Complex sentence: The famous disasters tend not to kill many people--humans can be warned not to drink or go near polluted water--but cyanide spills can have a devastating effect on rivers, killing everything for several miles downstream.
Split into simpler sentences: The famous disasters tend not to kill many people, as humans can be warned not to drink or go near polluted water. However cyanide spills can have a devastating effect on rivers, killing everything for several miles downstream.