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The River Coln rises at an altitude of about 200m AOD near Sevenhampton in Gloucestershire and flows from the limestone Cotswold Hills in a south-easterly direction to Lechlade where it joins the River Thames at an altitude of about 75m AOD. There are no major tributaries. The sampling site is located in Lechlade about 70m above the confluence with the Thames.The source of the river is in the Inferior Oolite aquifer in which it flows for the first few kilometers, but most of the river runs on the Great Oolite aquifer. Both of the limestone aquifers are sources for water abstration; a total of 55 ml per day are consented from the catchment. The river crosses Oxford Clay before running into the Thames. The catchment is mostly rural, with farming the main industry. The upper catchment is mainly grazing land, and there are large areas of deciduous woodland in the south-west. The upper two-thirds of the catchment is within the Cotswold Area of Outstanding Natural Beauty (AONB), and around Fairford the river has been designated as a Nitrate-sensitive area.
There are no large conurbations on the upper catchment, although Cheltenham, from where surface water drains into the limestone above the river's source, has a population of over 100,000. The Coln catchment supports a population of around 9,000. The main sewage inputs to the river are from works at Andoversford, Bibury and Fairford. Bibury Tout farm is the largest discharge into the river, although most of this is 'on-line' through fish-ponds. The river has been subject to various enhancement schemes to improve ecology and fisheries. Water quality was recorded as 'good to fair' in the 1995 General Quality Assessment survey; the classification varies throughout the river, due to the effects of both discharges and low flows.