### structure | barrier:

Dam
* All dams are types of embankments
  - hold back water and release water, in varying amounts and for different purposes
* Many dams lack fish ladders.
* Most dams are VERY nervous and upset at birthing time, particularly the first whelp
  - consist of mud
  - fill with water
* Most dams have walls
* Most dams impede channel erosion
  - prevent erosion
  - release cold water
  - typically have some sort of lights that cast luminescence upon the water
* Some dams affect stability
  - are formed by earthquakes
  - create areas
* Some dams eliminate habitats
  - nest habitats
  - sandbar habitats
* Some dams have dams
  - virtually no water reserve storage capacity
  - shed the bacteria in their colostrum and milk
  - support invertebrate communities.
  * generally serve the primary purpose of retaining water. The tallest dam in the world is the 300 meter high Nurek Dam in Tajikistan
* also affect water temperature
  - allow water to be contained and saved for use in dry years
  - cause migrating fish to spend more time in slower and warmer waters created by reservoirs
* also create barriers to fish migration
  - impoundments with large surface areas for foraging and transport of food
  - physical barriers to species migration and dispersal
  - develop quite an appetite soon after whelping
  - displace cities and people, because many people choose to live near rivers
  - ensure adequate oxygen for the aquatic life
  - have other uses
  - impede young fish as they migrate downstream on their way out to sea
  - impose societal costs
  - influence stream processes downstream and the migration of fish throughout the watershed
  - interfere with the ability of fish to travel along streams
  - limit lungfish movement, preventing the migration of adults to spawning areas
  - make lakes where thousands of migrating birds stop for a rest on their long flights
  - play an important role in flood control through routing of floods
  - provide flood control, supply water to cities, and can assist river navigation
  - reduce the supply of phosphorus and silicon
  - result in changes in concentrations of dissolved oxygen and nitrogen
* also serve as barriers to migrating trout and salmon
  - people by reducing or preventing floods
* also store water for farm use
  - in big lakes called reservoirs
* alter the flow, temperature and sediment in river systems
  - geography and displace people and wildlife
  - rivers in many ways
* always carry the danger of collapse due to earthquakes, flooding or sabotage
  - seem to create problems while solving others
* are a very common occurrence on American rivers, and the number of dams seems astounding.
* are an essential component of farming
  - important source of electrical power in many countries
  - built across rivers to store water and make electric power
  - centers of energy for the living
  - especially harmful to trout streams because they slow and warm the water
  - important structures for human use of fresh water
  - major obstacles to the movement of fish, either upstream or downstream
  - merely man-made obstacles where female fish are forced to wait and ripen
  - one cause in the decline of the salmon
  - responsible in part for reducing suitable habitat and can cause population isolation
  - structures built on rivers that control the flow of water
* are the cleanest, most environmentally friendly source of mass-produced electricity
  - main reason one-fifth of the world's freshwater fish are now endangered or extinct
  - most largest man-made structures on the planet
  - primary cause of their demise
  - very damaging to rivers
* benefit people by providing usable, reliable water sources.
* block fish from reaching spawning areas, and polluted runoff degrades water quality
  - migration and alter the natural pattern of floods and sediment flows
  - passage upstream to areas where salmon once reproduced and spent their early lives
* block the path of salmon migration
  - transport of organic material, such as large wood and vegetation detritus
* build snow-dens in which they give birth and nurse.
* can affect both the habitat and hydrology of a stream
  - also affect the quality of the river water both above and below the dam
* can also be a problem to fish because they stop the fish from migrating up stream
  - lethal to salmon
  - generate electricity and help control floods
  - help with transportation
* can be a phenomenal attractor for a variety of fish at certain times of the year
  - arch, gravity, or embankment dams
* can create danger if outdoor recreationalists fail to use caution and obey warning signs
  - large reservoirs over what used to be dry land, producing many problems
  - serious safety hazards
  - deprive coastal areas of needed sediments
* can displace people and disturb the ecosystem
  - populations and are often destructive to the environment they are in
  - enhance wetlands and support healthy fisheries
  - only help control floods
  - prevent fish from accessing spawning grounds and destroy habitat
* can provide habitats for many species of wildlife
  - hydropower, irrigation and flood control
  - retain placentas, develop eclampsia, go into uterine inertia or die during a c-section
  - serve many different purposes
  - stop regular annual floods but often fail to hold back exceptionally large floods
  - trap water to be released through turbines
* cause additional damage by blocking fish migrations.
* change the chemical, physical, and biological processes of river ecosystems.
* choke every major river.
* collect sediments as well as fresh water carried by rivers.
* commonly serve a number of purposes.
* contribute in a major way towards wiping out populations of fish.
* create deep reservoirs and can also vary the flow of water downstream
  - pools of still or slow moving water
* destroy spawning habitat and prevent salmon from migrating up and downstream.
* devastate river ecosystems and undermine the rights and livelihoods of affected communities
* die shortly after they abort.
* disrupt sediment transport needed to maintain delta wetlands at river mouths
  - the natural flow of water and disrupt ecosystems upstream and downstream
* fundamentally change the biological, physical, and chemical functions of rivers.
* greatly change the river environment.
* have both positive and negative ecological impacts
  - limited life spans, both structurally and economically
  - major downstream effects in settings throughout the United States
  - profound but varied impacts on the rivers that they harness
* help maintain water depths under the ice, allowing the beavers to stay active all winter
  - to reduce erosion
* hold back silt, debris and other nutrients that create healthy environments for river species
  - debris, and nutrients
  - further erosion
* includes bases
  - sections
* interfere with fish migration, and some still block it completely.
* interrupt the natural rhythm of fish life cycles as they swim from river to ocean and back.
* kill people because they spread diseases such as malaria and because they break
* often cause flooding damage in rural and suburban areas
  - slow and sometimes block salmon from moving upstream
* permanently change where rivers flow and how much water passes through.
* present a significant barrier to migrating fish.
* prevent the natural movement of the species and adversely affect their spawning habits
  - seasonal flooding that create species-rich flood plains
* producing electricity create extreme fluctuations in river levels downstream.
* provide cheap, clean energy but also destroy unique ecosystems
  - clean, pollution free energy, but they can also harm the environment
  - electricity by guiding water down a chute and over a turbine at high speed
  - electricity, water, flood control and irrigation
  - excellent habitat for predators that feed on migrating fish slowed by dams
* reduce the number of juvenile salmon that migrate downstream to the ocean.
* require maintenance
  - treatments
* restrict fish passage to the ocean and back to spawning grounds.
* retain the lubricating layer essential to maintaining flow.
* significantly slow the currents, their turbines also grind up many juveniles.
* slow the current, and piranhas like to breed in the slowest-moving waters.
* stop the flow of water.
* threaten fish.
* transform landscapes and create risks of irreversible impacts.
* wash out important wildlife habitat and block migratory fish from their spawning grounds.