Beavers can help clean polluted rivers and reduce soil loss from farmland, according to a study by Exeter University and the Devon Wildlife Trust.
They looked at a captive colony of beavers in Devon, analysing water before and after it passed through their dams.
They discovered that the dams act as a filter, trapping soil which has run off from farmers' fields, and which contains nitrates and phosphorus from fertilisers..
These chemicals are not allowed into the drinking water supply.
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