The river is one of many in England and Wales whose biological and chemical quality ratings have improved to good or fair from poor after 15 billion pounds of investment between 1995 and 2000.Agency Chairman John Harman said in a statement: "The overall quality of river water has improved dramatically over the last 10 years. As a result, otters, salmon and an abundance of fish and birds have returned to waterways, including many in urban and industrial heartlands."
The agency said 94 percent of rivers were of good or fair quality in 2000, against just under 85 percent in 1990.
Biological quality improved two percent and chemical quality three percent compared with 1999.
"Good water has the means to support a trout fishery," said John Seager, the agency's head of environmental quality. A "fair" rating means water is suitable to be extracted and treated for drinking water.
In 2000 some 7,000 salmon migrated through the Tees Estuary, considered biologically dead in 1970.
Increased flow, particularly after floods last autumn, helped to clean the rivers, and a decline in heavy industry has reduced pollution.
In the Northwest, 16 percent of rivers are still classified as poor or biologically dead, though this is an improvement from 37 percent a decade ago.