Simplify the following text for a Grade 9 native English-speaking person.
Original: The Los Angeles River also forms the northern boundary of the mountains from the easternmost part of the San Fernando Valley to Elysian Park , where the river turns south , thereby defining the easternmost part of the mountain range .
Simplified: Beginning at the western end of the San Fernando Valley the river runs to the north of the mountains . After passing between the range and the Verdugo Mountains it flows south around Elysian Park defining the easternmost extent of the mountains .