Whisper Falls Question - How does the waterfall work?

How does the waterfall work? What are the time travel rules?

The simple explanation: The waterfall serves as an express elevator between years.

I based "time travel" in this series on an ancient theory of time known as spiral time.  In this theory, time is not linear. It does not move in a straight line (linear). It spirals--like a coil, spring, or "slinky."

Another way to model time in the Whisper Falls series is to view it like a round building. Each floor represents one year. (Or--each floor is 365 days in circumference.) In the first book, there are 220 floors stacked on top of each other. The bottom floor is 1796. The top floor is 2016. 

There are 6 rules that the waterfall follows to allow Mark and Susanna to travel through time.

1. Travel always occurs on the same day between centuries. If it's July 4th in 1796, then it is July 4th in 2016. You can think of it as an "express elevator" in the round building metaphor. If you get on the December 9th elevator in the 19th century, then you get off on December 9th in the 21st century. 
2. Time progresses at the same pace between centuries.  If Mark visits 1796 and stays for 24 hours, then 2016 has moved forward 24 hours as well. For instance, if Mark traveled to the past on June 2nd and stayed one full day, then he'll need to ride the June 3rd "express elevator" back up to 2016.
3. Present-day is always the destination of any travel to the future. Susanna cannot choose to travel to a future year other than Mark's world. In the round building metaphor, Mark's life always awaits them on the top floor. For books 1 and 2, that is 2016. For book 3, it's 2017. The express elevator only travels up to the 21st century.
4. The waterfall allows Susanna to suggest a year in the past that she wants to travel to. Susanna can ask the waterfall to change the "bottom floor" to years later than 1796.  For instance, if she asked to go to 1800, Whisper Falls could take her there.
5. The waterfall can refuse to take Susanna somewhere if it deems that too dangerous. If she wanted to visit the American Civil War, for example, the waterfall would've refused.
6. Once Susanna has visited a date in the past, she can't go further back.  She can only ask for dates  that are moving forward.  For instance, if she visits October 1, 1800, she may never again travel to a time that is earlier than October 1, 1800.

[I often get questions about the Whisper Falls series.  I'll answer each of the most commonly asked questions in a separate blog post. You can navigate to them in the FAQ on my website too.]