Abstract: At 1:15 p.m. on October 12, 2017, a diver, co-diver, and line attendant were monitoring and collecting mussels in the Mississippi River near Cassville, WI. The diver and co-diver entered the water from a floating dock. The dive location was approximately 20 feet from the shoreline and approximately 9 feet deep. The water temperature was approximately 57 degrees Fahrenheit. The air temperature was 58 degrees Fahrenheit. The speed of the river current at the dive location was unknown. The line attendant was on the dock while the divers where in the water. The divers were attached to one another by a 17 foot tether. The 17 foot tether was secured to a shackle on an 87 foot tending line. The tending line was secured to a cleat on the dock. The tending line was monitored and adjusted by the line attendant. Both divers were in the water for approximately fifteen minutes. The diver motioned via hand signals and line tugs to the co-diver there was low oxygen. The diver started to ascend to the surface first. The co-diver then ascended to the surface and realized that the diver had not surfaced. The co-diver followed the tether to the bottom of the river and found the diver unresponsive. The co-diver rescued the diver and was able to be resuscitated. The diver was taken to a hospital and placed in an induced coma to assist with recovery. The diver was released after three days and made a full recovery. It was determined post incident that there was approximately 15-20 percent oxygen available in the diver's tank when they were diving. Both divers were trained and certified in diving.