The Olona (Western Lombard: Ulona, Urona or Uòna) is an Italian river belonging to the Po Basin, long, 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. 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. 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. 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.
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. The Olona river consortium (it. Consorzio del fiume Olona), that is founded in 1606, is the oldest irrigation consortium in Italy.
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". 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.
Physical geography Sources
The principal source of the Olona is in the Fornaci della Riana locality, at the Rasa of Varese, part of the homonymous provincial capital, at the Sacro Monte di Varese, within the Campo dei Fiori regional park. The Fornaci della Riana owe their name to some ancient limestone furnaces that remained active until 1972.
In addition to the main source, the river also flows from five other small springs, two in Val di Rasa and three in Valganna; these springs give rise to two branches joining downstream from Bregazzana (fraction of Varese).
The branch that is born to the west, in Val di Rasa, is the most important; the two springs that originate this waterway are located at the Varrò pass (between Monte Legnone and Monte Pizzella) and on Monte Chiusarella.
The main source, and the one located at the Varrò pass, join upstream from the inhabited area of the Rasa di Varese, while the spring that flows from Monte Chiusarella flows into the Olona downstream. The branch of the Valganna, which is located to the east, was born instead south of Monte Martica.
The branch of the Rasa is fed by seven small tributaries (more precisely, the torrents Legnone, Grassi, Boccaccia, Brasché, Pissabò, Valle del Forno and Sesnini), while the branch of Valganna is increased by four torrents (the Margorabbia, Valfredda, Valpissavacca and Pedana della Madonna).
The Valganna branch also gives rise to the Fonteviva lake, dedicated to sport fishing, and to the Valganna waterfalls, which in winter, due to the harsh climate, are often frozen. They are found in the municipality of Induno Olona and are close to the famous and homonymous caves. On the waterfalls, which were artificially created at the beginning of the 20th century to improve water withdrawal, one can admire the phenomenon of travertine surfacing.
Path
After the initial stretch, the river begins to travel the valley of the same name, the Valle Olona. This valley originated from the Olona and the retreat of the glaciers during the last ice age; it looks like a valley deeply engraved with the inhabited centers located on the hills overlooking the river bed, the so-called pianalti.
The slopes are mostly covered with woods, while in the valley bottom there are cultivated areas, meadows and heaths. The main tributary on Olona in the province of Varese is the Bevera; other important tributaries of this stretch of the river are the Vellone, the Gaggiolo (also called Rio Lanza, Ranza, Anza or Clivio), the Quadronna, the Selvagna, the Mornaga, the Riale delle Selve and the Tenore.
In Gorla Minore the river branches off into the Olonella, which joins the main riverbed after 1200 m. In this stretch, many artificial canals are born in the service of agriculture and of the industries that re-enter the Olona before Castellanza. After passing Castellanza, the waterway leaves the Valle Olona and heads towards the Po Valley. Once there was also another natural branch which was indicated by the name of Olonella and which crossed Legnano passing behind the basilica of San Magno. The natural island that was formed by the two branches of the river was known as "Archbishop Braida". This legnanese branch was buried in the first part of the 20th century.See the two topographies of Legnano (dated 1925 and 1938) which are present in the text of D'Ilario on p. 352 and on p. 353.
After crossing San Lorenzo (fraction of Parabiago) and Nerviano, the Olona river receives its two main tributaries, the Bozzente and the Lura, at Rho. In Rho there is also the "deviator of the Olona", completed in the 1980s, which flows into Lambro Meridionale. This work has always been a source of controversy: on the one hand it has not avoided all the floods as it was designed and, on the other hand, it carries polluted water towards the Ticino. In November 2002, in particular climatic conditions, the Olona, the Seveso, the spillway and the Ticino overflowed.
From Lucernate (fraction of Rho) onwards the river no longer flows into the natural riverbed, but follows the path deviated by the ancient Romans towards the Bozzente. Entering Pero, after an initial stretch still outdoors, the Olona begins to flow under the road surface and reaches Milan by first crossing the Gallaratese, Lampugnano and QT8 districts, where it collects the water of the Merlata river (also called Fugone), to then skirt the southern slope of Monte Stella. Once in Piazza Stuparich it receives the confluence of the Pudiga (known as Mussa in his Milanese section).
Merlata and Pudiga are the water collectors that come from the area north of Milan, the so-called "groane". The Olona then runs along the Lampugnano and San Siro districts, and then continues under the avenues of the ring road bypass.
The path under the ring road, which was designed for the first time on the General Plan of Milan in 1884 (the so-called Piano Beruto), was channeled in the first two decades of the 20th century and covered in a period from 1950 to 1970. At the exit from this covered stretch, after passing under the Naviglio Grande, the Olona ends its course flowing into the Lambro Meridionale.
Until the entry into operation of the water purification system of Milan (2005), the Lambro Meridionale was a drain collector which collected the results of the western part of the city sewerage system. Subsequently, these sewage were diverted to the purifiers of San Rocco and Nosedo. From the same unloader, the Lambro Meridionale has its "clean source".
Basin
The Olona is long and has a drainage basin of . The drainage basin of the Olona extends over part of the province of Varese, the Metropolitan City of Milan and, to a lesser extent, province of Como, also affecting part of Switzerland. A small part of the Gaggiolo basin, its tributary, in fact belongs to the Canton Ticino.
With its , the Olona drainage basin occupies 5% of the Lombardy area and hosts approximately 1,000,000 inhabitants (which corresponds to around 10% of the residents in the region). The catchment area of Olona instead measures 370 km².
Any hydrographic engineers describe the Olona and the Lambro Meridionale as a single stream that flows into the Lambro at Sant'Angelo Lodigiano and which has an overall length of .
Original bed and the two Olona
One of the most important studies on hydrography of Milan was carried out by engineer Felice Poggi. In 1911 Poggi affirmed that the two Olona, the one that flows into the Lambro Meridionale and the stream that flows into the Po at San Zenone al Po, constituted until the first years of the Common Era a single river that had a total length of . This hypothesis has also been confirmed by subsequent studies.
The place where the river was diverted to Milan by the ancient Romans is Lucernate, a fraction of Rho. From here, to find the ancient bed of the Olona following the minimum undulations and the very small altimetric variations of the terrain, we arrive at Cascina Olona (a locality of Settimo Milanese; the toponym is indicative), in Baggio and in Corsico, with a possible variant that from Settimo Milanese would lead to Muggiano and Trezzano sul Naviglio (or Cesano Boscone).
In the neighboring towns of Cesano Boscone, Corsico and Trezzano sul Naviglio, all three of which rise on the Naviglio Grande, it is possible to identify two waterways that could flow - towards the south - into the ancient natural riverbed of the Olona to Binasco: at Trezzano sul Naviglio and Cesano Boscone would be the Belgioioso canal, while in Corsico would be the Vecchia roggia.
From Binasco, with the derivation of the Canale Ticinello, we arrive then a little further south, in the territory of Lacchiarella, where the Colombana and Carona irrigation channels bring water to the irrigation network giving rise to the Roggione. The Roggione, when at the Settimo di BornascoRegional Technical Map 1: 10,000 Milan-Pavia. receives the Olonetta canal, changes its name to lower or Olona. The Olonetta, together with the Misana canal, comes from a fountain in Misano Olona, a few kilometers upstream. Taking the old course and its name, the southern Olona flows into the Po at San Zenone al Po.
The two Olona do not have an autonomous hydrography: at Rozzano, from the Lambro Meridionale, a branch take off towards the south-west, gaining vigor thanks to the water supply provided by springs and artificial canals. This stream then flows into the southern Olona. The reconnection of the two Olona is being planned with the construction of an artificial riverbed that would resume the ancient course of the river.
Detour to Milan
Until ancient Roman times, at La Maddalena, today's quartier of Milan, the Olona was diverted towards the city with the aim of bringing water to it: in ancient Roman times it flowed into the moat of the republican defensive walls of the city of Mediolanum then, from the 12th century, in the defensive moat around the medieval walls and later (1603) in the Darsena of Porta Ticinese.
In particular Olona river, during the Middle Ages, flowed into the moat of medieval defensive walls of the city in correspondence of the modern Piazza della Resistenza Partigiana, while in ancient Roman times it continued the city route reaching the modern Piazza Vetra, where it poured its water into the moat of Roman walls thanks to the homonymous canal, the Canale Vetra. The Cerchia dei Navigli then originated from the medieval moat of walls, while the two branches of the Roman moat became the Grande Sevese and the Piccolo Sevese, two canals still existing today in the Milan underground.