The Menindee lakes are overflow lakes, as is the Anabranch an overflow for the Darling. They naturally filled in flood times. A campaign began in the 19th century, later supported by Henry Lawson, to make them artificial reservoirs. This was done, with weirs and the Copi channel, so when I was growing up they were always full. I sailed on Lake Menindee and we camped alongside the lakes never thinking that they might empty. The decision to run them empty was based on concerns about the evaporation loss and salinity which affected downstream, including the Murray into South Australia. I am in favour of keeping them full whenever possible, but I don't understand the big picture. I suspect that irrigation elsewhere is a much, much bigger problem.