According to the Orlando Sentinel:
Walt Disney World is draining and filling in the pool at its long-closed River Country water park.
The 330,000-gallon Upstream Plunge was a major attraction at the now abandoned water park. It is surrounded by manmade boulders for jumping and diving platforms.
The green and white kidney-shaped area in the Google Earth image is the pool, which is especially close to the still functional Mickey’s Backyard BBQ and Pioneer Hall. Although fences separate the closed water park from the rest of Fort Wilderness, intrepid explorers have managed to get inside and snap some eerie pictures of the abandoned area, much to the consternation of WDW management.
It is not known what will become of the rest of the abandoned water park, although rumors as far back as 2010 indicated the area was being explored as a possible site for a new DVC property.
River Country held its Grand Opening on June 19, 1976 as part of the “Bicentennial Summer” of 1976 at the Walt Disney World Resort. In the 1980s, River Country was very popular but after Typhoon Lagoon opened in 1989, attendance waned, and ultimately River Country was closed permanently in 2001 in the wake of declining tourism after 9/11.
The standing water in the pool could be a breeding ground for mosquitoes, the known carrier of the Zika virus. There have recently been documented cases of the virus just miles from Orlando in Tampa, FL, making the threat of the virus very real in the Orlando area.
Disney has not given any official word on why the pool is being filled in, but many have speculated the Zika threat as the reason.