Geomorphic and Ecological Disturbance and Recovery from Two Small Dams and Their Removal

Dams are known to impact river channels and ecosystems, both during their lifetime and in their decommissioning. In this study, we applied a before-after-control-impact design associated with two small dam removals to investigate abiotic and biotic recovery trajectories from both the elimination of the press disturbance associated with the presence of dams and the introduction of a pulse disturbance associated with removal of dams. The two case studies represent different geomorphic and ecological conditions that we expected to represent low and high sensitivities to the pulse disturbance of dam removal: the 4 m tall, gravel-filled Brownsville Dam on the wadeable Calapooia River and the 12.5 m tall, sand and gravel-filled Savage Rapids Dam on the largely non-wadeable Rogue River. We evaluated both geomorphic and ecological responses annually for two years post removal, and asked if functional traits of the macroinvertebrate assemblages provided more persistent signals of ecological disturbance than taxonomically defined assemblages over the period of study. Results indicate that: 1) the presence of the dams constituted a strong ecological press disturbance to the near-downstream reaches on both rivers, despite the fact that both rivers passed unregulated flow and sediment during the high flow season; 2) ecological recovery from this press disturbance occurred within the year following the restoration action of dam removal, whereas signals of geomorphic disturbance from the pulse of released sediment persisted two years post-removal, and 3) the strength of the press disturbance and the rapid ecological recovery were detected regardless of whether recovery was assessed by taxonomic or functional assemblages and for both case studies, in spite of their different geomorphic settings

Geomorphic and ecological disturbance and recovery from two small dams and their removal.

Dams are known to impact river channels and ecosystems, both during their lifetime and in their decommissioning. In this study, we applied a before-after-control-impact design associated with two small dam removals to investigate abiotic and biotic recovery trajectories from both the elimination of the press disturbance associated with the presence of dams and the introduction of a pulse disturbance associated with removal of dams. The two case studies represent different geomorphic and ecological conditions that we expected to represent low and high sensitivities to the pulse disturbance of dam removal: the 4 m tall, gravel-filled Brownsville Dam on the wadeable Calapooia River and the 12.5 m tall, sand and gravel-filled Savage Rapids Dam on the largely non-wadeable Rogue River. We evaluated both geomorphic and ecological responses annually for two years post removal, and asked if functional traits of the macroinvertebrate assemblages provided more persistent signals of ecological disturbance than taxonomically defined assemblages over the period of study. Results indicate that: 1) the presence of the dams constituted a strong ecological press disturbance to the near-downstream reaches on both rivers, despite the fact that both rivers passed unregulated flow and sediment during the high flow season; 2) ecological recovery from this press disturbance occurred within the year following the restoration action of dam removal, whereas signals of geomorphic disturbance from the pulse of released sediment persisted two years post-removal, and 3) the strength of the press disturbance and the rapid ecological recovery were detected regardless of whether recovery was assessed by taxonomic or functional assemblages and for both case studies, in spite of their different geomorphic settings

NMS ordination plot according to the functional community structure (based on 20 benthic insect traits) of A) 3 upstream and 3 downstream riffles associated with the Brownsville Dam removal on the Calapooia River over three collection years, and B) 11 upstream and 11 downstream transects associated with the Savage Rapids Dam removal on the Rogue River over four collection years.

<p>Note that the 11 transects from reach R-DS2 have been removed from the figure to reduce clutter (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108091#pone-0108091-t004" target="_blank">Table 4</a> for MRPP analysis of these data, including the R-DS2 reach). The symbols follow the same explanation as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108091#pone-0108091-g002" target="_blank">Figure 2</a>. A) Final stress for the two-dimensional solution = 6.4. B) Final stress for the two-dimensional solution was 13.2.</p

Site locations.

<p>Each dot represents a macroinvertebrate sampling location: 3 riffles per reach on the Calapooia River, and 11 transects per reach on the Rogue River.</p

Mean %clingers (expressed as relative abundance, +/−2 standard errors) per reach across A) riffle sample units on the Calapooia, and B) transects sampled on the Rogue.

<p>In both panels, year is on the x-axis, and lighter-colored bars represent the upstream reaches. Vertical dashed line shows timing of dam removal. Note that, in the interest of space, the second downstream reach is not plotted for the Rogue due to the lack of change over time.</p

Characteristics of the study reaches and sediment reservoirs.

<p>Barrier height varies with season due to installation of flashboards (Brownsville Dam) and stop logs (Savage Rapids Dam). Erosional efficiency was calculated as dimensionless ratio of the volume of sediment eroded from the reservoir to the volume of streamflow delivered to the site across the study period. D50_R = median grain size of the reservoir sediments. D50_D = median grain size of the river bed downstream of the dam.</p><p>Characteristics of the study reaches and sediment reservoirs.</p

NMS ordination plot according to the taxonomic structure of A) 3 upstream and 3 downstream riffles associated with the Brownsville Dam removal on the Calapooia River over three collection years, and B) 11 upstream and 11 downstream transects associated with the Savage Rapids Dam removal on the Rogue River over four collection years.

<p>Note that the 11 transects from reach “DS2” have been removed from the figure to reduce clutter (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108091#pone-0108091-t003" target="_blank">Table 3</a> for MRPP analysis of these data, including the DS2 reach). A) Final stress for the two-dimensional solution = 10.1. B) Final stress for a three-dimensional solution was 15.2, and the unpictured third axis had R² = 0.21.</p

Results of MRPP analysis for differences among taxonomically defined assemblages in both rivers.

<p>The first row of statistics for each group structure represents the test across the all groups comprised in that dataset, and the “pairs” column shows statistics for pairwise comparisons within the respective group. <i>A</i> is chance-corrected within-group agreement. Bolded p-values indicate significance at alpha = 0.05 after corrections for multiple comparisons.</p><p>Results of MRPP analysis for differences among taxonomically defined assemblages in both rivers.</p

Mean %EPT (expressed as a proportion, +/−2 standard errors) per reach across A) riffle sample units on the Calapooia, and B) transects sampled on the Rogue.

<p>In both panels, year is on the x-axis, and lighter-colored bars represent the upstream reaches. Vertical dashed line shows timing of dam removal. Note that the second downstream reach is not plotted for the Rogue River due to the lack of change over time.</p