Early factories were built near rivers because the river water was able to power the machinery that the factories needed and it was also a convenient place to discard waste. The problem with these early factories is that they led to a large pollution problem in many of America's waterways.
Many rivers in city areas today continue to have pollution problems due to early factories and to continued waste from modern factories. As the years progressed, factories would no longer require rivers for power and would begin using coal power. This new power allowed factories to appear in areas that were not near waterways.