They are the Janon, upstream Gier, Onzion, Dorlay, Egarande, Couzon, Grand Malval, Mezerin and Combe de l'Enfer.

They are the Langonand, which flows into the Janon, Mornante, Ruisseau des Arcs, Faverge, Durèze, Collenon, Féloin and Bozançon.

[10] Gier has seasonal fluctuations typical of rivers of the French Massif Central that are partly fed by melting snow.

Flow is lower in summer, from July to September, with a decrease of the average monthly rate to the level of 1.14 cubic metres (40 cu ft) in August.

B. Chambeyron, the historian of Rive-de-Gier, talks of a flood in 1684 where the waters from Saint-Chamond to Givors rose to the first floor above ground level.

[4] The Barrage du Piney, a dam, was built on the Gier at 45°25′32″N 4°31′38″E / 45.425632°N 4.527321°E / 45.425632; 4.527321 near La Valla-en-Gier between 1953 and 1955, with a capacity of 1,900,000 cubic metres (67,000,000 cu ft).

The Barrage de Soulages, another dam, was built on the Gier at 45°26′17″N 4°30′44″E / 45.437934°N 4.51221°E / 45.437934; 4.51221 between 1968 and 1970, with a capacity of 2,600,000 cubic metres (92,000,000 cu ft).

[16] The main axis of the Gier valley today holds an almost continuous urban and industrial area from Terrenoire in Saint-Étienne to Givors on the Rhone.

[5] The main towns from west to east, with their 2010 populations, are Saint-Chamond (35,793), La Grand-Croix (5,070), Lorette (4,498), Rive-de-Gier (14,996) and Givors (19,118).