When I took the keys to the kingdom tour I was told that they can not drain the lagoon perminately because the magic kingdom was designed so that the water is pumped from the reedy creek to a treatment plant, then the treated water is pumped into the lagon and then flows through pipes to the moat in front of the castle which flows into the jungle cruise ride, then is piped over to the lake next to thunder mountain, then flows through a canal to main lake in front of the magic kingdom which is man made, then flows into the natural lake on the other side of the contemporary and then back into the reedy creek.