The largest of a group of reservoirs in Inverclyde, Loch Thom lies to the west of the Gryfe Reservoirs, 3 miles (5 km) southwest of Greenock. It was built 1825-7 as the principal part of a scheme to provide water to Greenock. Greenock was growing rapidly in the early years of the 19th century and there was great demand for water to power its mills. However, the growth also brought overcrowding and insanitary conditions which gave rise to a succession of epidemics of cholera, typhus and smallpox and thus a desperate need for clean water.
Designed by engineer Robert Thom (1774 - 1847) the scheme, in its entirety, cost £51,000. At first known as the Great Reservoir or Little Caspian it was later renamed in honour of Thom. Originally occupying 119 ha (295 acres) and containing 8182 million litres (1800 million gallons) of water, the embankments retaining the reservoir were raised in 1872 to increase its size to the modern-day capacity of 11365 million litres (2500 million gallons) of water and a depth of 12.8m (42 feet).
Water leaves Loch Thom through the Compensation Reservoir to the west and is then conveyed to its destination by an aqueduct known as the Greenock Cut. It is now operated by Scottish Water.
