The Arga begins in the hills of Quinto Real, where it is regulated by the Eugui reservoir, which, with a capacity of 21 Hm2 is the main drinking-water reservoir for the Pamplona District. In Huarte, it has a year-round, average flow of 7.18 m3/sec. Just as it reaches the Basin of Pamplona, it is joined by the River Ulzama, from the hills of Ulzama-Velate, which, with a year-round, average flow of 8.48 m3/sec, means that it doubles in size.
On entering the Basin, the river takes on a meandering form, receiving waters successively from the Rivers Elorz, Juslapeña and, above all, the Araquil. In Pamplona, the river’s flood meadow ranges from 500 to 1,500 metres wide and its varied flow means that it can rise quite spectacularly, affecting large areas of land.
Consequently, the Arga should be considered a highly dynamic river with erosion on outer faces of meanders and banks, sedimentation and deposits on inner faces, and with a natural tendency to change course over its alluvial plain. Its natural dynamism has been affected and almost totally eliminated by the presence of numerous infrastructures, constructions (buildings, dams, defences, bridges, etc.) and uses (market gardens, sports clubs, resources, etc.), meaning that its course may now be considered invariable.
