Reintroduced beavers have spent the past decade doing what beavers do, which is to say building dams in small streams without consulting anybody about it. Hydrologists have been following them with measuring equipment.
The dams slow water in headwater catchments, holding it in ponds and wet ground rather than sending it downstream in a single pulse. In modelling terms it resembles a great many very small reservoirs, built and maintained at no public cost.
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