The River Yarrow is in Lancashire, with its source at an area called Will Narr at Hordern Stoops, along Spitlers Edge - the Chorley/Blackburn boundary - on the West Pennine Moors.

The dam works by monitoring the river level in croston and either lowers or raises gates to control the flow of water.

It may have the same origin as the River Yarrow in Selkirkshire in Scotland,[3] and therefore be derived from the Brittonic element garw, meaning "rough, harsh, rugged, uncultivated".

[3] However, it may also be related to the River Arrow in Warwickshire (Brittonic *ar-w-ā-),[3] and derived either from Brittonic *ar, an ancient river-name element implying either horizontal motion, "flowing", or else "rising" or "springing up",[3] or *arβ, *arw, found in the Celtic languages as Welsh irfin and Breton irvin, both meaning "a wild turnip".

[3] A relationship with the River Arrow in the Welsh marches is also possible,[3] deriving therefore from a form of Brittonic arɣant, meaning "silver, white, bright".

[5] Removal of Himalayan Balsam, Japanese Knotweed and Rhododendron plants is undertaken to allow low-growing native species to flourish.

Limestone Brook — has its source near to Devils Ditch, and flows via Shooting Huts on Anglezarke Moor where it joins at the Meeting of the Waters.

Reasons for the quality being less than good include sewage discharge, physical modification of the channel and poor nutrient management of agricultural land.