Pollution and reduction of rivers are threatening specific plants and animals including Fremont cottonwood (Populus fremontii) and Goodding's willow (Salix gooddingii), the threatened Gila trout (Oncorhynchus gilae), the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus).
Populus fremontii (Fremont cottonwood) occurs in California east to Utah and Arizona and south into northwest Mexico; it is similar to the eastern cottonwood, differing mainly in the leaves having fewer, larger serrations on the edge, and small differences in the flower and seed pod structure.
Populus | Populus tremuloides | Populus tremula | Populus alba | Populus nigra | Populus fremontii | Populus sect. Aigeiros | ''populus Romanus'' (Roman people) | ''Populus nigra'', originating in the United States from plantings by André Michaux | Populus lasiocarpa | Populus deltoides |