So how do waterfalls get formed?
Answering this question is really a lesson in geology (the study of rocks or the earth) and the water cycle.Have a comment or contribution?
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GEOLOGYHaving established what constitutes a waterfall and what types of waterfalls can exist, we're now in a position to discuss how geology influences how waterfalls are formed.
Waterfall formation is based around the basic principle that there is a watercourse (realize that water is an erosive agent) traversing over different layers of rock each with different rates of erosion. In other words, you have a river or stream flowing over hard rock (where erosion is slow) and also flowing over soft rock (where erosion is more rapid).
Over time, the soft rock is further cut into by the water ultimately making the watercourse steeper beyond the hard rock layer. This steepening effect also accelerates erosion as the influence of gravity on the water increases the water's speed (thanks to the increasing slope as a result of the accelerated erosion). Typically, cascades and rapids, like the Waitavala Water Slide in the Fijian Island of Taveuni, are at this stage of waterfall development.
Eventually, the watercourse steepens until it's either nearly vertical or completely vertical. At this point, you have a bon-a-fide waterfall!
But the story doesn't end there!
With the watercourse continuing to cut into the softer rock, the waterfall gets taller, the plunge pool (where the waterfall lands) gets deeper, and the soft rock directly beneath the hard rock gets undercut. Bridal Veil Falls in New Zealand, I think, is a pretty good example of a waterfall at this stage. Notice how the watercourse plunges the cliff and doesn't even make contact with the cliff wall. This suggests that the hard rock layer is overhanging.
It's the undercutting action that results in waterfalls where you might be able to go behind it! Steinsdalsfossen in Norway is an example of such a waterfall.
As the undercutting continues, eventually the overhanging hard rock gets unstable and collapses into the base of the waterfall. The net result of this action is that the waterfall retreats further upstream to the remaining lip of the hard rock layer. With its high volume of water, Niagara Falls continues to retreat about a whopping 3ft per year! Look at the overhanging wall in the photograph, which is further evidence that this process is still going on!
The undercutting still continues until you run out of the hard rock layer. At that point, the watercourse will probably go back to being a stream or rapid. Figure 1: The basic waterfall formation principle
This entire waterfall formation process is perhaps best illustrated by this animation
This animation illustrates something similar to the above except it shows a little more depth, and it also helps you visualize how waters can carve out gorges and canyons as well!
If you're more of a hands-on person, you can demonstrate at home or at the local park the waterfall formation process explained above.
To do this, make a fairly large sand pile. This sand pile represents the soft rock layer.
Then, insert a board or anything relatively flat and hard with some degree of thickness (the thicker the better) somewhere into middle of the side of the sand pile. This hard, flat material you've just inserted represents the hard rock layer.
Finally, pour water onto the slope of the sand pile (the same side you inserted the hard, flat item) and observe the water flow over the sand and hard object.
If done right, the water should be flowing down the slope of the sand pile eventually cutting into it. The water will flow over the hard object before falling over its edge and back onto the sand. The water will cut into the sand below the hard object and it's this part of the moving water that ultimately results in the waterfall. If you run this experiment long enough, you should be able to form your own little waterfall (Figure 2)!Figure 2: Here's something you can try at home
Once you accept the idea and science behind the waterfall formation, you might be able to imagine how you can end up with different shapes or types of waterfalls simply by varying the orientation and combination of hard and soft rock with water cutting through them.
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THE WATER CYCLEIt's no secret that you need water to have a waterfall.
But where does that water come from?
Below (Figure 3) is a simplified diagram (perhaps too overly simplified) of the water cycle. This is the dominant source (perhaps the only source) of all inland freshwater. Figure 3: The oversimplified water cycle
Even though the earth is nearly 70% water, the vast water supply is stuck at sea level in the oceans. So how does water get into the higher elevations on land? After all, you need elevation to even have a waterfall drop, right?
Well, the sun's energy essentially turns liquid water molecules into water vapor (or water molecules into a gaseous form) in a process called evaporation. This process, by the way, is the very process that separates all the salt and other things with the water from the water gases themselves resulting in the drinkable freshwater.
The water vapor (by a process I don't fully understand though it's been said that mountains tend to be rainmakers) eventually coagulates or comes together in the form of clouds. Then, the clouds can move inland according to the whims of air currents, the rotation of the earth, and other poorly understood atmospheric events.
When the water vapors rise up against mountains, plateaus, or an updraft, or runs into a cold air mass, the clouds get cooler and the water gases condense as mist, rain, or snow (or for a fancier term: precipitation).
As the water precipitates from the sky, gravity takes over and the water collected on land ultimately drains back into the oceans.
Now that you know how the water cycle works, keep this in mind the next time you see industrial waste, sewage, or other junk that gets flushed out into the oceans. Guess where all that junk goes? That's right, to the bottom of the oceans where most of life on earth exists! Just something else to worry about as Global Warming deservedly gets all the headlines, but Oceanic Acidity (i.e. the oceans are becoming more like acid through a combination of dissolved carbon dioxide and waste) might be the pink elephant in the room! Anyways, that's another topic but at least the water cycle sheds some light about this very troubling issue.
Finally, let's delve deeper into how the water moves from mountains to oceans.
When water precipitates from the sky, the liquid (rain) or solid (ice/snow) water collects on the ground. The water then descends to lower elevations while carving out channels that become watercourses such as rivers, streams, brooks, springs, etc. The places where the precipitation collects and drains towards the watercourses are called drainages or watersheds or water catchments or basins (Figure 4). You can think of drainages as a sort of tilted funnel where the output of the funnel is your major river that has collected all that water and melted snow or ice and is sending it all back to the ocean.Figure 4: An illustration of a drainage funneling water to the ocean
The following animation by the Michigan Technology University illustrates the way terrain determines how a drainage collects and funnels precipitation into a watercourse.
If the watercourse just so happens to cut through differing layers of rock with at least one of the layers being harder than the rest (as described above), then you have a waterfall!
So what determines the distribution of precipitation? That's dependent on climatology and meteorology (in other words, climate and weather). Unfortunately, the atmosphere as well as its interaction with the earth's geology is immensely complex. Delving deep into this topic to try to gain an understanding is beyond the scope of our discussion.
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