Loch Lomond is the largest area of freshwater in Britain (7,108 ha) and is drained into the Clyde estuary by the River Leven.

Within Cashel, water courses to the south of Bcinn Bhreac flow into Loch Lomond (i.e. into the Clyde catchment), whereas those to the north flow into the Forth catchment.

Cashel lies in an area where the critical loads of acidity for freshwater are exceeded (0-0.5 keq H+/ha/yr). The area has a high level of precipitation and is relatively close to the Glasgow conurbation where annual sulphur dioxide and nitrogen oxide emissions are each up to 10,000 tonnes per 10 km Grid Square (1989 estimates).

Within Cashel the most significant watercourses are the Blair Burn, which follows the approximate line of the southeast boundary, and the Cashel Burn, which in its lower reaches bisects the property. Numerous feeder streams on the higher ground feed both burns. The only permanent standing water of any size is in two small lochans on the north east side of Beinn Bhreac.