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

Abstract

The Little Bow River (LBR) in western Canada naturally displayed an intermittent flow regime; the channel dried up most summers, excluding the development of riparian woodlands within this semi-arid ecoregion. Around 1900, the Little Bow Canal was excavated to divert water from the adjacent Highwood River and with flow augmentation the LBR flow became continuous through the growth season. We hypothesized that the continuous regime enabled riparian woodland establishment and assessed conditions with sequential aerial photographs and field observations. Supporting the hypothesis, a few woodland groves established near the Highwood River where balsam poplars (Populus balsamifera) provided an abundant seed source. To investigate the basis for woodland development, we analysed historical hydrology of the LBR and assessed the four larger woodland groves, which included mature poplars, trembling aspen (P. tremuloides) and willow shrubs (Salix bebbiana and S. exigua). Each location had some bank excavation with channelization or gravel mining, and tree ageing through ring counts indicated gradual colonization and pulses of establishment after floods in 1920 and 1942. Thus, the conversion from an intermittent to continuous flow regime enabled woodland development which also benefited from excavations that created barren colonization sites. The study revealed four requirements for riparian woodland colonization in a dry region: (a) seeds, (b) barren sites, (c) bank saturation with higher river flow, and (d) sufficient river flows for tree and shrub survival and growth. While water withdrawal commonly degrades riverine ecosystems, flow augmentation can provide the opposite outcome, enhancing the river and riparian environments.

No evidence for cottonwood forest decline along a flow‐augmented western U.S. river

“…For example, on the Middle Fork of the Flathead River, Montana, cottonwoods grew faster on a reach with upwelling nutrient‐rich groundwater than on a losing reach (Harner & Stanford, 2003). On the Little Bow River, Alberta, balsam poplars grew faster at a site with nutrient inputs from livestock operations (Bigelow et al, 2020). Though investigating such site‐level factors was beyond the scope of this study, this could be addressed in future research.…”

“…Ecosystem decline is not observed in all managed rivers, and riparian forest or wetland expansion has been observed in a few cases. In the Canadian prairie, flow augmentation for conveyance of irrigation water resulted in woodland development on The Little Bow River (Bigelow et al, 2020). In some developed settings local water availability has been increased or restored via urban storm drain outflows, municipal effluents, and leakage, supporting ecologically important wetlands (e.g., Villarreal et al 2012, Palta et al, 2017).…”