Simplify the following text for a Grade 4 native English-speaking person.
Original: The 500 kmÂ swamp , a floodplain for the Lachlan , joins the Murrumbidgee River to the south .
Simplified: Water from this big ( 500 kmÂ ) swamp finally goes into the Murrumbidgee River . Water from the Lachlan is used for farming .