1. c. 28), authorized Thomas Steers and William Squire to make the River Douglas navigable to small ships between Wigan and its mouth.

The Douglas is derived from the Brittonic elements dūβ-, meaning "black",[4] and *glẹ:ss, "stream, rivulet, watercourse" (Welsh du-glais).

[5][6] The Douglas descends rapidly, dropping below the 430-foot (130 m) contour before flowing into the Lower Rivington Reservoir, just north of Horwich.

Below the dam, the river's flow is maintained by compensation water released from the reservoir, marking the county boundary between Greater Manchester and Lancashire.

It flows in a generally southwesterly direction and is crossed by the Leeds and Liverpool Canal,[6] via a grade II listed aqueduct designed by John Rennie in the 1790s.

[7] The river then bypasses Worthington Lakes, three reservoirs built in the 1860s to provide drinking water for Wigan, and now managed as a country park by United Utilities.

[8] The river ceases to form the county boundary just before the lakes and re-emerges from a tunnel below the reservoir dam, passing east of Standish.

White Bridge at Rufford is a grade II listed structure with three spans, constructed of squared sandstone.

[15] Upgrades to the Wigan and Skelmersdale Treatment Works have been carried out, to improve the quality of the final effluent before discharge into the river system, while in Wigan, the sewage system has been improved, to reduce the amount of raw sewage discharged into the river by combined sewer overflows.

[15] Another joint initiative between the Environment Agency and the Royal Society for the Protection of Birds has seen sea defences improved and 370 acres (150 ha) of saltmarsh created as part of the Hesketh Outmarsh Coastal Realignment project.

Reasons for the quality being less than good include sewage discharge for most of the river, together with physical modification of the channel and poor nutrient management of agricultural land on the lower sections.