Every so often Folsom Lake empties enough to see portions of the original Morman Island locale -- good luck getting to this place the rest of the time! This cache will generally be 25 feet under water and consists of a water tight (I hope) container, a tether and anchor to hold it in place. Intrepid divers might be able to find it, but I wouldn't recommend it.
Mormon Island was a mining town along the American River which sprung up during the goldrush of 1840s/1850s California. To see more about Morman Island, read the wiki entry: http://en.wikipedia.org/wiki/Mormon_Island .
Congrats to Soft298 FTF and Kappa65 STF!!