To have a 'Rocky Waterfall' is to simultaneously urinate while having a dump.
Example 1. "I was so desperate for a wee, that when I was shitting I pissed at the same time. It looked like a rocky waterfall."

Example 2. "I was desperate for some really kinky porn, so I looked up golden showers with scat. What I got can only be described as a rocky waterfall"
by Lord Dangleballs August 13, 2016
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