The large river best known to the ancient Greeks was the Nile of Egypt. They spoke of the river with admiration and called Egypt “the gift of the Nile”. The reason for this was, first, that the Nile brought water to a rainless desert and, second, that once a year, the river overflowed its banks, leaving, as the water went back, a new layer of fertile soil.
The flood waters carry in them soil (called silt) from the upper parts of the river valley to the lower parts, and so to the sea. But as the river meets the sea, the sea acts as a barrier and forces the river to drop the silt it is carrying.
There are no tides in the Mediterranean to carry the silt away, so year after year it collects at the mouth of the Nile, and the river must find its way around islands of silt to the always more distant Mediterranean. In this way, a vast area of fertile soil has been built up at the mouth of the Nile and out into the sea. The river water splits up to form small branches winding across the area. To the ancient Greeks, the mouth of the Nile looked like the drawing.
Now we sometimes name things after the letters of the alphabet they resemble: a U-turn, an I-beam a T-square, an S-bend, and so on. The Greeks did the same. The triangular area of land built up at the mouth of the Nile looked like the fourth letter of the Greek alphabet delta (Δ) and so this was the name they gave it. The word is now used for all areas of land formed at the mouth of rivers which flow into tideless seas, even when they are nor triangular in shape. The Mississippi delta, for example, is not shaped at all like the Greek delta, as you will see if you look at a map.
埃及尼罗河在古希腊人中享有盛誉。古希腊人每每提起尼罗河，总是满怀崇敬，把埃及称为“尼罗河的恩赐”。个中缘由有二：其一，尼罗河为一片久无甘露的沙漠带来了河水，其二，每年尼罗河水位上涨，会溢出河岸一次，退潮过后，留下的就是一层肥沃的泥土。
河水把泥土(或叫淤沙)从上游河谷带到下游地区乃至入海口。当与大海交汇时，由于受到海洋阻力，顺水而下的淤沙便停滞下来。
然而地中海潮水不会把淤沙带走，年复一年，淤沙便在尼罗河的入海口处堆积，尼罗河为此不得不绕道而行，距海洋越来越远。于是在尼罗河和地中海之间形成了一大片肥沃的土壤，尼罗河水也分成一股股细流，蜿蜒穿过这片土地。古希腊人觉得，尼罗河口的形状与三角图形很是相似。
如今我们会因为事物的形状酷似某个字母，便以此为之命名。比方说：U形转弯，I形标，T形广场，S形弯等等。希腊人也这样取名。尼罗河口的这片土地看起来很像希腊字母的第四个Δ(Delta)，尼罗河三角洲便由此得名。现在任何在河流入海(无潮海)口形成的流域，不管其形似与否，都可以用Delta三角洲命名。如果你对照地图就会发现，其实密西西比河三角洲根本就不像三角形。
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