What does 10 TRILLION gallons of water even equate to?

I've wondered, with California and the drought that they have been through, why can't they create more holding areas (reservoirs), to hold back the water when it does rain. I realize there would be a tremendous cost associated with something such as this, but wouldn't it be cheaper than failed crops, landslides, failed highways, deaths, etc.? It does seem like a waste now, it's just dumping into the ocean. We have a similar, but definitely smaller issue here on Vancouver Island. If they can build a series of small dams, then we could spread out the water over time.