Bringing back the old dashboard
As I typed this I discovered the answer to the question I had, so I thought I'd share it.
If anyone else was dismayed after installing El Capitan to find that the Dashboard no longer behaves as an overlay but instead slides in from the left and is only dismissible by clicking an obscure, small arrow key (why? Why not just click to dismiss anywhere on the screen?) you can change this behavior in the Mission Control area of System Preferences. There's a drop down menu that allows you to choose displaying Dashboard as a Space (the new, awful behavior) or as an overlay (the previous perfectly good behavior that they had to "improve" by making worse).
iMac