With time and money, new forests can be planted on deforested land, oil spills and toxic dumps can be cleaned up, stream habitats can be restored.
During the 2007-8 school year, 61 teenagers from Philadelphia-area high schools worked in three Walking the Walk groups, helping to clean up local streams, making blankets for the children of homeless mothers, collecting books for prisoners wishing to pursue their education and visiting a residence for the developmentally disabled.
Dr Jeremy Biggs, Pond Conservation's director, says: "It's hard to clean up our lowland streams, rivers and lakes so that they can support very high quality wildlife.
Mr. Obama, in a nine-minute address to E.P.A. employees, said he was proud of his administration's actions to improve car and truck efficiency, reduce toxic emissions from power plants and clean up rivers and streams.
Some waste managers say that China's new contamination standards are impossible to meet, while others are trying to clean up their recycling streams by slowing down their processing facilities, limiting the types of materials they accept or trying to better educate customers on what belongs in the recycling bin.
Such photos became part of that "clean up your room" stream of nagging from parents.
A combination of public and private financing is helping to clean up a polluted stream running through the development.
For the energetic, there was a five-kilometer run, as well as opportunities to monitor local streams, clean up a park, and plant trees.
They have been mapping the city's waterways since 2010, and also organise expeditions and weekend taskforces, bringing teams of volunteers together to clean up neglected rivers and streams.
