They weren't actively doing so. Scientists just used the declassified photos as data for a modern analysis.
> The scientists combined declassified US spy satellite images from the mid-1970s with modern satellite data to create the first detailed, four-decade record of ice along the 2,000km (1,200-mile) mountain chain.
> The spy satellite photographs used in the research had lain unused in archives for some years. But a computer tool developed by Maurer and colleagues enabled these 1970s photos to be turned into 3D maps.
You can be damned sure they monitor some natural resources, like oil. Water's probably on the list now, too.