Olona

Olona
The Olona river in Legnano
Location of the mouth of Olona river in Italy
Native name	Olona (Italian)
Location
Country	Italy
Regions	Lombardy
Provinces	Province of Varese Metropolitan City of Milan
Physical characteristics
Source	Rasa di Varese
• location	Varese, Lombardy, Italy
• coordinates	45°51′20″N 8°49′23″E / 45.85556°N 8.82306°E / 45.85556; 8.82306
• elevation	548 m (1,798 ft)
Mouth	Lambro Meridionale
• location	Milan, Lombardy, Italy
• coordinates	45°26′52″N 9°9′22″E / 45.44778°N 9.15611°E / 45.44778; 9.15611
• elevation	113 m (371 ft)
Length	71 km (44 mi)
Basin size	1,038 km2 (401 sq mi)
Width
• minimum	5 m (16 ft)
• average	8 m (26 ft)
• maximum	18 m (59 ft)
Discharge
• average	14 m3/s (490 cu ft/s)
• minimum	7 m3/s (250 cu ft/s)
• maximum	18 m3/s (640 cu ft/s)
Basin features
Progression	Lambro Meridionale→ Lambro→ Po→ Adriatic Sea
Waterfalls	Cascate di Valganna
Average slope	6 ‰

The Olona (Olona in Italian; Ulona,^[1] Urona or Uòna^[2] in Western Lombard) is an Italian river belonging to the Po Basin, 71 kilometres (44 mi) long,^[3][4][5] that runs through the Province of Varese and Metropolitan City of Milan whose course is developed entirely in Lombardy.

The river born at 548 meters above sea level in the Fornaci della Riana locality at the Rasa of Varese, at the Sacro Monte di Varese, within the Campo dei Fiori Regional Park.^[6][7] After crossing the Valle Olona and the Alto Milanese, the Olona reaches Rho where it pours part of its water into the Canale Scolmatore Nord Ovest.^[8] After passing Pero, the river enters in Milan, where, at the exit of its underground route, it flows into the Lambro Meridionale, that flows into the Lambro at Sant'Angelo Lodigiano, in the quartier of San Cristoforo ending its course.^[9] Along the way, the water system formed by the Olona and the Lambro Meridionale crosses or laps 45 towns receiving the water of 19 tributaries.^[10]

The Olona is known for the waterfalls and caves of Valganna and for having been one of the most polluted rivers in Italy. The valley carved by the river, thanks to the system of water wheels that exploited the driving force originated by the water, was one of the cradles of Italian industrialization.^[11] The Olona river consortium (it. Consorzio del fiume Olona), that is founded in 1606, is the oldest irrigation consortium in Italy.^[12]

The river is sometimes also referred to as "northern Olona" for the homonymy with another Olona, who was born in Bornasco and flows into the Po after having crossed the Province of Pavia. This second Olona, in turn, is designated as "inferior" or "southern".^[13] The homonymy is not of imitative or etymological origin, but it is due to the fact that originally it was two trunks of the same river, diverted by the ancient Romans in its upper stretch towards Milan to bring water to the moat of the defensive walls of the city.^[14]