BUTTE, Mont. — The recent death of thousands of snow geese on a toxic artificial lake here high in the Rocky Mountains once again underscored the devastating environmental legacy of more than a century’s worth of copper mining.

In the middle of the migratory season, some 3,000 to 4,000 of the large white geese unwarily spread their black-tipped wings and settled down into the acidic wastewater that fills the lake, a 700-acre former open pit mine.

It is not known yet exactly what killed the enormous flock. But the last time large numbers of birds died here — 342 in a 1995 incident — it was because they drank the toxic reddish-brown water and it damaged their brains and other organs.

In a natural spectacle, huge flocks of snow geese have been migrating south from Canada to the southwestern United States, passing through Montana by the tens of thousands. The skies are so crowded in some places that people complain the collective din of flapping wings and honking keep them awake at night.