I'm mapping water bodies in my region with Sentinel 2A Level 2-A imagery. I used ArcGIS Pro Raster Calculator to take (Band 3 - Band 8) / (Band 3 + Band 8) to get Normalized Difference Water Index. The process ran smoothly and at 0.05 and greater values, I can see all rivers, ponds, and storm basins.
However, there is a man-made basin that is used for recreation (jet skiing, weddings, concerts, etc.) that has me confused. The pixel values for only this basin range from -0.95 to -0.98.
What could cause that? Concrete-lined bottom? Total overgrowth of algae? Drained? I created a true color composite and the basin looks very dark, similar to nearby canals and ponds.