Plant root architecture reveals the sources of water and nutrients but tree root systems are large and difficult to analyze. With riparian (floodplain) trees, river cut-banks provide natural hydraulic excavation of root systems and this presents a unique study opportunity. Subsequently, we developed the 'Cut-bank Root Method', a simple, quantitative approach for analyzing the distribution of coarse roots, based on analyses of photographs of river cut-banks. These reveal the vertical extent of roots and median root depths (R d ). We applied this method along six rivers draining the Canadian Rocky Mountains and observed tenfold difference in R d . The floodplain forests were dominated by cottonwoods and from mountain to prairie zones we observed progressively deeper roots of Populus trichocarpa (black cottonwood, R d * 0.3 m), P. balsamifera (balsam poplar), P. angustifolia (narrowleaf cottonwood), and P. deltoides (prairie cottonwood, R d * 0.9 m), which had R d similar to P. fremontii (Fremont cottonwood) in Nevada, USA. Roots were shallower for co-occurring facultative riparian trees, with R d * 0.1 m for P. tremuloides (trembling aspen) and Picea glauca (white spruce). Across the Canadian sites, R d for cottonwoods were strongly associated with a growth season moisture index (May through September precipitation-potential evapotranspiration; R 2 = 0.97, P \ 0.001). Thus, in wetter climates, riparian cottonwoods were shallow-rooted and would be more dependent upon rain than stream flow. Conversely, in the drier semi-arid regions the cottonwoods were phreatophytic, with deeper root systems in the capillary fringe above the alluvial ground-water table. These phreatophytic cottonwoods would be highly dependent upon stream flow and vulnerable to declining river flows due to river regulation or climate change.

Although it is recognized that streamside vegetation can reduce river bank erosion, the relative effectiveness of forest versus grassland has been unclear. To compare erosion resistance of the two vegetation types, we studied the free-flowing Elk River in British Columbia, Canada from 1993 to 2014, including major floods in June 1995 and 2013. Interpretation of aerial photographs from 1994 and 2000 were used to examine the correspondence between floodplain vegetation and the extent of channel change after the 1995 flood. Along a 23 km reach with alternating forest and grassland, 15 locations displayed substantial change as the river moved a channel width (45 m) or more with meander migration, or up to 200 m with channel avulsion. All ten locations with major change (>75 m) occurred where the floodplain zones were occupied by grasslands, sometimes with small shrubs. In contrast, channels flanked by forest were minimally altered (<15 m), and deciduous (black cottonwood, Populus trichocarpa) or mixed deciduous-coniferous groves were effective at resisting erosion. Some changes accompanied the 1995 flood and further changes followed as the destabilized banks were vulnerable to smaller floods in 1996 and 1997. Providing another comparison, a position that was dramatically scoured in 1995 when it was grassland had subsequent cottonwood colonization, and the 4 m trees resisted erosion from the 2013 flood. Thus, trees were more resistant than grassland to flood-associated bank erosion. We recommend that riparian forests should be conserved to provide bank stability and to maintain an equilibrium of river and floodplain dynamics.

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.