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。