Why rivers are essential to the expansion of the Russian Empire?

The steppe and forest-steppe of Ukraine and southern Russia is good agricultural land, but it was traditionally held by pastoral nomads. Any state that could drive off the nomads and fill the land with tax-paying peasants would expand its power enormously. During the period 1500–1800, this region was taken under Russian control.

"The history of Russia is the history of a country being colonizedmigration and colonization of the country have been fundamental facts of our history.." Vasily Klyuchevsky, Kurs russkoy istorii, I, 20–21.

In the absence of a good map, locations will be given as approximately so many kilometers directly south of Moscow, and then so many kilometers east or west of that line. Thus, Kiev is about 600 south and 500 west, while Perekop at the head of the Crimean peninsula is 1100s and 250w. For contrast, France is not quite 1,000 kilometers from north to south and Moscow is about 1,000 kilometers south of the White Sea. Since these numbers are estimates, they should not be cited or copied.