Early Vegetation Development on an Exposed Reservoir: Implications for Dam Removal

Abstract

The 4-year drawdown of Horsetooth Reservoir, Colorado, for dam maintenance, provides a case study analog of vegetation response on sediment that might be exposed from removal of a tall dam. Early vegetation recovery on the exposed reservoir bottom was a combination of (1) vegetation colonization on bare, moist substrates typical of riparian zones and reservoir sediment of shallow dams and (2) a shift in moisture status from mesic to the xeric conditions associated with the pre-impoundment upland position of most of the drawdown zone. Plant communities changed rapidly during the first four years of exposure, but were still substantially different from the background upland plant community. Predictions from the recruitment box model about the locations of Populus deltoides subsp. monilifera (plains cottonwood) seedlings relative to the water surface were qualitatively confirmed with respect to optimum locations. However, the extreme vertical range of water surface elevations produced cottonwood seed regeneration well outside the predicted limits of drawdown rate and height above late summer stage. The establishment and survival of cottonwood at high elevations and the differences between the upland plant community and the community that had developed after four years of exposure suggest that vegetation recovery following tall dam removal will follow a trajectory very different from a simple reversal of the response to dam construction, involving not only long time scales of establishment and growth of upland vegetation, but also possibly decades of persistence of legacy vegetation established during the reservoir to upland transition.

Synthesis of Common Management Concerns Associated with Dam Removal

“…However, consistent with Orr and Stanley () and Auble et al . (), we found no clear evidence (Figure ) of nonnative species richness and abundance increasing with time since removal.…”

Monitoring the colonization of alluvial deposits using multitemporal UAV RGB‐imagery

“…(), Orr and Stanley (), and Auble et al. (), who observed rapid and dense revegetation after dam removal. Cross‐tabulation maps revealed that colonization peaked between March and May, when the colonized area increased from 30% to 65% (Figures and ) as soon as weather conditions were suitable.…”

“…Restoration project managers need to monitor the effectiveness of recolonization to avoid erosion of non-vegetated sediments. The few studies focusing on colonization after dam removal showed that initial colonization is characterized by its pioneer vegetation (mostly herbaceous) and short duration of this period of initial colonization (usually one or two years (Shafroth et al, 2002;Gurnell, Boitsidis, Thompson, & Clifford, 2006;Orr & Stanley, 2006;Auble, Shafroth, Scott, & Roelle, 2007;Stephens, 2017). These results highlight that managers have a short window of opportunity for action.…”

Geomorphic and process domain controls on riparian zones in the Colorado Front Range

“…The role of hydrology in shaping riparian vegetation is well established: rates of stage change influence seedling germination and recruitment (Mahoney and Rood, 1998); species present at a site reflect inundation period (Auble et al, 2005); change in moisture status correlates with change in species composition over a time span of several years (Auble et al, 2007); plant communities differ by elevation with respect to the floodplain (Chapin et al, 2002); vegetation type varies in relation to flow frequencies that determine sediment mobilization and effective discharge (Pike and Scatena, 2010). This study adds to this knowledge by demonstrating correlations between flood stages of different recurrence intervals and mean stage of the riparian edge for various process domains.…”