A lesser river of northern Beleriand that rose in the mountainous regions southwestward of Dorthonion, and flowed southward for some seventy miles to meet the great river Sirion on the borders of Brethil. It formed the eastern border of the land of Dimbar, separating that land from the perilous valley of Nan Dungortheb to the east.

The dangers of Nan Dungortheb arose from the creatures of spider form, the spawn of Ungoliant, that haunted the valleys of Ered Gorgoroth along its northern flanks. In more peaceful times, before Ungoliant returned to Middle-earth with Melkor, a road had been made that passed from west to east across Beleriand, crossing Mindeb some halfway along its course. In later times the terrors of Nan Dungortheb meant that the Mindeb river-crossing was rarely used.

In the years following their victory in the Nirnaeth Arnoediad, Orcs of Morgoth found their way through the northern mountains, travelling down through the Pass of Anach past the springs of Mindeb. These upper reaches of the river were said to form a dangerous and difficult path down from the heights, but the Orcs nonetheless made the journey in large numbers. Following the stream of Mindeb southwards, they infested the land of Dimbar to the river's west.