For most of its long history of building dams and straightening rivers, the U.S. Army Corps of Engineers hasn't had to worry much about endangered species. But that's changing. This spring, the Federal Fish and Wildlife Service released a sweeping document known as a "jeopardy opinion." It says the corps' operations on the Upper Mississippi River threaten two endangered species, including one called the pallid sturgeon. Fish and Wildlife is expected to issue another document soon, saying corps operations threaten the pallid sturgeon on the Missouri river as well. Now, the Fish and Wildlife Service wants the corps to make some changes in how it runs the nation's largest river system. But change for the Corps does not come easy. Minnesota Public Radio's Mary Losure reports.