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.

A functional flows approach to selecting ecologically relevant flow metrics for environmental flow applications

“…These included peak magnitude flows, spring recession flows, dry season low flows, and wet season initiation flows. Similar flow components have been identified in other seasonal river systems, such as the Waterton River in Canada (Foster, Mahoney, & Rood, ), the Spol River in Switzerland (Robinson, Siebers, & Ortlepp, ), and both inland and coastal rivers in Australia (Koster, Dawson, O'Mahony, Moloney, & Crook, ; Webb, Koster, Stuart, Reich, & Stewardson, ). These functional flow components are likely relevant for other seasonal climates and can provide a starting point for assessment in locations with limited data availability.…”

Flows for floodplain forests: Conversion from an intermittent to continuous flow regime enabled riparian woodland development along a prairie river

“…The study included a dendrochronologal approach to assess the timing of establishment of poplars relative to the historical flow patterns. The findings contribute to the understanding of the ecohydrology of riparian woodlands, and are applicable for the prescription of environmental flows along other regulated rivers (Foster, Mahoney, & Rood, 2018; Glenn, Nagler, Shafroth, & Jarchow, 2017; Hall, Rood, & Higgins, 2011; Rood et al, 2003; Shafroth et al, 2017; Shafroth, Auble, Stromberg, & Patten, 1998).…”

“…This ideally positions the seeds right along the recently exposed river margin, where the saturated substrate enables imbibition and rapid germination (Bradley & Smith, 1986; Kalischuk et al, 2001; Dixon, 2003). The period of seed maturation and dispersal is generally limited to a few weeks and peaks around mid‐June in southern Alberta, which is the typical interval for the post‐peak river recession (Kalischuk et al, 2001; Karrenberg et al, 2002; Foster et al, 2018; Figure S2). With the Highwood River as the adjacent seed source, the uppermost zones along the LBR would be much more likely to receive poplar seeds.…”

“…This assessment of colonization requirements could provide guidance for the conservation and restoration of riparian woodlands along other rivers (Foster et al, 2018; Glenn et al, 2017; Shafroth et al, 1998) by revealing limitations in particular colonization requirements. It would also assist in more complex challenges such as along the lower St. Mary River in southern Alberta (Rood et al, 2016).…”

Selection of Poplar Genotypes for Adapting to Climate Change

“…The reported values of stomatal conductance for poplars have been shown to be as high as 1000 mmol m −2 s -1 [44,45], which is usually related to low WUE [11,46], and generally reflects the status of poplars as phreatophytes and riparian species that typically grow on moist alluvial soils after floods or other disturbances [20,[47][48][49]. Our study, carried out in a more xeric habitat of north-central Alberta, revealed that stomatal conductance indeed strongly affects intrinsic WUE (negative correlation, Table 2, Figure 1), but the mean values for all tested genotypes (296 mmol m -2 s -1 ) were much lower than reported, which shows adaptive plasticity of our studied clones to water stress conditions.…”