This was fun to write, but I had to go to the bathroom afterwards.
Claim: When he had arrived at Therma, Xerxes quartered his army there. Its encampment by the sea covered all the space from Therma and the Mygdonian country to the rivers Lydias and Haliacmon, which unite their waters in one stream and so make the border between the Bottiaean and the Macedonian territory. [2] In this place the foreigners lay encamped; of the rivers just mentioned, the Cheidorus, which flows from the Crestonaean country, was the only one which could not suffice for the army's drinking but was completely drained by it.*
Analysis:
So above, Herodotus claims that the army Xerxes, king of Persia, led to Greece was so large that is drank the river Cheidorus (also known as the Echedoros; now called the Gallikos) dry.
Such a claim seems incredible, and so I want to find out if it could be true.
First, the size of the army. Herodotus gives the insane figure of 2.5 million troops, plus an equivalent number of support personnel. A contemporary poet gives 4 million. Another historian says there were 800,000 troops.
Modern scholars generally agree that the total number was probably about 200,000-500,000. However, let's give Herodotus the benefit of the doubt and go with 5 million total men. (Wolfram-Alpha tells me that the heat production of the crowd would be approximately 350 megawatts.)
Second, the size of the river. I couldn't find any figures on the volume of the Echedoros's flow, but here is a picture of it today. It looks small, so maybe the Persians could drink it! But I don't know how big it was in 480 BC, so the picture isn't much help.
If I can't figure out how big the Echedoros was then, maybe I should extrapolate based on other rivers. I live in Oregon, and the most familiar river to me is the Molalla. Right now (10/9/21) its flow is 179 ft^3/sec.
Let's say the Persians have two hours to drink the river. 179 ft^3/sec approximately equals 644400 ft^3/hour. In two hours, that's 1288800.
How much could the army drink? Herodotus seems to indicate that they were drinking from more than one river, but let's give the Persians the benefit of the doubt once again.
Humans can safely drink a few liters an hour, but more than that is unsafe. Let's say the Persians were very, very thirsty and could drink 2.0 liters an hour. That's 0.07 ft^3, or 0.14 ft^3 in two hours.
All right, here's the math:
Water per Persian x number of Persians = total water drank
0.14 ft^3 x 5 million = 700000 ft^3
700000 is less than 1288800.
No, they probably couldn't do it.
Sources for claim:
*Herodotus, The Histories, book 7, verse 127. Can also be found here.
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