The River Allen starts as a winterbourne at Wyke Farm in Monkton Up Wimborne, on the chalk downland of Cranborne Chase.

It flows roughly southwards on the dip slope of the downland through Wimborne St Giles, and is joined by the Gussage stream at Gussage All Saints and the Crichel stream south of Moor Crichel, before passing through Witchampton and Hinton Parva, where the landscape transitions from chalk downland to broad valley.

The amount of water taken was having a detrimental effect on the river flow whilst at the same time the salmonid population dropped significantly.

[9] The Allen had the strongest population of white clawed crayfish in Dorset, and measures such as tree planting and slowing the flow of the river were implemented to help the species.

[8] As well as an abundance of brown trout the river is home to minnow, dace, grayling, salmon, perch, roach, pike, bullhead and brook lamprey and the critically endangered European eel.