The pilot stated that shortly after taking off from a river in a float equipped Piper J3C, he was informed by radio that he had departed without his phone. The pilot returned to land, and during the landing, the airplane touched down on the right float, and the right wingtip subsequently impacted the water. According to a Federal Aviation Administration inspector, the pilot stated that after takeoff, he stalled the airplane while in a turn and then "crashed in the lake." He also stated that the "density altitude got the best of me." The pilot did not report any mechanical anomalies.