The mighty Anduin, the broad Entwash, the sparkling Snowbourne are but a few of the rivers and watercourses that bisect the Riddermark. 

The Anduin serves chiefly as the country's eastern border and plays little part in creating the abundant grasslands of the Mark. The Isen and Adorn, on the Mark's western margins, serve primarily as borders although the Isen and its tributaries help irrigate the lush grasslands of the Westfold.

The Entwash, which springs forth from Treebeard's home in Wellinghall, is the chief river of Rohan, and divides the country into two distinct regions: The West and East Emnet (plains). 

This mighty river, and its many tributaries, bring bounteous supplies of water from their sources in the Misty Mountains and Ered Nimrais (particularly in spring when the winter snow melt occurs.) 
Thus, although Rohan's terrain is chiefly made up of porous strata, such as chalk and lime, adequate water reaches the plains and results in the country's rich pasturelands and meads.

The Snowbourn, which begins its course at the neck of Harrowdale in the Ered Nimrais, supplies water and abundant fish-stocks to a number of villages (Dunharrow,  Upbourn, Underharrow), the capital city, Edoras, and the network of farms and homesteads which surround these settlements.