Though the river Tay belongs to Perthshire, one of its main affluents, the Isla, flows for at least two-thirds of its course through Angus. Forty-five miles from its junction with the Tay, far up in the heart of the eastern Grampians, the Isla finds its source in two streams, one descending from the corry of Caenlochan and the other from Glen Cannes. At first it flows due south through a narrow, tortuous glen, but soon turns south-east through a wider valley. Further on, it plunges into the Den of Airlie, one of the grandest ravines in this part of Scotland. After curving to the west, the Isla winds through the heart of Strathmore to join the main stream. Many hill burns, as the Brighty and the Newton Burn, pour into the Isla. Lower down it receives the Burn of Alyth and the Melgum. The Dean from the Forfar Loch, and the Ericht, known in one of the main branches of its upper course as the Blackwater—a stream nearly as long as the Isla itself and flowing through a valley parallel to it—are tributaries that join it in the Strath.

The Reekie Linn in the Den of Airlie is as imposing as any waterfall in the country. At this point the river, which has for some distance been hemmed in by precipitous banks, plunges over a rock 80 feet high in a cataract which when swollen is unbroken in its descent. The smoke-like spray which then enshrouds it gives its name to the fall. Scarcely less remarkable is the boiling Slug of Auchrannie, a rocky gorge of the river about a mile farther down the Den of Airlie. The waters of the Melgum flowing through the Loch of Lintrathen, from which Dundee derives its main water supply, have been partly impounded to increase the resources of that fine natural reservoir.

The central river of Forfarshire is the South Esk, a stream entirely within the county limits. It rises in Cairn Bannoch (3314 ft.), the extreme north-western point of Angus, is joined by a small hill burn that carries off the overflow of Loch Esk, often mistaken from its name as the source of the river, and under the sublime heights of Cairn Broadland, the Scorrie, and the Red Craig, receives the White Water from Glen Doll. The valley of the South Esk is known in its upper reaches as Glen Clova. For the first half of its course the river flows south-east. At Cortachy it enters a rocky channel and a richly wooded district, and after being joined by the Prosen, it sweeps eastwards into Strathmore and winds in a tranquil course to the sea, which it reaches through the wide estuary of Montrose Basin. Its main affluent on the left bank is the Noran from Glen Ogil. About six miles from Montrose Basin the river leaves the wooded policies of Brechin Castle and curves round the southern part of the ancient ecclesiastical city of Brechin. Lower down it passes the noble deer park and grounds of Kinnaird Castle.

Like the Isla and the South Esk, the North Esk is partly a highland, partly a lowland stream. For about half its course it forms the boundary of Kincardine. Its twin head-waters, the Lee and the Unich, join each other under the precipices of Craig Maskeldie, and after flowing through Loch Lee it receives the Mark on the left and the Eftock on the right. The valley of the Tarf, the main highland tributary of the Esk, spreads out like an extended hand to the north, each finger representing a hill burn descending from the frontier heights of the county. At the point where the Tarf joins the river is situated the hamlet of Tarfside, the headquarters of Upper Glen Esk. Towering above it in the west is the Craig of Migvie or Hill of Rowan crowned with an imposing monument to Fox Maule, Earl of Dalhousie. After forming a huge S-shaped curve, the axis of which lies east and west, the Esk enters a somewhat wider part of the glen, which it follows until it plunges into a ravine similar to those at Airlie and Cortachy. This, the most romantic part of its middle course, is crossed by the main road at the Gannochy Bridge, which presents fine views both up and down the river. Thereafter the Esk passes the pretty village of Edzell and continues its devious course to the sea, three miles north of Montrose. The Water of Saughs, known lower down its course as the West Water, is the longest tributary of the North Esk and joins it on the right bank a little below Edzell. The Cruick, a shorter stream flowing almost parallel to the South Esk, enters it in the grounds of Stracathro House.

Besides these larger rivers, four smaller streams fall to be noticed. The Lunan, with its tributary, the Vinney, rises in a chain of lochs east of Forfar and has a short course to Lunan Bay. The Brothock, a still smaller rivulet, enters the sea at Arbroath, a contracted form of Aberbrothock. A little to the west, the Elliot passes by Carmyllie and Arbirlot. The Dighty has a much longer course. It rises in the Lochs of Lundie under the central Sidlaws, flows east through Strathmartine and enters the lower estuary of the Tay at Monifieth.

The lochs of Forfarshire are less numerous and less extensive than they were, say, a hundred years ago.

Within that time many have been drained, with the double advantage of increasing the farming land and improving the climate of the county. In the case of lochs into which streams flow, silt is always being deposited, which through time decreases the area of the sheet of water. On the other hand, a loch is sometimes artificially enlarged and deepened for the purpose of supplying the wants of a town. Of this a notable instance is Lintrathen Loch.

There remain a few small lochs in the Lundie district. One by the roadside at Tallybaccart is the haunt of wild fowl and gulls. One or two pretty lochs lie deep in the neighbouring woods and at the head of Strathmartine is the small circular Pitlyel Loch in the midst of charming scenery. The Long Loch of Lundie directly under the highest parts of the Crags has been leased by the bleachers and millers of the Dighty, and at certain points its banks have been raised so as to secure at all seasons a sufficient supply of water for commercial purposes.

The Loch of Forfar, one mile and a quarter long by a quarter of a mile wide, lies close to the county town. Its surplus waters have been led into the Dean and so to the Isla. St Margaret’s Inch, an island in this loch, was the site of a religious house founded by Alexander II. To the east of Forfar stretches a chain of small lakes— Fithie, Rescobie, and Balgavies—-which are drained by the Lunan river.

The finest lochs are in the highland portion of the county. Every glen has its tarn. Drumore is under the heights of Mount Blair in the extreme west. In Glen Isla there is Auchintaple. Loch Esk lies high amongst the corries at the head of Glen Clova, and on the east side of the same glen, not far from the Kirkton but 1300 feet above it, are two deep mountain lochs, Brandy and Wharral, lying each under an amphitheatre of sterile and precipitous rocks. To the east of the Wharral ridge is Stony Loch, the source of the Water of Saughs. In Glen Esk there are two small lakes bearing the name of Carlochy, one in Glen Mark and the other in a deep embrasure of Craig Maskeldie. Loch Lee is, however, the chief mountain lake of Angus. A mile and a quarter in length by half a mile in breadth, it is in its noble surroundings a typically highland loch. Heather clad or bare rocky mountains rise to a great height on its two sides and its upper end, and except where the Lee is gradually silting it up on entering its waters, it is of great depth. Like so many more of the mountain lochs of the district, it is stocked with fine trout.

In 1870^ the Water Commissioners of Dundee were empowered by Act of Parliament to purchase Lintrathen Loch as a source for supplying water to their city.

Where bleach works and mills have not contaminated the purity of their waters, the lakes and streams of Forfarshire afford excellent trout fishing, which is particularly fine in the highland section of the county.