Conceptualizing Ecological Responses to Dam Removal: If You Remove It, What's to Come?

Abstract

One of the desired outcomes of dam decommissioning and removal is the recovery of aquatic and riparian ecosystems. To investigate this common objective, we synthesized information from empirical studies and ecological theory into conceptual models that depict key physical and biological links driving ecological responses to removing dams. We define models for three distinct spatial domains: upstream of the former reservoir, within the reservoir, and downstream of the removed dam. Emerging from these models are response trajectories that clarify potential pathways of ecological transitions in each domain. We illustrate that the responses are controlled by multiple causal pathways and feedback loops among physical and biological components of the ecosystem, creating recovery trajectories that are dynamic and nonlinear. In most cases, short-term effects are typically followed by longer-term responses that bring ecosystems to new and frequently predictable ecological condition, which may or may not be similar to what existed prior to impoundment.

Keywords: conceptual models; dam removal; disturbance; ecological modeling; river restoration.

Figure 1.

Spatial domains influenced by dam removal: (a) upstream of the reservoir, (b) within the reservoir or former impoundment, and (c) downstream of the dam. The boxes on the right represent the dominant processes that influence ecological responses in each domain.

Figure 2.

Causal-loop diagram depicting the cause-and-effect links and associated feedback loops influencing dam removal responses upstream of the former reservoir. Following dam removal, mobile organisms such as fish can recolonize upstream habitats, increasing upstream species richness. Recolonization is self-reinforced by feedback loops that promote productivity and diversity of upstream habitats. The shaded shapes indicate key ecological parameters. The arrows indicate the direction of influence, and the plus and minus signs indicate whether the influence is positive or negative. When they are positive, the variables change in the same direction (when causal variable increases the effected variable also increases or vice versa). When they are negative, the variables change in the opposite direction (when causal variable increases the effected variable decreases or vice versa). Causal links that control responses at short time scales (hours to years) and long time scales (years to decades) are shown in orange and yellow, respectively.

Figure 3.

Causal-loop diagram depicting the cause-and-effect links and associated feedback loops influencing dam removal responses within the former reservoir. Sediment erosion and changes in channel hydraulics alter the environment from one that favors pelagic production and lentic fish assemblages to one that favors benthic production and lotic fish assemblages. The shaded shapes indicate key ecological parameters. The arrows indicate the direction of influence, and the plus and minus signs indicate whether the influence is positive or negative. When they are positive, the variables change in the same direction (when causal variable increases the effected variable also increases or vice versa). When they are negative, the variables change in the opposite direction (when causal variable increases the effected variable decreases or vice versa). Causal links that control responses at short time scales (hours to years) and long time scales (years to decades) are shown in orange and yellow, respectively.

Figure 4.

Causal-loop diagram depicting mechanistic links and feedback loops influencing dam removal responses downstream of a former dam site. Release of sediment, nutrients, and organic matter from the former reservoir effect aquatic organisms and riparian vegetation via numerous causal pathways. Initial deposition of sediments, for example, can bury benthic and riparian organisms, but as this initial sediment pulse is eroded, new habitats for aquatic organisms are created (e.g., spawning gravel for fish). The long-term recovery of species is facilitated by the reestablishment of the natural flow, temperature, sediment, and nutrient regimes to which native organisms are adapted. The shaded shapes indicate key ecological parameters. The arrows indicate the direction of influence, and the plus and minus signs indicate whether the influence is positive or negative. When they are positive, the variables change in the same direction (when causal variable increases the effected variable also increases or vice versa). When they are negative, the variables change in the opposite direction (when causal variable increases the effected variable decreases or vice versa). Causal links that control responses at short time scales (hours to years) and long time scales (years to decades) are shown in orange and yellow, respectively.

Figure 5.

Ecological-response trajectories in upstream, reservoir, and downstream reaches following dam removal. Three hypothetical trajectories are presented for each location, with rationale that explain why these alternative trajectories might emerge. The vertical line on each plot indicates the time of dam removal, and the horizontal line represents the ecological condition that existed prior to dam construction. Recovery to predam conditions are unlikely if natural flow, temperature, sediment, and nutrient regimes remain altered by other dams, if reservoir sediment contains contaminants, and if nonnative species are present. The colored sections of the trajectories indicate the short-term (orange) and long-term (yellow) ecological responses to dam removal.

Figure 6.

Quantitative models can be used to simulate ecological responses to dam removal. In this case, a river food-web simulation model was used to predict downstream trophic responses to a hypothetical dam removal. The top panel (a) shows how physical and chemical responses to dam removal (shaded graphs) affect the dynamics of the modeled food web (e.g., water turbidity influences the amount of light that reaches the streambed to fuel periphyton production). The bottom panel (b) shows the resultant biomass dynamics of fish, aquatic invertebrates and periphyton. Abbreviations: D₅₀, median particle size of benthic substrate; FNU, formazin nephelometric units; SRP, soluble reactive phosphorus.

Figure 7.

Environmental data (a–g) used to parameterize the aquatic trophic productivity model for dam removal on the Elwha River (Washington state), and simulated outputs (h–k) for fish, invertebrate, periphyton and detritus biomass. Abbreviations: AFDM, ash-free dry mass; D₅₀, median particle size of benthic substrate; DIN, dissolved inorganic nitrogen; FNU, formazin nephelometric units; SRP, soluble reactive phosphorus