I thought about asking this question on English Language and Usage, but decided not to because I want answers from people who are familiar with cascades and waterfalls in reality, not just in the abstract. Moreover, the definitions aren't very helpful.
From The Oxford English Dictionary, a cascade is:
Usually, a small waterfall; esp. one of a series of small falls, formed by water in its descent over rocks, or in the artificial works of the kind introduced in landscape gardening.
And from the OED, a waterfall is:
A cascade of water falling from a height, formed when a river or stream flows over a precipice or ledge; (also) a garden water feature resembling this.
Much of my hiking has been done in the Sierra, especially Yosemite, where waterfalls and cascades of every size abound. I have always thought of a waterfall as water that flows over a lip and then falls freely. And a cascade as water that may flow quite steeply downhill but remains in contact with the underlying rock or streambed. I would never describe Nevada Falls, for example as a cascade, nor would I describe the cascades that flow over the bare granite at roughly the 8,000 foot level of Yosemite Creek as waterfalls.
Is there a difference between the two in TGO in other parts of the world, or have I made up a distinction that is not commonly used in TGO?