Abrupt downstream forest decline following river damming in southern Alberta

Abstract

The influence of river damming on the abundance of riparian poplar forests was investigated by comparing forest abundances on the dammed St. Mary and Waterton rivers of southern Alberta with the neighbouring undammed Belly River. Forest distributions were determined by estimating the linear river distance of forests from airphotos taken in 1961 and 1981, both upstream and downstream from the dams. During the 20-year interval, a 48% reduction occurred downstream from the St. Mary Dam, which was completed in 1951. Downstream from the Waterton Dam, completed in 1964, riparian forests declined 23 %. Poplar abundance along the downstream region of the undammed Belly River was relatively unchanged over the study period, showing less than a 1 % reduction. The upstream (undammed) portions of all three rivers underwent a slight forest decline during the 20-year interval; declines of 4.7, 4.6, and 6.1 % occurred along the St. Mary, Belly, and Waterton rivers, respectively. Thus, the damming of two rivers in southern Alberta has been followed by a rapid and dramatic downstream decline in riparian forests. Further, the close association between the location and extent of the forest decline supports a causal relationship between the river damming and the forest decline. Possible causes of the decline are discussed, including drought-induced mortality, particularly of seedlings.