Source: USGS Landsat
All 8 million Landsat scenes are available to download from EarthExplorer, GloVis, and the LandsatLook Viewer.
As the world’s longest continuously acquired collection of space-based, moderate-resolution land remote sensing data, the USGS Landsat archive adds over 1,100 scenes each day.

Above the Earth, Below the Surface: Landsat’s Role in Monitoring Water Quality
Safeguarding freshwater resources is crucial, and while scientists use a variety of ground-based techniques to gauge water quality, the Landsat program has provided water quality data from orbit for decades.