River MonstersAnimal Planet‘s best-performing series returned Sunday with the network’s most-watched season debut ever — 1.7 million viewers. That includes the network’s best debut stats among 25- to 54-year-olds (915,000). Following the two-hour River Monsters season premiere, at 11 PM, Animal Planet’s first live aftershow, River Monsters: Live With Jeremy Wade, clocked 1 million viewers. The season premiere saw an 85% increase in Twitter activity compared to last season’s premiere, no doubt because theanimal-planet-logo two-hour episode was called “Amazon Apocalypse” and purported to investigate the 1981 sinking of the Sobral Santos II, which Animal Planet says is “one of the Amazon’s largest maritime disasters” with a many as 200 passengers losing their lives to “something mysterious in the waters.” Maybe not so mysterious: According to press reports the Sobral Santos II, overladen with more than 500 passengers and cargo, capsized in piranha-infested waters on the Amazon.  But Animal Planet bills River Monsters as “a weekly murder mystery” in which Wade travels the world in search of man-eating fish, in the hope of proving reports “as tall tales or frightening facts.”