With large, semi-aquatic rodents, of course!
Rivers and streams that have been diverted by humans are designed to remove water quickly from the watershed, destroying local habitats for animals and making it more difficult for an ecosystem to recover from drought. Beavers build infrastructure which help to slow the flow of water, letting it recharge local aquifers, and preventing erosion which helps keep plants alive.
In the article Gizmodo references, one of the specific benefits beaver dams provide is pretty striking:
In several stream systems in the region, says Beesley, the only places where salmon — especially endangered Coho — have survived after four years of below-average rainfall are beaver ponds.
Another Gizmodo article covers how, apparently, beaver waterworks can also help clean the water of excess nitrogen from agricultural runoff:
Biologists at the University of Rhode Island were studying the nitrogen content of streams and noticed something odd: whenever there were beaver ponds upstream, nitrogen levels dropped. Beaver ponds slow down river water, and they mix it with organic matter, which must have an effect on river chemistry, but scientists didn’t know exactly what was happening in that murky water.
So they made soda-bottle-sized “ponds” that let them study variations on the conditions the beavers set up in their real-life ponds. And they found a kind of reverse nitrogen fixation process was occurring — call it “denitrification.” Bacteria in the dirt and the plant debris turned nitrates into nitrogen gas. The gas bubbled up to the surface and mixed with the atmosphere once more. In some cases, the level of nitrogen in the water dropped 45%.
Not covered in any of these articles was what method might be used to reintroduce beaver into areas where the animals have been driven out, but I suspect it won’t be via parachute, as was done in Idaho in the 1950s (do read the article at Boise State Public Radio, too, for a fuller story).