Final answer:
Habitat destruction, largely from damming, is the most widespread ecological problem with rivers in the USA, affecting river flow and habitats. This has resulted in the decline of many species adapted to natural river conditions.
Step-by-step explanation:
The most widespread ecological problem with rivers in the USA is habitat destruction, specifically from the damming of rivers, which causes a series of issues affecting the river ecosystem. Damming alters river flow and habitat connectivity, leading to a decline in species, particularly fish and amphibians that need to complete life stages in different habitat types. Many species have declined due to the 91% of river lengths that have been modified in some way through damming, levees, or rerouting for human development or agricultural purposes. In addition to habitat destruction, overharvesting and pollution from a variety of sources such as agriculture and industry are also significant threats to river environments, compounding the issues faced by aquatic species and ecosystems.