 The irrigation was accomplished by the use of "shaduf", canals, channels, dykes, weirs, and reservoirs.The frequent violent floods of the Tigris, and less so, of the Euphrates, meant that canals required frequent repair and continual removal of silt, and survey markers and boundary stones needed to be continually replaced.The government required individuals to work on the canals in a corvee, although the rich were able to exempt themselves.