et . ▁The ▁D ry ▁last s ▁for ▁ 6 ▁months ▁between ▁April ▁and ▁September ▁with ▁an ▁average ▁ra inf all ▁of ▁ 2 4   mm , ▁whereas ▁the ▁W et ▁last s ▁between ▁October ▁and ▁March ▁with ▁an ▁average ▁month ly ▁ra inf all ▁of ▁ 2 5 4   mm / month ▁( acc ording ▁to ▁the ▁Bureau ▁of ▁Meteor ology , ▁ 1 9 9 9 ). ▁The ▁majority ▁of ▁the ▁rain ▁falls ▁between ▁December ▁and ▁April . ▁Run off ▁var ies ▁between ▁ 2 5 0 – 1 0 0 0   mm . ▁ ▁Pe ak ▁flow ▁for ▁the ▁Elizabeth ▁River ▁occurs ▁in ▁February ▁with ▁ 3 8 9 M l / day , ▁after ▁which ▁it ▁slowly ▁decre ases ▁until ▁July ▁when ▁there ▁is ▁no ▁fresh water ▁input ▁into ▁Dar win ▁Har bour ▁until ▁the ▁on set ▁of ▁the ▁following ▁wet ▁season ▁( Pad ovan ▁ 1 9 9 7 ). ▁Cy clone ▁frequency ▁is ▁low ▁to ▁moder ate . ▁ ▁The ▁Finn iss , ▁El z abeth ▁and ▁Howard ▁R ivers ▁have ▁a ▁mean ▁annual ▁out flow ▁of ▁, ▁ ▁Water ▁quality ▁▁ ▁The ▁water ▁quality ▁at ▁the ▁upper ▁est u ary ▁monitoring ▁sites ▁of ▁the ▁Northern ▁Territ ory ▁Government ▁was ▁ 2 0 1 1 ▁in ▁excellent ▁condition . ▁Water ▁quality ▁at ▁the ▁fresh water ▁monitoring ▁sites ▁was ▁in ▁very ▁good ▁condition ▁in ▁ 2 0 1 1 . ▁ ▁Ge ology ▁▁▁ ▁The ▁underlying ▁l ith ology ▁is ▁domin ated ▁by ▁Perm ian