The Entwash Vale of Rohan (somewhat conjectural)

The broad green valley that marked the passage of the River Entwash through the grasslands of Rohan. For most of its length, the vale ran north to south through the central regions of Rohan, and the river marked the boundary between the two great divisions of the realm, the Westemnet and the Eastemnet. As it approached the White Mountains, the river veered eastward. Here, its valley ran for a while between the mountains to the south and the Emyn Muil to the north, before flattening out as the Entwash flowed into the Anduin, where wide fenlands formed at the Mouths of Entwash and the Wetwang.

As a wide, flat valley, the Entwash Vale was a region of marsh and wetland, and prone to flooding by the Entwash itself (hence, indeed, the -wash of the river's name). The Vale's most famous flood came in the spring of III 2759, when the meltwaters of the Long Winter caused flooding across Rohan. These floods played an important part in driving the invading Dunlendings out of the land.