Complex sentence: It derives its name from the San Gabriel River that flows southward through the center of the valley, which itself was named for the Spanish Mission San Gabriel Arcángel originally built in the Whittier Narrows in 1771.
Split into simpler sentences: It derives its name from the San Gabriel River that flows southward through the center of the valley. The river itself was named for the Spanish Mission San Gabriel Arcángel, which was originally built in the Whittier Narrows in 1771.