Dam removals Flashcards
(25 cards)
Why do we build dams? And what ecosystem services does that provide?
To impound and control water
- Hydropower production
- Irrigation
- Drinking water
- Industrial cooling
- Navigation
- Flood control
How many dams are there in Europe and how many of these are obsolete?
1.2 million dams, 15% (200.000) dams are considered obsolete and unrequired
How many dams are there in Sweden in total and in active use?
10.701 dams, 2.757 are used for regulation (whereby approx. 1800 are active)
What are some ecological effects of dams?
- Water reservoirs creats a lake with deeper, still water causing a shift from lotic (flowing water) to lentic (standing water) systems
- Much larger water area
- Traps sediments and nutrients
- Changes of the bottom sediment, smaller gravel and sand are often removed
- Obstruction of passage up/down stream
- Regulated river flows which causes short and long term changes to the natural hydrograph
What are the differences between an unaffected system and a strongly affected system?
Unaffected system:
High species richness of naturally occuring species, high habitat complexity
Strongly affected system:
Low species richness of naturally occuring species, changed species communities, simplified habitat complexity
Name 5 arguments that supports dam removal.
- Structural safety,
when concrete ages it weakens and becomes unsafe to operate. Removal is cheaper than restoration. - Reservoir siltation,
siltation reduces the dam’s ability to store water and produce electricity - Marginal benefits,
poor design, inefficient turbines or changing societal needs have made some old dams obsolete. - Restoring fish migration routes
- Increase attractiveness for sport fishing or recreation
What economic benefits are possible for dam removals?
Sport fishers/recreational fishers appreciated natural rivers over regulated rivers with dams much more, and were willing to pay more for fishing permits if dams were removed
What is one of the main issues with dam removals?
Dams are removed at an increasing rate, but monitoring and evaluation of the effects of dam removal is severely lacking. Only 5% of removals have been evaluated between 1970-2000.
What happens when a dam is removed?
Massive amounts of sediments that have been captured in the reservoirs get released down stream, this causes:
- clogging in spawning areas
- killing/stressing aquatic organisms
- changing nutrient conditions, nutrient loaded sediments –> nutrient lacking down stream habitats
- Spreads toxic substances down stream, that before was contained in the reservoir
What are some factors that can limit the recovery of fish populations, despite dam removal?
Recovery depends on wheter the fish species is limited by migration barriers or available habitat.
If the fish is limited by available habitat geomorphological processes (how the bottom is shaped by natural processes) is important
Can dam removal be considered either good or bad?
- It depends on whether you look at down stream effects or effects in the former reservoir
- Depends on what type of organism you look at
- Depends on what you measure (community composition or species/taxon richness)
- Recovery can take a long time, and the timing after removal when new measurments of the targets are taken can cause drastically different results in some cases.
Name 4 types of ecosystem services and give a few examples for each type.
- Supporting
- Nutrient cycling
- Soil formation
- Primary production - Provisioning
- Food
- Fresh water
- Wood and fiber
- Fuel - Regulating
- Climate regulation
- Flood regulation
- Disease regulation
- Water purification - Cultural
- Aesthetic
- Spiritual
- Educational
- Recreational
What are 5 arguments opposed to removal of dams?
- Loss of pedestrian access, the dam could have provided a bridge over the water
- Loss of reflecting pond and bathing place when reservoir is removed
- Cultural heritage of old mill, dam or power station
- Reservoirs support fish farming, causing economic loss if removed
- Benthic fauna and riparian vegetation are harmed by lower water level when reservoir is removed.
Which 3 strategies are preferred when restoring conditions in a regulated river to better support biodiversity?
- Removing dams
- Managing dams to re-create important features of natural flow regimes
- Reconnecting rivers to floodplains by removing levees (sv. vallar) that hold back the water
Removal of a dam may imply both ecological risks and ecological benefits. What are the main ecological benefits hoped for when a dam is removed? (6 benefits listed)
- Restoration of Fish Passage and Aquatic Organism Migration
- Restoration of Natural River Flow Regimes
- Restoration of Sediment Transport
- Improvement of Water Quality
- Restoration of Riparian Habitat
- Increased Habitat Connectivity and Biodiversity
How does dam removal help:
1. Restoration of Fish Passage and Aquatic Organism Migration?
Dams are significant barriers to the movement of fish and other aquatic species, preventing them from accessing upstream spawning grounds, foraging areas, and diverse habitats. Removing a dam allows for the reconnection of fragmented river systems, facilitating the natural migration patterns of species like salmon, steelhead, eels, and herring. This can lead to rapid recovery and increased populations of these species, which are often vital to the broader ecosystem.
How does dam removal help:
2. Restoration of Natural River Flow Regimes?
Dams alter the natural flow of rivers, creating stagnant reservoirs upstream and often leading to reduced and regulated flows downstream. Dam removal allows the river to revert to its natural hydrological processes, including seasonal fluctuations in water level, velocity, and temperature. This restoration of a free-flowing, dynamic river system is crucial for the health of aquatic and riparian (streamside) ecosystems.
How does dam removal help:
3. Restoration of Sediment Transport?
Dams trap vast amounts of sediment behind them, starving downstream reaches of the gravel, sand, and fine particles essential for creating and maintaining diverse riverbed habitats. This sediment starvation can lead to increased bank erosion downstream and a loss of important spawning and foraging areas for fish. Dam removal allows for the natural transport of sediment, replenishing downstream habitats, rebuilding deltas, and nourishing floodplains. While there can be an initial surge of sediment after removal, rivers typically recover and stabilize within a few years.
How does dam removal help:
4. Improvement of Water Quality?
Reservoirs created by dams often experience higher water temperatures, lower dissolved oxygen levels, and increased nutrient buildup, which can lead to algal blooms and other water quality problems. By restoring natural flow and eliminating the stagnant impoundment, dam removal can improve water quality, leading to healthier conditions for aquatic life.
How does dam removal help:
5. Restoration of Riparian Habitat?
The areas along a river (riparian zones) are intimately connected to the river’s flow and sediment dynamics. Dams can degrade these habitats by altering water levels, preventing natural flooding that nourishes floodplains, and changing the types of vegetation that can thrive. Dam removal helps to restore these crucial riparian habitats, which provide diverse conditions for a wide range of plants, insects, birds, and other wildlife.
How does dam removal help:
6. Increased Habitat Connectivity and Biodiversity?
By addressing all of the above (1-5), dam removal fundamentally re-establishes the longitudinal connectivity of the river. This allows for the movement of not just fish, but also nutrients, organic matter, and other organisms, leading to a more interconnected and resilient ecosystem with increased biodiversity.
In essence, what does dam removal aim to achieve?
In essence, dam removal aims to allow rivers to recover their natural dynamism and ecological functions, leading to a cascade of positive effects on the physical, chemical, and biological aspects of the river and its surrounding environment.
Based on ecological assessments of dam removals, describe the main observed effects on fish. Consider the time needed for recovery in your answer .
Observed Effects:
Rapid Reconnection: Immediate access to upstream habitats for migratory species (e.g., salmon, eels).
Species Shift: Decline of stagnant-water (lentic) species; increase of flowing-water (lotic) native riverine species.
Improved Habitat: Restoration of spawning gravels and better water quality.
Nutrient Cycling: Re-establishment of nutrient flow (e.g., marine-derived nutrients from salmon).
Time for Recovery:
Weeks-Months: Upstream passage, initial recolonization.
1-5 Years: Significant fish community shifts, evidence of spawning success.
5-15+ Years: Full population recovery and stable riverine community.
Based on ecological assessments of dam removals, describe the main observed effects on macroinvertebrates. Consider the time needed for recovery in your answer .
Observed Effects:
Initial Disturbance: Short-term decline due to sediment release.
Lotic Species Increase: Recovery and shift towards flowing-water indicators (e.g., EPT orders).
Habitat Diversification: More diverse riverbed substrates support a wider variety of species.
Time for Recovery:
Months-2 Years: Initial recovery of sensitive taxa and community structure.
2-7 Years: General recovery in taxonomic richness and similarity.
