The river drains Coniston Water from its southernmost point and flows for about 6 miles (9.5 km) in a southerly direction before joining the upper estuary of the River Leven at Greenodd.
In the 19th century numerous industries including a cotton mill and iron foundry flourished along the river, making use of the fast flowing water to drive machinery.
A bobbin mill at Spark Bridge, which manufactured wooden bobbins for the Lancashire cotton industry, used water-powered lathes to turn the wood.
The factory survived into the 1970s using Gilkes water turbines to generate electricity and turn the lathes.
The Allan Tarn at the beginning of the river became the Octopus Lagoon.