Whether to build a new bridge across the Bitterroot River between South Avenue and Blue Mountain is still a raging debate. What isn't debatable is there will be impacts to wildlife. Riparian areas are the most productive habitat in Montana, and the easiest to damage with development.
The footprint of the bigger bridge will likely require the toppling of hundreds of trees and shrubs, from ancient ponderosa pines and cottonwoods to birch, hawthorns and willows. This is habitat that supports deer, bear and a high diversity of avian species, from hummingbirds to bald eagles, and which we have less of every year in the valley. I've observed over 30 species of birds breeding here, and many more species migrating through.
When the old steel bridge is removed so too will disappear one of the deepest holes in this stretch of the river, a key refuge for fish through hot summers. The proposed bridge will be constructed where O'Brien Creek hits the Bitterroot, a place where Montana Fish, Wildlife and Parks has invested funds for restoring this "extremely important spawning tributary" (FWP comments, 2013) for trout. The nearby hunting perches of osprey and eagle will be lost too, as hydrologists state that some of the approximately 10-acre island upstream of the historic bridge will rapidly erode after removal.
This new backdoor into Missoula will cause higher vehicle volumes and speeds through the Blue Mountain Recreation Area, so more collisions with deer, bear, turkeys, elk (and probably even pets and pedestrians). Presently the traffic across the old Maclay Bridge is so leisurely that I've watched deer amble across the one-lane span, while motorists happily wait. A full environmental impact statement is needed to fully assess if the South Avenue project can go forward.