A pristine Wye or Usk stream is usually bounded by a mixture of grasses and trees.

Provided there isn’t complete shading, each spring and summer there is a growth of vegetation which effectively narrows the stream. This tends to maintain the speed of flow and depth.

Each winter, this is reversed as flows increase in intensity. The summer growth is knocked back and stream width adjusts to the prevailing flow, only to reverse and narrow again during the growing season.