Lakes were a prominent - almost ubiquitous - element of the empire's geography. Along with rivers, they are among the few features included on the small map insets on the cards. We only attached the "lakes" tag in cases where they were specifically mentioned on the cards. (Salt lakes are treated separately.)
There is only so much you can learn about Russia's lakes from a set of playing cards. That said, they do include nuggets of information here and there. We learn, for example, that Novgorod Province has 3,220 lakes, and Lifliand has 1,000.