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Control water's Flood option lets you:

cause the water level of all standing water in the area to rise by as much as 20 feet. 

If you do raise the water level above the apex of its container (say a river above its river bed or a barrel of water beyond its capacity), what happens to the excess water? The spell states:

If the area includes a shore, the flooding water spills over onto dry land.

Does the edge of an artificial container count as a "shore"? If not, what happens to a filled container when the water level rises 20 feet? If it does count as a shore, does the water fill the external space 20 feet above the apex, or is the expected volume of water conserved (as if the apex had been 20 feet higher)?

Essentially, what happens if the container for the water is insufficient to contain the new volume?

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    \$\begingroup\$ I guess the core of the question is whether (and to what extent) the flood option can create additional water if used on a body of water with limited volume. \$\endgroup\$ – Ryan Thompson Dec 17 '18 at 17:38
  • \$\begingroup\$ @RyanThompson All bodies of water have limited volume (save perhaps the Plane of Water which is unclear). I'm certain it creates additional volume. Otherwise the water level could not rise in some clear examples (like a river in a canyon). I'm more interested in the mechanics of this volume increase when the container is more clearly defined and limited below the 20 feet height. \$\endgroup\$ – David Coffron Dec 17 '18 at 17:43
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    \$\begingroup\$ Related and possible dupe: What the heck is freestanding water? \$\endgroup\$ – NautArch Dec 17 '18 at 17:45
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First and most importantly, the "Control Water" spell does not create more water.

"Until the spell ends, you control any freestanding water inside an area you choose that is a cube up to 100 feet on a side."

The spell then gives you four options that you get to choose on each of your turns as well as the turn that the spell is cast as long as you maintain concentration. Those options are "Flood", "Part Water", "Redirect flow", and "Whirlpool" When read fully, none of these options actually create more water than what was originally there.

I start out with this because your main question "what happens if the container for the water is insufficient to contain the NEW VOLUME?" (all caps the part I am first addressing) implies that you can create more water with the "Control Water" spell, which you can not.

2nd, Now as far as the Flood option, lets dig in. Flood gives you 2 more options based on the area of water you are controlling. You can either cause the existing water to rise 20 feet up (so you could say safely allow your fellow party members to scavenge the bottom of a body of water) or IF you selected a large body of water you can create a 20ft tall wave that then crashes at the destination of your choosing (Fun for naval fights)

Now as you pointed out "If the area includes a shore, the flooding water spills over onto dry land." If you keep reading 2 paragraphs down it then says "The water level remains elevated until the spell ends or you choose a different effect. If this effect produced a wave, the wave repeats on the start of your next turn while the flood effect lasts"

Conclusion

So it is up to your DM to define what shore is in his/her campaign however generally speaking I would say that the only case in which the water would spill over when originating from an artificial container is if you raised water from say a marine tank like the kind in our world designed to hold killer whales, manatees, and other large marine life forms. The reason as to why I say this because due to the limitation of "cube up to 100 feet on a side" you can control up to 7,480,000 gallons of water at a time since there are 7.48 gallons per cubic foot. (equation pulled from https://sciencing.com/calculate-water-fill-rectangular-tank-7686198.html)

When the water is elevated, the container previously holding it becomes empty. If the container still has water in it when you choose this effect the remaining water inside the container would level itself out. So to address your examples, water from a barrel would remain elevated, the river would flood onto the land and soak itself into the ground as water from upriver would then refill the space shortly afterwards, if the edge of an artificial container counts as a shore then when the flood option is chosen the water being controlled will come crashing down onto "shore" and the remaining water in the container will level out, if the edge of an artificial container doesn't count as a shore then the water remains elevated and any remaining water levels out in the container.

I'll gladly clarify anything that doesn't make sense in my answer.

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    \$\begingroup\$ It sounds like you’re saying “flood” makes the water levitate up 20 feet. Is that a correct understanding of this post? \$\endgroup\$ – SevenSidedDie Dec 19 '18 at 22:47
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    \$\begingroup\$ I think @Seven is picking up on a key issue. The spell says "the water level rises" not "the water rises". The use of the word "level" strongly suggests the mundane meaning: that supplemental water is added such that the top of the water volume is displaced upward, and not in the sense of magical levitation as you argue. That said, I think your point about it coming down to DM interpretation is probably right, regardless. \$\endgroup\$ – Rykara Dec 20 '18 at 0:04
  • \$\begingroup\$ You are correct that the spell as written does say "the water level rises" and does not say levitate as I did intend. However, due to the nature of how floods occur naturally as the result of heavy rains or melting snow, given the magical nature of the origin of the flood you are creating it then comes down to the dm's decision: Does the spell actually create more water than already is despite the fact the spell never states as such or is the existing water changing how it fills its container causing it to spill over? \$\endgroup\$ – KDodge Dec 20 '18 at 23:42
  • \$\begingroup\$ Due to the spell very clearly stating "If the areas includes a shore, the flooding water spills over onto dry land" In can easily be reasoned that if there is no dry land or shore, water level rises but doesn't spill over. And considering it rises by 20ft, that's quite the powerful spell that can create 100 feet by 100 feet by 20 feet out of thin air. As such was the basis of my argument that the existing water is what rises and not the creation of more water. \$\endgroup\$ – KDodge Dec 20 '18 at 23:46

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