What is the MAIN issue surrounding the Nile, Tigris , and Euphrates rivers ?

In the spring, when melted snow from nearby mountains flowed into the Tigris and Euphrates and the rivers overflowed their banks, the crops that were ready for harvest could be destroyed. Often, the floods ruined an entire harvest. Livestock, including cows, pigs, and sheep, were often drowned by the floodwaters.

Originating in Lake Hazer in Turkey—a region characterized by high mountains—the river flows parallel with the Euphrates River. The two eventually join, emptying into the Persian Gulf in the lowlands in an area known as the Shatt Al-Arab. More than half of the Tigris can be found in Iraq.

Explanation:

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places of higher salinity. there are parts of the ocean where hardly any rain falls but warm dry winds cause lots of evaporation. this evaporation removes water – when water vapour rises into the atmosphere, it leaves the salt behind, so the salinity of the seawater increases. this causes the seawater to become denser.

a is the answer i believe.