The number that Colorado River would have to downcut per year to form the mile-deep canyon in 4 million years is 0.40 millimeter per year.
Depth of canyon = 1 mile
Time taken = 4 million years
Based on the above information, the rate will be:
= Depth / Time taken
= 1 mile / 4 million years
= 0.25 mile / million year
Note tha<em>t 1 mile = 1609344 millimeter</em>
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Therefore, the rate will be:
= (0.25 × 1609344) / 1000000 years
= 0.40 millimeter / years
In conclusion, the rate is 0.40 millimeter per year.
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Answer: The Mexican Cession
Explanation: If we are measuring this impact by land area, the mexican cession definitely had the biggest impact, since it was the first land acquisition that connected the US to the Pacific and truly accomplished manifest destiny.