This child item contains files representing Particle Image Velocimetry (PIV) processing masks which excluded regions of invalid velocities from the PIV results. Masks typically are used to screen out velocities or prevent the creation of velocities for regions of an image where computed PIV velocities would be nonsensical or invalid. For example, near or on the channel banks, where a tree overhangs the channel, or the presence of a boat or other object in the water. By using masks, these regions can be excluded from analysis. The PIVLab software allows for the designation of a rectangular Region of Interest (ROI). For five of the field sites, which were located at engineered canals, a rectangular ROI was sufficient to exclude areas in an image scene which were invalid such as the channel banks. However, for three of the sites located in natural rivers, a rectangular ROI was not sufficient to screen invalid regions in the image, so polygonal masks were used in conjunction with the ROIs to segment valid regions for the PIV analysis. <br> The mask files included here are named with a prefix indicating which field site the mask is for. In two of the sites, multiple masks were used. Mask files are simple, headerless comma-delimited text files consisting of x,y pixel coordinate pairs which outline a polygon on the PIV image. <br/> Each Field Site is abbreviated in various files in this data release. File and folder names are used to quickly identify which site a particular file or dataset represents. The following abbreviations are used for masks: <ul> <li>ACR: Androscoggin River, Auburn, Maine, USA</li> <li>AFR: Agua Fria River, near Rock Springs, Arizona, USA</li> <li>GLR: Gila River near Dome, Arizona, USA</li> </ul>