[3] Discharging from Loch Rannoch, it flows east to a point near the Falls of Tummel, where it bends to the southeast, a direction which it maintains until it falls into the River Tay, just below Logierait, after a course of 58 miles (93 km) from its source in Stob Ghabbar (3,565 ft (1,087 m)).

Its only considerable affluent is the Garry, 24 miles (39 km) long, an impetuous river which issues from Loch Garry (2.5 mi (4.0 km) and 1,334 ft (407 m) above sea level).

Some 2 miles from its outlet from Loch Rannoch the river expands into Dunalastair Water (or Dunalastair Reservoir), a man made loch formed by a weir, part of the Tummel Hydro Electric power scheme.

[4] Loch Tummel was previously 4.43 km (2.75 mi) long and 39 m (128 ft) deep, but with the construction of the Clunie Dam in 1950, the water level was raised by 4.5 metres,[5] and Loch Tummel is now approximately 11 km (7 mi) long.

[7][8] The chief places of interest on the river are Kinloch Rannoch; Dunalastair, a rocky hill in well-wooded grounds, the embellishment of which was largely due to Alexander Robertson of Struan, the Jacobite and poet, from whom the spot takes its name (the stronghold of Alexander); Foss; Faskally House (beautifully situated on the left bank); Pitlochry; and Ballinluig.