The transboundary St Mary River drains Glacier National Park, USA, and was progressively dammed and diverted over the 20th century to support agricultural irrigation in northern Montana and southern Alberta, Canada. Following reduced instream flows, the riparian cottonwoods collapsed, and by 2000, few parental trees remained to provide seeds for cottonwood replenishment. As a novel twofold restoration strategy we: (1) worked with the dam operators to deliver a functional flow regime, a regulated instream flow pattern intended to recover some ecological function and specifically seedling recruitment, and (2) delivered cottonwood seeds by direct spreading and by sticking cuttings with seed catkins to allow gradual seed dispersal. The combination of river regulation and seeding enabled cottonwood colonization, and around 1.5% of the applied seeds produced seedlings after the first summer, at sites without livestock or heavy recreational use. Around 15% of those seedlings survived through the fourth summer, with mortality due to drought stress and flood scour, and establishment and survival were higher for the prairie cottonwood, Populus deltoides, than the narrowleaf cottonwood, Populus angustifolia. This study confirmed that the lack of seed source trees limited cottonwood colonization and demonstrated that the twofold restoration strategy provides promise for severe situations where parental trees have been lost. However, this would require substantial effort, and it would be more efficient to provide survivable instream flow patterns that avoid cottonwood collapse. Copyright © 2015 John Wiley & Sons, Ltd.

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With river regulation, water withdrawal is common, reducing instream flows. The opposite alteration, flow augmentation, is less common and could reveal a mechanistic coordination between flow regime, channel form, and riparian ecosystems. The Little Bow River, a naturally intermittent prairie stream in Southern Alberta, has experienced flow augmentation since the late 1890s, and the Little Bow/Highwood Project of 2004 enabled a tripling of diversion flows from 2.9 to 8.5 m3/s. We investigated the subsequent responses by assessing the channel form and riparian vegetation based on aerial photographs taken in 2000 versus 2010, and riparian birds were assessed between 2005 and 2013 to investigate associations with riparian vegetation. Following recent flow augmentation, the mean channel width increased from 12.2 to 13.5 m, while sinuosity was relatively unchanged. Streamside zones with true willows (especially Salix exigua and Salix bebbiana) increased from 7 to 11% of the river corridor, and the facultative riparian wolf willow (Elaeagnus commutata) zones increased from 16 to 20%, while grassy zones decreased from 64 to 52%. Avian species richness and Shannon–Wiener index increased, while species evenness was relatively unaltered, suggesting an increase of rarer bird species in response to the increased habitat structure and diversity following the expansion of riparian shrubs and woodland. This study revealed responses to the recent flow augmentation over the first decade of implementation, and alterations following flow augmentation would likely continue for decades until the river and riparian zones adjust to the new flow regime. Copyright © 2016 John Wiley & Sons, Ltd.

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.