The Tarff rises in a bog in Carrick and, flowing southward as the boundary between the parishes of Kirkcowan and Glenluce, flows across the Blood Moss, past Laggangarn with its ancient cross – adorned standing stones and the ruined chapel of Kilgallioch with its three bubbling wells, and swings eastward beyond its junction with the Mulniegarroch or Purgatory Burn.
It continues to move eastward for about a mile and a half, skirting the north side of Horse Hill, before resuming its generally southward course at Drumbays, passing through a Valley with Eldrig Fell to the east and Brough Hill, Artfield Fell, and Green Top to the west.
During its course of about six miles from Drumbays to the first Tarf Bridge, the countryside to the west is open, somewhat hilly moor and marshland, while, to the east, there are large, fairly recent coniferous plantations.
About two and a half miles south of the first bridge, which carries a minor road from Glenluce to Knowe,
Drumpail Burn joins the Tarff and about a mile further on the river takes a slightly more easterly direction as it flows past Airy ligg with its ford and footbridge.
Turning south again after a couple of miles, the river flows over stepping stones at the foot of Long Hill before passing under the main Dumfries to Stranraer road at Barlae. Again it veers to the east , once more over stepping stones, going past Muil Plantation of the north bank. It turns south under the Tarff Railway Bridge, and, at Kenmore, opposite which there is the ruin of an ancient chapel, it turns east for the last time.
Here the river runs wholly in the parish of Kirkcowan. At this place, Lincuan, the Laird of Craichlaw in the 17th century had a good salmon fishing with nets.
Over the last two miles of the river’s course, it drops quite steeply, thus providing a very swift current, one of the reasons that the mill owners of the Tarff (south bank) and Wauk (north bank) Mills chose to build their establishments there.
After flowing over the weir and past the mills, the Tarff courses under Johnston’s Bridge, over another series of weirs, and joins the Bladnoch about a mile east of the bridge near the place where, according to legend, the ancient chapel or church and settlement of St. Couan was established.