Short answers, pulled from the story.
The Gardon is 127.6 km long from its source in Saint-Martin-de-Lansuscle to where it joins the Rhone near Comps. Measured from the furthest source of its tributary the Gardon de Saint-Jean, the total length extends to 133 km.
The Gardon rises in the commune of Saint-Martin-de-Lansuscle in the Cevennes mountain range. It flows south and empties into the Rhone as a right-side tributary at Comps, north of Beaucaire.
Two historic bridges cross the Gardon: the Roman aqueduct Pont du Gard and the 16th-century Pont Saint-Nicolas. The Gorges du Gardon canyon ends at Pont Saint-Nicolas and is a popular recreation area.
It is possible to kayak or canoe under the Pont du Gard, though at times the river level is not high enough to allow passage. Rental agencies in the village of Collias, about 12 km downriver from Pont Saint-Nicolas, bus paddlers upriver; the trip from Collias to the Pont du Gard takes roughly an hour and a half.
The Gardon flooded at record levels in September 2002 and again in December 2003. Both events damaged multiple bridges along the river, including the Pont Saint-Nicolas, which has since been fully restored.
The department of Gard takes its name from the Gardon river. The river is known in both Occitan and French as the Gardon or Gard.