Its length is approximately 18 miles from source to the Thames, which it meets at Lechlade, giving a name to that town.
The Leach springs from the limestone uplands of the Cotswolds. In parts of its course it becomes a seasonal bourn, only running above ground when there is sufficient rainfall.
It seems likely that this spring in a sheltered valley on the Cotswold uplands was the original reason for the existence of the village of Hampnett. According to residents, this section runs all year round.
After running a mile down its limestone valley, the Leach reaches Northleach, the first village to which it gives its name. The river enters Northleach to the southwest, where it gushes out of a Victorian conduit just below the Fosse Way.
The site of the first watermill on the river is in a part of Northleach called Mill End. A section of the river is confined into mill race type stonework, close to the churchyard and runs behind houses marking the town boundary. The river can next be seen at a road bridge at the end of the town. It is still little more than ditch-sized, and as such continues down the valley to the hamlet of Eastington running alongside a lane before passing through a culvert and away through grazing land.
The river runs behind Lodge Park, a National Trust property, and through Larkethill Wood, where it flows under another lane, before passing Kilkenny Farm.
The Leach valley shows as a substantial dip in the B4425 Cirencester to Burford road. The river has an embankment at the bottom of the valley, but has not increased in size. Through all this section the river is often nearly invisible under summer vegetation. It then passes Swyre Farm with another road crossing, then Dean Farm, before reaching a very picturesque winding valley, crossed by the Westwell-Hatherop road.
This section is notably a winterbourn in an unremarkable depression. It normally runs in the winter, but often disappears in the summer, and this section remained dry throughout the dry winter of 2007, and ran in the summer instead.
Between the Cirencester road and the paired villages of Eastleach Martin and Eastleach Turville is the point at which the bourn becomes a river flowing all year. Here it is crossed by a stone slab clapper bridge on a footpath close to a wide marshy area. A small spring rises close to the bridge.
By the time the river passes between the Eastleach villages, each with their parish churches just a few hundred yards apart, the Leach has become a "proper" river, several yards wide. Between the villages a road bridge and another "clapper" footbridge cross the river. It passes another old mill between the Eastleaches and Southrop then skirts the hamlet of Fyfield. At the bridge at Southrop it divides into several channels, making a very attractive effect.
Final section and confluence
There are large watermills (none working) on the river in fairly quick succession, firstly at Little Faringdon close to Lechlade the town named after the river, then at "Lechlade Mill," and finally Priory Mill which is situated on the Kelmscott road. A few hundred yards later the Leach discharges from the northern bank into the Thames on the reach above Buscot Lock. It joins at the corner of the camping field behind "The Trout" inn.