Complex sentence: San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean.
Split into simpler sentences: San Francisco Bay is a shallow, productive estuary that drains water from approximately forty percent of California. Water from the Sacramento and San Joaquin rivers, from the Sierra Nevada mountains passes through the Bay to the Pacific Ocean.