The Forth Estuary is divided into two parts, the narrow part to the west of Queensferry which is wholly within the area of the Forth River Purification Board and in which there is a substantial tidal variation in salinity, and the broader part to the east of Queensferry which is divided between the Forth and Lothians Boards and is virtually open sea.

There is no record of extensive biological appraisal of the narrow part of the Forth and the quality is assessed largely by general physical and chemical observations. The quality is largely determined by discharge of degradable organic matter in the form of sewage, sewage effluents and trade effluents, the bacterial oxidation of which deoxygenates the water, more or less, according to the combination of fresh water flow, meteorological conditions and the monthly tide cycle. The condition may be aggravated by thermal waste discharge.

In the broader part of the estuary on the north shore, pollution problems are limited to the littoral zone. In a number of places there is local beach pollution from sewage solids but mining pollution in the form of washery slurry pit redd and ferruginous pit waters has extensively affected conditions over a six mile stretch of shore.