But we noticed a few weeks ago
that our water was getting more shallow
and eventually we could only find a small place to splash.

Now it is totally empty!!! WHO PULLED THE PLUG???
We do find nice stinky things in the drying mud,
but there is no more swimming. 
but a vernal pool.
That means it's a pond that fills with melted snow or rainfall and makes you think you have a pond until sometime in the early summer when it dries up until the fall.
Since it can't support fish, it's a great place for tadpoles and frogs to breed without being eaten by fish.