The Desert, Flooded?

So how is it possible that a desert can be flooded? I was recommended to visit the Joshua Tree State Park in California before I left for my trip out west.  I had my heart set on the trip after looking at numerous images of Joshua Trees, and amazing images taken by different photographers. I was very geared up and ready for my own little adventure, minus the snakes and such. So we drove a short 45 minutes from where we were staying in Palm Springs out to the southern entrance of Joshua Tree.  I had to whole crew with me and we were ready, ready for some amazing scenery and locations to photograph.  We turned off of I-10, drove up the bump, rock, and dirt filled road which lead directly into the park, and as soon as we got to the entrance, we were greeted with some lovely orange and white striped signs.  Image my bewilderment when I figured out that the sign proclaimed that the park was "flooded".  

What?  How is this possible?  In the middle of the desert on a day when the thermometer claims it is it 100 degrees Fahrenheit, wouldn't the water just evaporate if there was any? So we parked the car and just photographed around the entrance, unfortunately without seeing any Joshua Trees, beautiful cliffs, or large red rocks.  It was beautiful, but I guess this means I will just have to make another trip out to the desert one day soon.  Maybe in the not to distant future the desert will only be "flooded" with sunshine.