The River Isla rises among the Grampians, at an altitude of 3100 feet, 1½ mile NE of the meeting-pint of Forfar, Perth, and Aberdeen shires, and 6 1/8 miles SSW of Lochnagar.
It then winds 29½ miles south-south-eastward, then 17 1/8 miles south-westward, till, after a total descent of 3000 feet, it falls into the Tay 11½ miles NNE of Perth, and 4½ WSW of Coupar-Angus.
Its chief tributaries are Melgam Water, the Burn of Alyth, Dean Water, the Ericht, and Lunan Water and it traverses or bounds the parishes of Glenisla, Lintrathen, Airlie, Ruthven, Meigle, Bendochy, Blairgowrie, Coupar-Angus, Cargill, and Caputh. One of Scotland’s most spectacular waterfalls, Reekie Linn, can be found along its course. It is liable to great freshets or floods. Historical reports write of the occasion of the thunderstorm of 17 July 1880, when the water rushed down it in the form of a moving embankment 10 feet high, and, spreading over the valley, buried crops of all kinds in sand, and swept away sheep and lambs. The damage caused by another flood, in Sept. 1881, was estimated at £10, 000, including £2000 for renewal of embankments. The great traveller of the early 19th century, Dr Macculloch apparently said ‘three yards of the Isla and its tributaries are worth all the Tweed put together. ‘
In the Irish Nennius version of Historia Brittomum there are the lines ‘Atá dno glenn i n-Aengus, & eigim cacha h-aidchi Luain and, & Glend Ailbe a ainm, & ni feas cia do gni fuith’ (There is a valley in Aengus, in which shouting is heard every Monday night; Glen Ailbe is its name, and it is not known who makes the noise).