What is the primary difference between the environmental impacts of impoundment vs. run-of-the-river systems?

Impoundment systems cause massive habitat loss through flooding and significantly alter downstream flow, whereas run-of-the-river systems maintain more natural flow patterns and require much less land flooding.

How does tidal power impact marine ecosystems differently than river-based hydro?

Tidal power can disrupt coastal sediment transport and poses a risk of mechanical injury to marine mammals and fish from underwater turbines, whereas river hydro primarily impacts freshwater migration and terrestrial flooding.

Why is 'low carbon' a more accurate description of hydroelectricity than 'zero carbon'?

While the operation doesn't burn fuel, the construction uses carbon-intensive materials, and the decomposition of flooded vegetation in reservoirs can release methane ($CH_4$) and carbon dioxide ($CO_2$).

What is a common misconception regarding the 'cleanliness' of reservoir water released downstream?

Many assume it is just water, but it is often lower in dissolved oxygen and higher in heavy metals or different temperatures than the natural river, which can harm downstream species.

Why is it incorrect to say that fish ladders completely solve the problem of fish migration?

Fish ladders only assist some species and individuals; many fish still fail to find the entrance, and the ladders do not help juvenile fish migrating downstream through dangerous turbines.

What error is made when evaluating the lifespan of a dam without considering siltation?

Siltation (sediment buildup) gradually fills the reservoir, reducing its storage capacity and energy potential over time, eventually making the dam useless if not managed.

Define 'Siltation' in the context of hydroelectric dams.

Siltation is the accumulation of sediments (sand, dirt, organic matter) behind a dam, which would naturally have flowed downstream to nourish deltas and floodplains.

What is a 'Fish Ladder'?

A structure built alongside or around a dam to allow migratory fish to bypass the barrier by swimming and leaping up a series of low steps.

Define 'Thermal Stratification' in a reservoir.

The separation of reservoir water into layers of different temperatures, typically with a warm surface layer and a cold, dense, oxygen-poor bottom layer.

How does the anaerobic decomposition of organic matter in reservoirs contribute to climate change?

When plants are flooded, they rot in an environment without oxygen, which produces methane ($CH_4$), a greenhouse gas with a much higher warming potential than $CO_2$.

Environmental Impacts of Hydroelectricity | College Board AP Environmental Science

Revision Notes

College Board

Environmental Science

Unit 6: Energy Resources & Consumption

Environmental Impacts of Hydroelectricity

Summary

While hydroelectricity is a renewable and low-carbon energy source, its implementation involves significant environmental trade-offs. These range from the physical transformation of terrestrial habitats into aquatic reservoirs to the alteration of downstream water chemistry and the disruption of biological migration patterns.

1. Ecological and Habitat Transformation

Habitat Loss and Fragmentation: The construction of large-scale impoundment dams requires the flooding of vast upstream valleys, which permanently destroys terrestrial ecosystems and displaces local flora and fauna.
Disruption of Migration: Dams act as physical barriers that prevent the movement of migratory fish species, such as salmon, which must travel between freshwater and saltwater to complete their life cycles.
Reservoir Ecology: The transition from a lotic (flowing water) to a lentic (still water) environment favors different species, often leading to a loss of native biodiversity and the potential introduction of invasive species.

Diagram showing a dam creating an upstream reservoir and blocking fish migration paths.

2. Water Quality and Physical Alterations

Thermal Stratification: Large reservoirs can cause water to stratify by temperature; when deep, cold, oxygen-poor water is released through turbines, it can shock downstream aquatic life.
Sediment Trapping (Siltation): Dams prevent the natural flow of nutrient-rich silt to downstream floodplains and deltas, leading to increased erosion and loss of soil fertility in those areas.
Dissolved Oxygen Levels: The decomposition of organic matter in newly flooded reservoirs consumes oxygen, potentially creating hypoxic conditions that are lethal to many aquatic organisms.

3. Greenhouse Gas Implications

4. Socio-Economic and Human Impacts

Human Displacement: The creation of reservoirs often necessitates the forced relocation of indigenous communities and local populations, leading to the loss of cultural heritage and livelihoods.
Downstream Water Security: Changes in river flow can impact communities downstream that rely on the river for irrigation, transportation, or drinking water.
Risk of Failure: Although rare, the catastrophic failure of a dam poses a significant safety risk to all populations living in the downstream flood zone.

5. Key Distinctions: Impact by Plant Type

6. Exam Strategy & Tips