Why do so many crops grow in california central valley?

The Central Valley, also known as the Great Valley of California, covers about 20,000 square miles and is one of the more notable structural depressions in the world. Occupying a central position in California, it is bounded by the Cascade Range to the north, the Sierra Nevada to the east, the Tehachapi Mountains to the south, and the Coast Ranges and San Francisco Bay to the west. The Valley is a vast agricultural region drained by the Sacramento and San Joaquin Rivers. The Valley averages about 50 miles in width and extends about 400 mi northwest from the Tehachapi Mountains to Redding. Generally, most of the valley lies close to sea level and the land surface has very low relief, but is higher along the valley margins.