Simplify the following text for a Grade 9 native English-speaking person.
Original: Prior to development of the San Joaquin Valley that began in the 1850s by American settlers , the San Joaquin River and its tributaries tended to flood the valley in the winter months .
Simplified: Before humans began to develop the Central Valley in the 1850s , the San Joaquin River and its surrounding lands were one of the richest biological habitats in California . In winter , the river would flood the valley , leaving behind wetlands .