Functional flows: an environmental flow regime benefits riparian cottonwoods along the Waterton River, Alberta

Abstract

With drainage from the Waterton‐Glacier International Peace Park, the Waterton River was dammed in 1964 to trap spring flow and permit offstream diversion for irrigation. Field observations in the 1980s indicated some decrepit riparian woodlands suggesting drought stress of the black and narrowleaf cottonwoods (Populus trichocarpa, P. angustifolia) due to insufficient in‐stream flows. Subsequently, an environmental flow regime commenced in 1991 and provided “functional flows,” deliberately regulating in‐stream flow components intended to restore ecological processes and particularly (1) an increase of the minimum flow from 0.93 to 2.27 m3/s (mean discharge 21.9 m3/s) and (2) flow ramping, gradual recession after the spring peak. This study investigated the historic flow patterns and the growth, population age structure, and spatial distributions of riparian cottonwoods along the free‐flowing upstream and regulated downstream reaches over four dam operations intervals: the free‐flowing pre‐dam condition; the initial dammed interval to the mid‐1970s; a post‐dam and drought interval in the 1980s; and with the environmental flow regime. Analyses of sapling, shrub‐, and tree‐sized cottonwoods included tree ring analyses to determine ages and growth patterns, and distributions were assessed relative to streamside elevations and sediment textures. These indicated that there has been progressive cottonwood colonization after damming but the colonization band dropped in elevation with the reduced flow regime and the future woodlands could become narrower. The tree ring analyses indicated that the growth of established trees benefited from the functional flows and the increase in minimum flow was probably particularly beneficial to the riparian cottonwoods.