The answer to 'why is my downspouts water stains' is that it is likely caused by mineral buildup or dirt/debris accumulation in the downspout. Downspouts can develop water stains and discoloration over time due to the minerals and sediment in the water flowing through them. This is a common issue and usually not a sign of a serious problem.
The stains are typically caused by a combination of hard water minerals, like calcium and iron, that leave behind deposits as the water evaporates. Dirt, leaves, and other debris can also collect in the downspout and contribute to the staining. While unsightly, these stains are usually just a cosmetic issue and don't require immediate attention unless they become severe.