A screen capture from the YouTube video, embedded below, that was released by the Arkansas Commissioner of State Lands.

Last month, news reports circulated nation-wide about an Arkansas kayaker who drowned after being sucked into a newly formed whirlpool on the Spring River in northern Arkansas. The victim was 64-year-old Donald Wright, a resident of Searcy, Ark., where he was executive director of the nonprofit Life Recovery Center. According to various reports, Wright was an experienced kayaker leading a paddling trip with residents from the center on June 11, 2018, when the accident occurred.

While floating near Class II+ Saddler Falls, two group members in a canoe tried to bypass a challenging river-left chute called Dead Man's Curve when they were pulled into the whirlpool. Their canoe flipped, and one man swam to shore while the other man was pulled into the whirlpool. He was reportedly sucked underneath a bedrock shelf and emerged downstream.

Wright paddled over to help, but his kayak flipped and was pulled into the whirlpool. While exact details are uncertain, Wright's daughter later told a reporter that her father drowned after hitting his head against the empty canoe. The canoeist who was pulled into the whirlpool was treated for minor injuries at a hospital before being released. Shortly after the accident, local officials placed warning signs and buoys upstream of the whirlpool to keep paddlers away from the feature.

This section of the Spring River, known for frequent Class II ledges that span the channel, is a popular paddling destination due to its year-round flows from Mammoth Spring. The largest in Arkansas, and third-largest in the Ozark Mountains, Mammoth Spring has an average daily discharge around 320 CFS (cubic feet per second). With pervasive karst topography, the Ozarks are known for abundant freshwater springs and float streams. Also abundant are caves and sinkholes, formed by water eroding soluble limestone formations.

This photo from the Arkansas Game and Fish Commission shows the sinkhole on Northern Arkansas’s Spring River.

In the weeks that followed, the state of Arkansas assembled a research team with expertise in river geology, karst topography, and stream-channel construction. In a phone call with a contributing editor from Canoe & Kayak, state geologist Bill Prior said that examination of this undercut hole, including the dye-tracing test shown in the above video, has changed the general understanding of the riverbed, itself.

"The [Spring] river is known for these ledges," said Prior. "Each [ledge] is two to three feet high. You would think it's bedrock being exposed. But where the accident happened, these features are actually a rim-stone structure."

A screen capture from the YouTube video (embedded above) released by the Arkansas Commissioner of State Lands

Prior explained that the rim-stone structure on the Spring River, which forms the ledges at Saddler Falls and likely elsewhere along the river, is comprised of two geologically distinct layers, one atop the other. The underlying layer, which collapsed to create the whirlpool, is tufa. Comprised of calcium carbonate, tufa is similar to travertine, a dense stone that precipitates out from mineral-rich warm waters. However, tufa forms under cooler water conditions and it is much more porous, and weaker, as a result.

"This tufa is pretty spongey and full of holes," explained Prior.

Above the tufa formation in the Spring River, the ledge's rim was comprised of a dense layer of cobbles which tumbled down the river long ago and cemented into place from precipitated lime. Slowly, over time, water eroded the underlying tufa until a tunnel was cut beneath the cobblestone rim in the center-right channel near Saddler Falls.

A USGS gauge, located in the town of Hardy about 10 miles downstream from the accident, shows the Spring River was running just under 700 CFS on June 11th, which is about 200 CFS below the statistical mean for that date. Meanwhile, the river was on a steady decline after a high flow of 3500, due to a rain event, around June 1st.

"When the rim-stone went dry [around June 11th] that diverted more water flow into the hole," said Prior. "Acting like a super large drain, the hole [became] so much larger and continued to grow."

Photo courtesy of Arkansas Commissioner of State Lands

On July 11th, the Arkansas Commissioner of State Lands announced in a press release that the sinkhole feature had been collapsed. In the photos and video provided, a track hoe can be seen dropping a large boulder into the middle channel near Saddler Falls.

This action should end the recent hazard at Saddler Falls. And hopefully, in the future, a new understanding of Spring River geology will help paddlers and officials spot any new undercuts that develop in the river bed.

— Mike Bezemek is the guidebook author of Paddling the Ozarks.