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This one is rather straightforward. A character falls into a magically-induced coma for about 10,000 years and then wakes up to find the world around him different in quite a few ways. There's not a dramatic technological difference between the past and present, though there have been some slight advancements here and there. Question is (assuming relatively accurate mapmaking before and after his coma), do the maps look significantly different? Obviously, forests may grow or recede, but my main interest is in the bodies of water. Would rivers have enough time to significantly change course or widen/deepen?

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    $\begingroup$ This is anything but straightforward. I once lived in New Mexico, and until the Rio Grande river was dammed, old timers said they could go to bed with the river on one side of the town and wake up to find the river on the other side of town. That's just 10 hours. You need to define the size of the river, slope of the river, spring runoff conditions, and the nature of the ground it runs through. Then you get to hope nothing geologically happens anywhere near the river for 10,000 years (e.g. Mammoth hot springs, Colorado river, vis earthquakes, etc.). Nothing about this is straightforward. $\endgroup$ – JBH May 15 at 4:48
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    $\begingroup$ And all that is assuming the population during that time hasn't dammed the river, re-routed the river, covered over the river, tapped the river for irrigation such that it's a trickle, etc. The mind boggles what can happen to a river in 10,000 years. Or not, depending on where the river is, what value it has to population growth, what ground it flows through.... There's so much missing data that it's impossible to even give you a sensible list of what might happen. $\endgroup$ – JBH May 15 at 4:50
  • $\begingroup$ To take just two examples, try to research the evolution of the lower course of the Rhine and the of the mouths of the Danube during the last two thousand years, let alone ten thousand. $\endgroup$ – AlexP May 15 at 7:59
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10 thousands year is an awful lot of time for a river.

Just look at how the Netherlands changed in the last 600 years:

Netherland land reclamation

Or look at how the Po river delta changed over the past 300 years:

Po river delta

The Adriatic sea is called like this because of Adria, a city which was once a big port and now is several kilometers inland.

And the examples above only deal with the river/sea interface. While the river flows inland similar changes can occur: it just needs a flood to reshape a good part of the landscape.

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You don't need 10,000 years to change the river course, It may also happen in a matter of a few weeks if there is a massive earthquake in the region it can change the direction of rivers. (It could be a country-wide impact)

for instance it happened to Mississippi

In 1811 and 1812, a series of earthquakes emanated from New Madrid, Missouri, and were felt as far away as Ohio and South Carolina. The soil beneath the Mississippi River rose, temporarily changing its course so that it flowed backward.

If the earthquake is big or a series of quakes, the changes could be permanent.

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If an Ice Age happens during the 10,000 years, rivers could change course drastically.

Glaciers can carve mountains in half, and leave behind hills hundreds of feet high.

The Great Lakes could just as easily drain to East Texas, Louisiana, or New York as to the gap between Labrador and Newfoundland.

Weather patterns can change. For example, the Gulf Stream can shut down, or California could become subject to monsoons. Thus, an increase or decrease in precipitation of 4x is quite possible. This changes the size and speed of rivers, and the volumes of lakes. Enormous lakes can form, disappear, or become attached to oceans in this much time. Think Lake Chad, the Baltic, the Black Sea, or even the Mediterranean!

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  • $\begingroup$ Ooooh, +1. This is a very good point. $\endgroup$ – JBH May 15 at 16:51
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A river can change its course overnight, if there is a violent storm. Such a storm hit southern England in February 1287, leaving some towns under the sea (Winchelsea), land-locking others that used to be ports (New Romney), and redrawing the course of the River Rother so that it now meets the sea near Rye, ten miles away from its former mouth at Dungeness.

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