This dataset is a digital image mosaic of sidescan for six sites where known rock structures exist in the St. Croix River. Sidescan sonar emits conical or fan-shaped beam down across a wide angle perpendicular to the path of the sensor through the water. The intensity of the acoustic reflections from the riverbed is recorded in a series of cross-track slices and when stitched together in a mosaic, form an image of the riverbed giving the user a view of the material and texture of the riverbed. Sidescan mosaics are being used in this project mainly as ancillary information to help view the rock structures and will also be used for investigating the use of hydroacoustic technologies for quantifying habitat for imperiled mussels Spectaclecase (Margaritifera monodonta) and Salamander (Simpsonaias ambigua) typically associated with rock structures (e.g., wing dams, revetment) in rivers. Sidescan can be recorded simultaneous with the multibeam data; and were collected using a Norbit integrated wide band multibeam system compact (iWBMSc) sonar unit for five of the sites. A recreational grade Humminbird Helix was used to collect sidescan of the one site too shallow for multibeam. All hydroacoustic data were collected in early May of 2021, and the image mosaics are provided in GeoTiff format.