ThelargeriverbestknowntotheancientGreekswastheNileofEgypt.They

　　ThelargeriverbestknowntotheancientGreekswastheNileofEgypt.TheyspokeoftheriverwithadmirationandcalledEgypt“thegiftoftheNile”.Thereasonforthiswas,first,thattheNilebroughtwatertoarainlessdesertand,second,thatonceayear,theriveroverfloweditsbanks,leaving,asthewaterwentback,anewlayeroffertilesoil. 　　Thefloodwaterscarryinthemsoil(calledsilt)fromtheupperpartsoftherivervalleytothelowerparts,andsotothesea.Butastherivermeetsthesea,theseaactsasabarrierandforcestherivertodropthesiltitiscarrying. 　　TherearenotidesintheMediterraneantocarrythesiltaway,soyearafteryearitcollectsatthemouthoftheNile,andtherivermustfinditswayaroundislandsofsilttothealwaysmoredistantMediterranean.Inthisway,avastareaoffertilesoilhasbeenbuiltupatthemouthoftheNileandoutintothesea.Theriverwatersplitsuptoformsmallbrancheswindingacrossthearea.TotheancientGreeks,themouthoftheNilelookedlikethedrawing. 　　Nowwesometimesnamethingsafterthelettersofthealphabettheyresemble:aU-turn,anI-beamaT-square,anS-bend,andsoon.TheGreeksdidthesame.ThetriangularareaoflandbuiltupatthemouthoftheNilelookedlikethefourthletteroftheGreekalphabetdelta(Δ)andsothiswasthenametheygaveit.Thewordisnowusedforallareasoflandformedatthemouthofriverswhichflowintotidelessseas,evenwhentheyarenortriangularinshape.TheMississippidelta,forexample,isnotshapedatallliketheGreekdelta,asyouwillseeifyoulookatamap.

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。