▁and ▁rapidly ▁flows ▁in ▁narrow ▁channels ▁during ▁or ▁immediately ▁after ▁heavy ▁ra ins ▁or ▁mel ting ▁snow , ▁removing ▁soil ▁to ▁a ▁considerable ▁depth . ▁ ▁R ivers ▁and ▁streams ▁ ▁Valley ▁or ▁stream ▁er os ion ▁occurs ▁with ▁continued ▁water ▁flow ▁along ▁a ▁linear ▁feature . ▁The ▁er os ion ▁is ▁both ▁down ward , ▁deep ening ▁the ▁valley , ▁and ▁head ward , ▁extending ▁the ▁valley ▁into ▁the ▁hill side , ▁creating ▁head ▁cut s ▁and ▁ste ep ▁banks . ▁In ▁the ▁earliest ▁stage ▁of ▁stream ▁er os ion , ▁the ▁er os ive ▁activity ▁is ▁domin antly ▁vertical , ▁the ▁valle ys ▁have ▁a ▁typical ▁V ▁cross - section ▁and ▁the ▁stream ▁gradient ▁is ▁relatively ▁ste ep . ▁When ▁some ▁base ▁level ▁is ▁reached , ▁the ▁er os ive ▁activity ▁switch es ▁to ▁later al ▁er os ion , ▁which ▁wid ens ▁the ▁valley ▁floor ▁and ▁creates ▁a ▁narrow ▁flo od plain . ▁The ▁stream ▁gradient ▁becomes ▁nearly ▁flat , ▁and ▁later al ▁de position ▁of ▁sed iments ▁becomes ▁important ▁as ▁the ▁stream ▁me and ers ▁across ▁the ▁valley ▁floor . ▁In ▁all ▁stages ▁of ▁stream ▁er os ion , ▁by ▁far ▁the ▁most ▁er os ion ▁occurs ▁during ▁times ▁of ▁flo od , ▁when ▁more ▁and ▁faster - mov ing ▁water ▁is ▁available ▁to ▁carry ▁a ▁larger ▁sed iment ▁load . ▁In ▁such ▁processes , ▁it ▁is ▁not ▁the ▁water ▁alone ▁that ▁er odes : ▁susp ended ▁ab ras ive ▁particles , ▁p eb bles ▁and ▁b ould ers