[jcwade]

Actually, the newest dam in Calif is holding back Diamond Valley Lake near Hemet and was completed in 2000. It holds 810,000 acre feet of water and is the largest and newest reservoir in SoCal.
The problem with the Central Valley farmers is not that they have historical water rights, it's what they grow.
Crops like almonds or pistachios use up to 1900 gallons of water to grow 1 pound.
Crops like strawberries or oranges only use about 12 gallons per pound.
The farmers need to grow more sustainable crops considering we live in a desert and are suffering near constant droughts.
For example, California grows 100% of the US almonds and 70% of the worldwide supply.