Riparian recruitment persists after damming: Environmental flows and coupled colonization of cottonwoods and willows following floods along a dryland river

Abstract

AbstractFor riparian woodlands, occasional floods provide geomorphic disturbance that creates barren colonization sites, and river stage patterns that enable seedling establishment. To investigate impacts from river damming and instream flow regulation on these processes, we studied the lower Red Deer River in the semi‐arid region of Alberta, Canada. Dickson Dam was implemented in 1983 and although major flood peaks in June 2005 and June 2013 were attenuated by about one‐quarter, substantial seedling establishment of plains cottonwoods (Populus deltoides) was observed. Cross‐sectional transects in 2014 and 2015 revealed the reproductively mature cottonwood band from 2005, while the 2013 colonization was limited to low elevations of 0.3–1.3 m above the base river stage. Seedling numbers and the elevational ranges were probably reduced by abrupt (>4 cm/day) river stage reductions in July 2005 and in July 2013. In addition to direct seedling establishment on barren sandbars, we observed “coupled colonization” with cottonwood recruitment within sparse patches of sandbar willows (Salix exigua). The flood‐tolerant willows stabilize the bars and increase aggradation through sediment trapping. Thus, cottonwood colonization persisted after damming, indicating that the pattern of downstream flow regulation was important, rather than damming per se. To sustain riparian recruitment along regulated rivers in dry regions, we recommend: (1) that floods be allowed as feasible, (2) higher river stages during seed dispersal, (3) flow ramping (gradual summer recession) for seedling survival, (4) sufficient growing season flows to avoid drought‐induced mortality, and (5) that willows be encouraged as well as cottonwoods.