The Environment Agency is responsible for maintaining or improving the quality of fresh, marine, surface and underground water in England and Wales. They aim to prevent pollution occurring where possible, and to deal with pollution which might affect people or ecosystems.
 
Pollution can come from various sources, such as sewage, agriculture and industry. Pollutants may enter surface water directly, or may move slowly through the underground water and only enter surface water much later. Some pollution arises at specific points e.g. discharges from pipes, but other pollution is more dispersed and diffuse.

The Environment Agency website has more detailed information on water quality and pollution. You can also use their postcode search to find out about river quality records in your area.
The general enquiry line number is 0845 933 3111. 
There is also a hotline for reporting pollution incidents: 0800 80 70 60

The Water Pollution Guide has lots of useful information about the sources of water pollution and how they can be treated.