: (a) the free‐flowing predam condition, (b) the initial dammed interval to the mid‐1970s, (c) a drought interval in the 1980s, and (d) with the environmental flow regime after 1991. Analyses revealed woodland reduction from 1961 to 1985 due to losses through bank erosion with major floods and apparent decline due to low flows following a regional drought and water withdrawal for irrigation. With the subsequent environmental flow regime, there was apparent woodland recovery, despite drought in 2000 and 2001. This study demonstrated that the correspondence between river flow patterns and the extent of riparian woodlands and the benefit from the environmental flow regime that probably reduced drought stress and mortality.