The San Pedro River and the surrounding riparian area are one of the major flyways in North America for migrating birds. Loss of this habitat would do incalculable damage to the ability of birds to continuing their ancestral migration patterns.
Because this is a desert environment, mammals are continually in search of water. The construction of an expressway that would bar them from ready access to the river would undoubtedly affect them adversely, even to the extent of causing extinction of some species.