Real-time water data from USGS NWIS stream / river / groundwater gauges within a bounding box around lat/lon. Returns each site's latest reading per requested variable: streamflow (00060, ft³/s), gage height (00065, ft), water temperature (00010, °C), precipitation (00045, in), and others. Each reading: site code + name, lat/lon, variable name + description + unit, value (or null if no data / sentinel), qualifier codes (e.g., 'P' = provisional, 'A' = approved), observed timestamp. Free public-domain data from waterservices.usgs.gov.