AP Environmental Science

## Learning Outcomes

- Understand why hydroelectric power is considered renewable but not impact-free
- Identify major environmental tradeoffs of hydropower dams, including habitat flooding and fish migration barriers
- Explain how dams change sediment transport, river flow, water temperature, and dissolved oxygen
- Distinguish between impoundment and run-of-river systems
- Evaluate why fish ladders and other mitigation methods reduce impacts but do not fully solve them
- Explain the tradeoffs involved in dam removal
- Apply cause-and-effect reasoning about hydroelectric impacts in AP Environmental Science questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand hydroelectric environmental tradeoffs, with a focus on the following syllabus points:

- Know that hydroelectric power generates electricity from moving water and is a renewable energy source with low direct air pollution during operation
- Identify major benefits of dams, including electricity generation, flood control, water storage, irrigation, and recreation
- Explain how dam construction floods upstream land and changes river habitatss
- Understand that dams can block migratory fish, especially anadromous fish such as salmon
- Explain how reservoirs trap sediment and reduce downstream sediment delivery
- Know that dams can alter flow regime, water temperature, and dissolved oxygen downstream
- Understand that reservoirs may emit carbon dioxide and methane when flooded vegetation and soils decompose
- Distinguish between impoundment systems and run-of-river systems in terms of storage and environmental disruption

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why can hydroelectric power be described as renewable but still environmentally damaging?
2. How does a dam interfere with fish migration even when a fish ladder is present?
3. Why does sediment often build up behind dams, and why does that matter downstream?
4. How can a reservoir change downstream water temperature and dissolved oxygen?
5. What is one major ecological benefit and one major short-term ecological cost of dam removal?

## Introduction

## Introduction

Hydroelectric power is one of the clearest AP Environmental Science examples of an environmental tradeoff. It produces electricity without the smokestack emissions associated with coal or oil, and it can also provide flood control and water storage. At the same time, it changes rivers in ways that can damage habitats, block migration routes, alter water quality, and disrupt food webs.

For the exam, the key is not to treat hydropower as simply “good” or “bad.” You need to show how a dam changes a river system step by step. A strong APES answer explains the mechanism: dam construction changes flow, storage, temperature, oxygen, and sediment movement, and those physical changes then affect organisms.

> **Key Term: hydroelectric power**
> Electricity generated when moving water turns turbines.

### Why hydropower is useful but still a tradeoff

Hydropower is classed as a **renewable** energy source because flowing water is continuously replenished by the water cycle. In operation, a hydroelectric plant has low direct air pollution compared with fossil-fuel plants. That matters for APES because it means hydropower can reduce emissions of sulfur dioxide, nitrogen oxides, particulates, and large amounts of carbon dioxide from electricity generation.

But “renewable” does not mean “no environmental cost.” The main tradeoff comes from the fact that most large hydroelectric systems depend on dams and reservoirs. A free-flowing river is converted into a managed water-storage system. That changes the habitat both upstream and downstream.

Benefits commonly linked to hydroelectric dams include:

- renewable electricity generation
- low operating costs once built
- flood control
- water storage for cities and agriculture
- recreation such as boating and fishing

Costs commonly linked to hydroelectric dams include:

- flooding of land behind the dam
- displacement of people and wildlife
- habitat fragmentation
- blocked fish migration
- trapped sediment
- altered downstream flow, temperature, and oxygen
- possible disease-vector habitat in slow-moving water
  > **Key Term: reservoir**
  > The artificial lake formed behind a dam where water is stored.

A very common APES mistake is to stop at “dams destroy habitat.” That is true, but too vague. Better answers specify **which** habitat is lost and **how** that loss affects species. For example, forest, grassland, or river habitat upstream may be flooded, while downstream habitat may be degraded by colder, low-oxygen releases or by reduced sediment supply.

> **Test Tip:** In FRQs, pair every impact with a mechanism. Do not write only “dams hurt fish.” Write “dams block access to upstream spawning habitat and can also injure downstream migrants in turbines.”

### Upstream flooding, habitat change, and reservoir effects

The first major environmental change happens when the dam is built and the reservoir fills. Land that was once river valley, forest, wetland, or farmland is submerged. Species adapted to terrestrial or fast-flowing river environments may be lost from that area.

This flooding can also create new habitat, but it is not the same habitat. A reservoir is usually deeper, wider, and slower-moving than the original river. Species that thrived in fast, oxygen-rich current may decline, while species better adapted to still or slow water may increase. This is why APES often treats reservoir creation as habitat conversion, not just habitat destruction.

Flooded vegetation and soils create another tradeoff. When organic matter decomposes underwater, microbes break it down. Where oxygen is limited, decomposition can produce **methane** as well as carbon dioxide. So hydropower is usually lower-emission than fossil fuels during operation, but some reservoirs are not greenhouse-gas free.

> **Key Term: impoundment**
> A hydropower system that stores water behind a dam in a large reservoir.

In warm regions or where lots of biomass is flooded, decomposition can be especially important. For exam purposes, be careful with wording: some reservoirs emit greenhouse gases; do not claim that all hydropower reservoirs are major methane sources.

Reservoirs also often have a larger exposed water surface than the original river channel. That can increase evaporation, especially in hot or dry climates. So a project meant to store water may also lose more water to the atmosphere than the undammed river did.

Another issue is public health. Slow-moving water behind dams can provide breeding sites for mosquitoes and other disease vectors. This is not always the main APES focus, but it is a valid environmental drawback because changes in water movement can change disease ecology.

### Fish migration barriers and why fish ladders are limited

Fish impacts are among the most tested hydroelectric tradeoffs. Dams break river continuity. Migratory fish that evolved to move upstream and downstream now encounter a barrier they cannot naturally pass.

> **Key Term: fish migration barrier**
> A physical obstruction, such as a dam, that prevents or reduces fish movement to feeding, rearing, or spawning habitats.

Salmon are a classic example. They are **anadromous** fish: they hatch in freshwater streams, migrate to the ocean, then return to freshwater to spawn. If a dam blocks access to upstream spawning habitat, adults may fail to reproduce successfully. Over time, populations can shrink or disappear from that river system.

Dams do more than block upstream travel. They also interfere with downstream movement. Juvenile fish moving downstream may be drawn into water intakes, carried through spillways, or sent through turbines.

> **Key Term: entrainment**
> The drawing of fish, eggs, or larvae into dam intakes or water-diversion structures.
>
> **Key Term: turbine mortality**
> Injury or death of fish passing through turbines due to blade strike, pressure change, turbulence, or shear stress.

Students often overestimate the value of fish ladders. Fish ladders can help some fish move upstream by creating a stepped route around the barrier. That can reduce the problem, but not eliminate it.

Why are fish ladders incomplete as a solution?

- not all species use them equally well
- some fish still fail to locate the ladder entrance
- ladders mainly help upstream passage
- they do little to solve downstream entrainment and turbine mortality
- delays at dams can increase stress, disease exposure, and predation

This matters because an APES answer that says “build a fish ladder and the problem is solved” is too simplistic. A stronger answer says fish ladders are a mitigation strategy with limited effectiveness.

Large reservoirs can also create slow, warm pools that differ greatly from river habitat. Salmon and other cold-water species may be delayed in migration, exposed to higher temperatures, and made more vulnerable to predators and pathogens.

> **Test Tip:** If an MCQ or FRQ mentions salmon, spawning, or dams, think in two directions: upstream access to spawning habitat and downstream survival of juveniles.

### Sediment trapping, altered flow regime, and water-quality changes

A natural river carries suspended sediment: silt, clay, and organic particles. When fast-moving river water enters a reservoir, its velocity drops. As the water slows, sediment settles out and accumulates behind the dam.

> **Key Term: sediment trapping**
> The settling and accumulation of sediment in a reservoir instead of transport downstream.

This has several consequences. First, the reservoir gradually fills with sediment, reducing storage capacity over long periods and sometimes requiring dredging. Second, downstream reaches receive less sediment than they did naturally.

Why does that matter downstream?

- floodplains receive less nutrient-rich silt
- spawning habitat may change
- river channels may become sediment-starved
- bank and channel structure can shift
- wetlands and downstream habitats may receive fewer replenishing materials

A strong cause-and-effect chain would be:

dam slows water -> sediment settles in reservoir -> less sediment reaches downstream floodplain -> reduced nutrient deposition and altered habitat quality

Flow regime also changes.

> **Key Term: altered flow regime**
> A change in the timing, magnitude, frequency, or duration of river discharge caused by dam operations.

Natural rivers rise and fall in seasonal patterns. Dams often flatten, delay, or abruptly change these patterns. In hydropower systems, operators may release more water when electricity demand is high. This can create **hydropeaking**, where downstream depth and velocity change quickly over short periods.

Such rapid changes can:

- strand fish eggs or juveniles in shallow areas
- disturb spawning cues
- erode stream banks
- reduce natural flooding of floodplains
- change which species can survive downstream

Water quality also changes because reservoirs are more lake-like than river-like. In deeper reservoirs, warm surface water and colder deeper water may separate into layers.

> **Key Term: thermal stratification**
> The layering of reservoir water by temperature, often with warm water near the surface and colder water at depth.

If a dam releases deep water, that water may be colder than the natural downstream river would be in summer. It may also have lower dissolved oxygen because deeper water is more isolated from atmospheric mixing and can lose oxygen as decomposition proceeds.

> **Key Term: dissolved oxygen (DO)**
> The amount of oxygen dissolved in water and available for aquatic organisms.

Low DO stresses fish and invertebrates. Temperature stress and oxygen stress often act together, making downstream habitat less suitable.

Conceptually, the sequence is:

reservoir forms -> water slows and stratifies -> deep water becomes colder and may have lower DO -> dam releases deep water -> downstream organisms experience changed temperature and oxygen conditions

This is why AP questions may ask about altered water temperatures or hypoxia below dams.

### Comparing hydropower systems and evaluating solutions

Not all hydropower projects create the same level of disruption. Large **impoundment** dams usually cause the greatest flooding and the largest changes in sediment storage and flow control. By contrast, **run-of-river** systems use river flow with little or no major reservoir storage.

> **Key Term: run-of-river**
> A hydropower system that uses flowing river water with limited storage and generally less flooding than a large reservoir dam.

A run-of-river system often has:

- less flooding of upstream land
- less large-scale reservoir formation
- less thermal stratification
- less storage for flood control or water supply
- less ability to smooth out seasonal flow variation

So the tradeoff is narrower environmental disruption in some ways, but also less controllable and less flexible power generation.

Mitigation measures for dam impacts include:

- fish ladders or fish passage systems
- spillway releases for safer downstream fish movement
- selective water withdrawal to improve release temperature
- water releases timed to support migration
- habitat restoration nearby
- sediment management
- in some cases, dam removal

Dam removal is another tradeoff question. Long term, removal can restore river connectivity, allow fish migration, and reestablish more natural sediment transport and flow patterns. But short term, removal can release stored sediment, increase turbidity, and disrupt existing reservoir habitat.

So a balanced APES judgement might be:

Dam removal may improve long-term ecological function by restoring migration routes and sediment flow, but it can also cause short-term sediment pulses and loss of reservoir services such as recreation or water storage.

The best APES answers do not assume one universal solution. They ask what the river is used for, which species are present, how large the dam is, and whether the main goal is electricity, flood control, or habitat restoration.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Hydroelectric power is renewable but still has major environmental tradeoffs
- Dams provide benefits such as electricity generation, flood control, water storage, and recreation
- Reservoir formation floods upstream land and converts river or terrestrial habitat into still-water habitat
- Flooded vegetation and soils can decompose and release carbon dioxide and methane
- Dams act as fish migration barriers, especially for anadromous fish such as salmon
- Fish ladders can improve some upstream passage but do not fully solve downstream entrainment or turbine mortality
- Reservoirs trap sediment, reducing downstream sediment and nutrient delivery
- Dams alter flow regime, including the timing and magnitude of discharge
- Reservoir thermal stratification can change downstream water temperature and dissolved oxygen
- Run-of-river systems usually cause less flooding than large impoundment systems
- Dam removal can restore fish passage and sediment flow but may cause short-term turbidity and sediment-release problems
- Strong APES answers explain the mechanism linking dam construction to ecological impacts

## Key Terms and Concepts

- hydroelectric power
- reservoir
- impoundment
- fish migration barrier
- entrainment
- turbine mortality
- sediment trapping
- altered flow regime
- thermal stratification
- dissolved oxygen (DO)
- run-of-river


Hydroelectric Power - Hydroelectric environmental tradeoffs