DNR report reveals cause of massive Top of the Rock sinkhole

Water seeping through a golf course pond’s clay liner likely caused a massive sinkhole to develop at Top of the Rock Golf Course in May, a state report concludes.

The report by the Missouri Department of Natural Resource’s Environmental Geology section also advised the golf course owners to fill the sinkhole with rocks and install a plastic liner on top of a clay barrier to prevent future sinkholes from forming.

“It is likely that the pond had a significant influence on the formation and rapid growth of this sinkhole, providing a source of water to carry soil particles into underlying bedrock conduits,” the report states. “This piping action likely caused the formation and rapid growth of a subsurface void that eventually collapsed.”

Bass Pro Shops owns the golf course at Ridgedale, south of Branson. Martin Mac Donald, Bass Pro’s director of conservation, issued this statement Thursday afternoon:

“At Top of the Rock, we’re still excited about the sinkhole and the geology beneath it,” he said. “We’re preparing an update on the sinkhole and we will have some exciting news to share soon.”

He declined to elaborate about what ongoing investigations at the site might have revealed. A week after the sinkhole formed, Mac Donald confirmed that water flowing into the sinkhole had made its way into a cave that Bass Pro Shops founder Johnny Morris explored on the property 12 years ago.

“Water from the sinkhole is filtering into the cave he discovered, and the speculation now is that there's a new cave system theoretically unearthed down there,” Mac Donald said on May 26.

The spectacular sinkhole, estimated to be 60 feet in diameter and 30 to 40 feet deep, drew national media attention after it formed in the early hours of May 22. The man-made pond was built on the golf course’s driving range, and portions of land around it were covered with artificial turf, some of which hung into the sinkhole from its edges.

The report also notes that a significant amount of pond water rushed downhill during the collapse, depositing stones and sand — and a large fish — along the way.

The fish was found in a smaller pond that had never been stocked, according to the report.

“The fish is presumed to have come from the larger pond that was breached by the collapse,” the report states. “The distribution of sand, rock and the fish suggests a significant amount of overland flow from the breached pond carried soil, sand and boulder-size rock downslope from within or near the collapse.”

Bass Pro Shops contacted DNR after the sinkhole developed for technical advice, and DNR geology experts visited the sinkhole on May 28.

The report indicates the northern section of Top of the Rock Golf Course was built “in a collapsed structure that previously contained a sinkhole pond” on either side of U.S. 65 highway. The sinkhole formed about 1,000 feet south of that sinkhole structure, according to the report.

DNR investigators also discovered four smaller “collapse/settlement areas” near the main sinkhole, three of them beneath artificial turf.

“The three turf-covered features were 5 to 10 feet in diameter,” the report states. “The fourth was partially exposed, about 20 feet in diameter, and appeared to be a collapse, though the bottom was not observed.”

DNR recommended that the collapsed area be filled with rock “to provide a buttress to further soughing.” The agency suggested overfilling the sinkhole with rock because more settling was likely to occur.

Doug Gouzie, associate professor of geology and geography at Missouri State University, was with DNR officials and a technical advisor from GeoEngineers, Inc., hired by Bass Pro, during their tour of the sinkhole.

“Much of the driving range had Astroturf over it and it makes sense that the original hole must have partly opened underneath the pond because no Astroturf was there,” he said.

He said he didn’t know if golf course managers had to pump more water into the pond as it slowly leaked through the pond’s clay liner ahead of the collapse.

He said it was clear from debris on the surface there was a lot of water that rushed downhill after the collapse happened— enough to carry rocks and a fish.

“The state and I felt that what slid downhill was a lot like what happens when you put too much toilet paper in one flush,” he said. “It creates a clog and if more water pours in, it fills up the bowl and overflows. We think the water and debris rushing into the sinkhole temporarily clogged it, and water from the pond filled it up and slid out the sides and downhill.”