Withywindle was a minor tributary of the River Baranduin or Brandywine that flowed through the Old Forest on the borders of the Shire. The river began in the Barrow-downs and flowed southwest through the Old Forest until merging with the Brandywine. It was the Withywindle valley that was said to be the heart of all the strange happenings in the Old Forest. At the mouth of the Withywindle there was a hythe - or haven - in the north bank called the Grindwall. The Grindwall was beyond the protection of the High Hay, so it was guarded and there was a fence extending into the water.
There was a small village named Breredon behind the Grindwall between the end of the High Hay and the Brandywine. The village of Haysend was also located at the mouth of the Withywindle. The Elvet-isle in the Withywindle was home to swans.
When Frodo Baggins and his companions entered the Old Forest on September 26, 3018, they found themselves drawn toward the Withywindle valley despite their intentions, and on the banks of the river they encountered Old Man Willow, an ancient tree whose hatred of all peoples who walked free on the earth permeated the woods. The Hobbits were lulled to sleep by Old Man Willow's spell. Frodo was pushed into the Withywindle and was held under by the willow's roots until Sam saved him, and Merry and Pippin were trapped inside Old Man Willow. They were rescued by Tom Bombadil.
Long ago, Tom had found Goldberry, the River-daughter, in a pool down the Withywindle, and at the end of every summer he would go along the river to gather water-lilies for her. He was on his last trip of the year when he encountered the Hobbits.
The word withy means "willow" and windle means "spindle" or "reel." In the real world, Withywind is a type of bindweed, or convolvulus, a harmful weed that twines itself around other plants. The word withywind means "flexibly strong, entangle."