Olona

Olona
The Olona river in Legnano
Country	Italy
Basin features
Main source	Principal source at Fornaci della Riana at 548 m (Rasa di Varese). Other springs are at Monte Martica (937 m), Passo Varrò (760 m), Alpe Ravetta (618 m), Pian Waldes (720 m) and Miniera Valvassera (450 m) 548 m (1,798 ft)
River mouth	Lambro meridionale.
Basin size	1,038 km2 (401 sq mi)
Physical characteristics
Length	71 km (44 mi)
Discharge	Average rate: 14 m3/s (490 cu ft/s)

The Olona (Ulona, Urona or Uòna in Western Lombard) is an Italian river, 71 kilometres (44 mi) long, that runs through the provinces of Varese, Milan and Pavia.

The Olona has more than one source; in fact it has six main springs. Three springs are located in the Rasa Valley, just above Varese, whereas other three springs pour water down from the southern side of pleasant Valganna - since 1877 these still clean and fresh mountain waters were historically used, and still are, to make beer in a renowned local brewery, Poretti, now run by Carlsberg.

The Olona river passes through the deep Olona Valley, cut in the poruous soils of the upper Po Valley, eventually flowing in the plain after Legnano. It is in the Olona valley part that the river gets much of the industrial pollution which has made it a byname for venomous waters for decades. Only in recent years attempts to restore the river's ecology have proven at least partially successful. After Legnano, the Olona river flows in the plain until Milan, usually contained in narrow artificial banks. New expansion basins are being constructed to prevent floodings, which are rare but can be devastating under conditions of sudden intense rain in the upper river flow.

In ancient times, the 131-kilometre (81 mi) long Olona continued its course south of Milan, through the Province of Pavia, eventually joining the Po. Due to massive hydraulic works, that started in the Roman Age and were continued until the 20th century, just before Milan the river leaves its natural bed and begins to flow under the street level, like a sewer, feeding and becoming part of the complex groundwater system of the city.

Immediately after passing through a tunnel under the Naviglio Grande, the Olona flows into the Lambro Meridionale, that carries its waters in the Lambro. Until the late 1980s, a branch of the Olona fed the Darsena of Porta Ticinese (the inner harbour of Milan). The water flow in this branch was first reduced, and finally completely cleared, when city authorities realized that the river's pollution began to threaten the Darsena and the Naviglio Pavese, which originates from it.