The afterbay pool is formed by Yellowtail Afterbay Dam, constructed on the Bighorn River 2.2 miles downstream from Yellowtail Dam. The dam is a 72 ft. high concrete gravity structure with embankment wings. The afterbay dam has a crest length of 1,360 ft. and consists of 21,600 cubic yards of concrete and 162,000 cubic yards of earth material and rip rap. Discharges are used to provide a uniform daily flow into Bighorn River, leveling the peaking power discharges from Yellowtail Powerplant.
Marine sedimentary rock of the Early Cretaceous, know as the Thermopolis shale for the concrete overflow structure. Ten to 15 feet of alluvium overlaying the shale forms the foundation of the embankment wings.