Over the past century the United States has led the world in dam construction.

There are at least 90,000 dams over six feet tall in this country,

and over 2 million shorter than six feet.

More than a quarter have passed their 50 year life expectancy. By 2020, that figure will reach 85%.

On average, we have constructed one dam over six feet tall,

every day since the signing of the Declaration of Independence.

Dans are set in place for irrigation, electricity generation, water storage.

They have lots of society benefits, but it's

undeniable that dams cause significant harm to natural ecosystems. They prevent fish migration and limit access to spawning

habitats. They decrease the flow of the river. Many fish such as salmon and river herring

depend on steady flows to guide them.

Irregular releases of water destroy natural seasonal flow that signals the beginning

of growth and reproduction cycles in many species. They trap massive

amounts of sediment, blanketing rock riverbeds where fish spawn. Larger

objects like rocks and logs get trapped, keeping them from creating complex habitats downstream.

Sometimes the river is stopped entirely behind the dam for periods of time, leaving the riverbed dry. the water gets warmer than it should, affecting sensitive species and leading

algae to bloom.

When water is released, it's often released from the deep, cold, oxygen-dep deprived depths.

Once past the half century mark, dams begin to decay.

The earth around them erodes and seeps, gates rust, concrete loses tensile strength,

and the accumulating settlement reduces the capacity of the reservoirs.

A feeble dam could fail, causing dangerous flow. of the

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