How much is enough?
Do you know how many dams there are in the world? What about in your country or your state? The numbers may surprise you.
In the United States, there are over 91,000 dams. They are present in all 50 states. In my home state of Colorado, there are at least 14 dams on the main stem of the Colorado River with hundreds more on its tributaries. There are at least 2000 dams in the Hudson River watershed in New York state. There are more than 140,000 dams in Kansas.
China has at least 98,000 dams. The Amazon Basin states have built around 412 dams. In terms of total storage capacity of dams, the top 5 countries are Canada, China, Russia, the US, and Brazil.
I say at least for these numbers because the exact number of dams is not known. Various sources cite different numbers. This is because dams are managed by multiple entities (government, public, and private), defined by size (small, large, mega) and function (energy production, irrigation, recreations, etc.), and some are decommissioned or abandoned but still standing.
Given these variations, the total number of dams in the world is estimated to be between 900,000 and 2.8 million.
As a result, over 500,000km of rivers are impacted and “many great rivers are little more than staircases of reservoirs” (see McCully 1996:6). Only 37% of long rivers (>1000km) in the world remain free-flowing and only 23% reach the ocean unimpeded.
The numbers of dams matters, but so does the size of dams. ICOLD, the International Commission on Large Dams, defines large dams as “A dam with a height of 15 metres or greater from lowest foundation to crest or a dam between 5 metres and 15 metres impounding more than 3 million cubic metres.” Small dams are roughly thought to be under 15 meters in height. While on the other end of the spectrum, Mega dams are those above 150 meters high.
There are at least 40,000 mega dams around the world (and around 800,000 small dams). Some mega dams include the Oroville dam in the US (the largest in the United States), the Three Gorges dam in China (the world’s largest dam), the Hoover dam in the US, and the Glen Canyon dam in the US (the last two both situated on the Colorado River).
Many of these dams are old and pose significant risk to peoples and ecosystems. The average age of US dams are 57 years, meaning they are likely not up to date in terms of standards or technology. Aging infrastructure is a serious problem throughout the United States. I am not clear as to how much of the US $1 trillion infrastructure bill will go towards needed updates but it will likely be insufficient.
It would cost $70 billion to rehabilitate all the dams in the US. In 2016, costs to update, repair, and rehabilitate dams were estimated at $60.7 billion, up from $34 billion in 2001 - these numbers are likely to increase. The 2021 Infrastructure report card states that at least 15,600 dams in the US are classified as high-hazard structures (loss of life likely in a dam failure). An additional 11,343 dams are considered to be of significant hazard-potential.
Even though we have upwards of 3 million dams, many argue that is not enough. 3700 dams are already in planning or construction stages despite the droughts and floods currently stressing dam systems.
How many will be enough?