Simplify the following text for a Grade 9 native English-speaking person.
Original: Before the 18th century , San Juan Creek and its tributary , Trabuco Creek , had indigenous names , but these are thus far unknown .
Simplified: Its biggest tributary , Trabuco Creek or Arroyo Trabuco , is about 21 m long , flowing south from the Santa Ana Mountains into San Juan Creek . Oso Creek is a smaller tributary to Trabuco Creek , but has a lot of water in it the entire year .