Problems caused by dams are the consequences of this massive engineering program have been devastating.the dams have wiped out species;flooded huge area of wetlands;forest and farmlands;blocked fish migration and many more.
The construction of large dams completely  change the relationship of water and land destroying the existing of ecosystem balance, which in many cases,has taken thousands of years to create. Currently,  there are around 40,000 large dams which abstract the World's rivers,completing changing their circulation system this is not going to occur without dire environmental impacts. 
Increasingly, there are attempts by dam builders and investors to ensure that dams are built in a more sustainable way. WWF is partnering with banks, industry, and other NGOs to develop and promote environmental standards and best practices for dams.