First, don't apply reverb to the actual WAV file. In the Multitrack view, apply your reverb with the little "FX" button on the track. Then look at the toolbar and find the button that says "Wet __/" (that's a picture of an envelope). Make sure it's pressed.
Now you should have envelopes on your track that control how much reverb it has.
You could also try out the FX Parameter Envelopes, where you can actually control aspects of your effects, like decay time, etc.