Introduction to the Physics of Waterfalls
The second law of thermodynamics says that things tend toward a more disordered state. Given that, what is creation and what is destruction? Is the second law saying that destruction wins over creation? Certainly not. It is saying that there is simply a tendency for things to move toward a more disordered state.
A waterfall, in my mind, satisfies all of these criteria, creation and destruction and the second law of thermodynamics, at once. After all, what is a waterfall? How was it created and how does it really work? This article examines these issues in detail.
The Top of a Waterfall: Just the Beginning
The Creation of a Waterfall
A waterfall is created when river water erodes the weaker earth, rock, or sand of its original stream bed, pushing the rock aside and along with the water flow over time (generally, eons). Gradually, a dip in the river is created. Destruction? Eventually, that dip became significant enough to be called a "waterfall": a new creation.
It's true that the river "destroyed" its original boundaries--its original stream bed and the material that was in it. This is in compliance with the second law of thermodynamics--things tend to a more disordered state. This "more disordered state" is, however, itself a creation in my view.
The original river was "destroyed" over a great period of time, however it simultaneously created something beautiful: the waterfall, where water reaches an edge in its stream bed then all of that water falls in a seemingly disorderly fashion down some distance before crashing into the bottom and then continuing on its way in its "newly created" riverbed.
A Waterfall is a Bit Like Billiards
To understand the physics of the waterfall, consider water molecules to be like billiard balls, knocking each other about.
As each molecule falls, it bumps into other molecules of water and sometimes of rock/mineral, until it reaches the bottom and hits, with force depending on the distance from which it fell. This force was caused by gravity pulling the molecule rapidly downward with all of the rest of the stream's molecules of water and some impurities. Impurities might be minerals eroded by the stream, perhaps even pieces of sand, wood or leaves or other vegetation, or humanity's litter that was floating or traveling along in the upper portion of the river.
Billiards and the Physics of Waterfalls Have Much in Common
Physics is All Around Us
The Bottom of a Waterfall Only Appears to be Chaotic
To the naked eye, the bottom of the waterfall appears to be chaotic. However, what does the water molecule hit when it reaches the bottom, all full of kinetic energy it gained from gravity and distance? It hits other water and mineral molecules that have recently made the same trip over the waterfall, also full of kinetic energy, or possibly the other impurities mentioned previously.
All of these molecules at the bottom of the waterfall are seen, by the naked eye, as a roiling, bubbling mass of water that looks as powerful and dangerously destructive/creative as it is. Why is the base of the waterfall so very powerful, much more powerful than the regular part of the stream? The base of the waterfall has gained tremendous kinetic energy in its acceleration down from the top of the waterfall.
It uses this kinetic energy to create a pit in the "new" stream bed, over time, at the base of the waterfall, since it erodes the solid ground materials with greater efficiency, giving up some or most of its kinetic energy in the process.
If a particular molecule does not directly hit the bottom surface containing the waterfall, or cauldron, then it hits another molecule, which may hit another, and so on--very much like the games of billiards and pool--until finally a molecule hits the bottom, possibly with enough force to dislodge one of the resident molecules of bedrock or whatever material is originally at the bottom of the waterfall.
A particular molecule may also, or instead, use its kinetic energy to bump other water molecules completely out of the stream, creating the familiar mist of water that most of us have felt on our faces, and cursed on our camera lenses, when standing in awe at the bottom of the waterfall. This would be akin to a billiard ball being accidentally shot completely off the table—a somewhat rare occurrence.
Another way in which the water molecule may use its energy is to push the earlier-fallen water molecules downstream faster, which is why the water moves onward: water cannot collect forever in the cauldron created at the bottom of the waterfall, eventually it runs out of room and energy to remain there, and so it moves on in the direction that it finds easiest to proceed in: along the river bed.
After the Waterfall, the River Continues
Why does the river at the bottom of the waterfall run in line with the top of the waterfall, even if the surrounding material might be softer and an "easier target" for the water molecules to erode? Because the water already has great momentum in the original direction, therefore it will tend to continue in that direction for some distance after the waterfall, unless very hard bedrock or some other diverter turns it astray.
The further away from the waterfall, generally the calmer the waters grow until they appear just as would any other stream given the depth and breadth of it with respect to the water flow.
A Few Words About Hydro Power
A typical, modern hydroelectric power plant works because of the same physics that we discussed above. It harvests some of the incredible energy of falling water, using it to turn turbines that, in turn, produce electricity for immediate use or for storage in enormous batteries.
In historical times, hydraulic power was used to turn a wooden paddle wheel which, in turn, directly powered a saw mill or grain mill. Such things may still be found in use in parts of the United States today, either as historical landmarks, reproductions of such, or in daily use by scattered Amish communities throughout portions of the United States.
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Yvonne on January 14, 2020:
This is so good and well written. Thank you for this. The physics behind waterfalls has always seemed like mystery to me but its now perfectly clear. Thumbs up!
Faiq on August 13, 2017:
Very nicely written article. Please explain the physics behind being jettisoned from the tip of the back of a rubber kayak over the roiling waters of a 6 feet water fall in upper Washington State. The other people in the kayak just sat pretty. The surrounding views before my ordeal were "breathtaking", but when I was tumbling at the bottom of the waterfall I thought that all my breaths were literally taken away forever. I have a general idea about what happened but would like to hear your explanation in physics laws acting on the kayak, the kayak responses, my trajectory in air, and in water. Thanks.
douglas wynn on July 16, 2015:
i am a fine artist-visual , mostly and also a student of aesthetics. i always try to read, which helps in the creative process of my works. i usually do landscapes and recently a waterfall. i've done them before ,but also recently have finished reading a book ,"art & physics" by l. shlain. i was surprised to see how closely related physics and art really are. wanting to bridge the gap a bit more between the waterfall, my art and physics i figured there might be something on the internet to help. thank you for your information. it will come in very handy bridging the gaps even more. sincerely, doug. email@example.com
CraftytotheCore on October 12, 2013:
Laura Schneider (author) from Minnesota, USA on October 12, 2013:
Thanks, CraftytotheCore! --GeekytotheCore
CraftytotheCore on October 11, 2013:
Very interesting. I especially liked the explanation of why water moves onward. I also like all the details and how you used the game of billiards as an example.
Laura Schneider (author) from Minnesota, USA on September 21, 2013:
You're welcome, susi10! Thanks for the compliment! :-)
Susan W from The British Isles, Europe on September 21, 2013:
This is a very well written article, you have explained the physics behind a waterfall superbly, I knew how waterfalls were formed geographically but I never thought of the actual physics behind it at a molecular level. Thanks for writing this, Laura!
~ Susan W.
Laura Schneider (author) from Minnesota, USA on April 26, 2013:
An excellent analysis! Thanks for commenting and for the compliments, lumen2light!
Neil Coulson from Dundee, Scotland on April 26, 2013:
Nice article: the laws of physics are derived from natural events which we then apply to other things. As for creation/destruction, when it comes to nature, I believe that there is no destruction as everything happens for a reason. As for the human race, we Create and destroy and it is human nature to apply our own theology to the working of nature.