Side-scan sonar (SSS) has been successfully used to detect West Indian and West African manatees, however confirmed sonar images of Amazonian manatees have not been obtained yet. One potential difficulty with this methodology is differentiating manatee images from other large aquatic vertebrates. While West Indian and West African manatees share habitat with small cetaceans in estuarine and marine habitats, their potential confusion in sonar images has been avoided because of their reluctance to approach the boat during surveys. In contrast, the Amazonian manatee shares most of its habitat with two cetaceans, the Amazonian River dolphin (ARD, Inia spp.) and the Tucuxi (Sotalia fluviatilis). In this study we characterize the sonar image produced by ARD and make notes on behavior observed during surveys for Amazonian manatees. Boat surveys using SSS were conducted during the high (July 15-30, 2017) and low (December 13-21, 2017) water seasons in Amanã Lake, Amazonas, Brazil. Sonar surveys were recorded and then analyzed with ReefMaster Sonar Viewer (v. 1.0.36). ARD produced a characteristic wavy tail in the shadow of the image. Unlike the acoustic images produced by manatees, ARD acoustic images contain sharper angles and the shadow is narrower. ARD in Lake Amanã repeatedly followed our boat for kilometers during several hours. They would swim under the boat and approach the SSS transducer, suggesting they could hear the sonar, but were not distressed by it. ARD were seen in small pods (1-4), however several pods would aggregate over time in relatively large numbers (10-20) around the boat.