The Valle Olona is a valley stretching from just south of Bregazzana, a hamlet of Varese, to Castellanza in northern Italy.
Carved by the Olona River, from which it takes its name, the valley formed during the Riss glaciation due to the erosive action of a glacier descending from Lake Lugano. Over subsequent centuries, the Olona River partially filled the valley's alluvial zones with gravel and sand, and later, through repeated flooding, deposited substantial sediment that now supports diverse plant life.
The Valle Olona typically features a valley floor devoid of settlements, except for (one of two hamlets of Castellanza, located at the valley's outlet) and the hamlets of Gornate Olona, namely Torba and San Pancrazio. Farther north, one of the last inhabited areas is the Mulini di Gurone, which also hosts a dam designed to protect downstream settlements from Olona River floods.
Scattered across the valley floor and nestled among numerous wetlands and broadleaf tree forests that cloak the surrounding slopes are remnants of and several , once vital to local economies.
The valley also encompasses the route of the Valmorea railway, once abandoned but now repurposed as a cycling and pedestrian path between Castellanza and Castiglione Olona. Part of the line was refurbished to accommodate a tourist train between Malnate and Mendrisio until 2013. Plans are underway for the Ciclovia Olona Lura, a 165 km loop connecting the valley to the Lura stream along the European EuroVelo 5 route.
Physical geography
Terrain
Located south of the Valmorea valley (carved by the Lanza stream) and the valley etched by the Bevera stream, the Valle Olona gathers waters from the Olona and its tributaries across the southeastern Province of Varese.
The Valle Olona begins south of Bregazzana, a hamlet of Varese, where the Olona branch from the converges with the branch originating in the Valganna, forming a single waterway that flows to Milan, where it joins the .
The aforementioned Lanza and Bevera streams merge with the Olona south of this confluence, as their initial courses are hydrologically distinct, with sources farther north and relatively distant from those of the Olona.
Besides its primary source at Fornaci della Riana in , a hamlet of Varese, the Olona emerges from five smaller springs, two in Val di Rasa and three in Valganna, which unite downstream of Bregazzana.
After cutting southeast through the Province of Varese, the Valle Olona ends at Castellanza, where its slopes level off. From here, the Olona flows across a plain, transitioning from the earlier hilly and prealpine terrain. The valley's lowest point is at Castellanza (216 m a.s.l.), while its highest is Pian Valdes, peaking at 718 m a.s.l.
The valley's perimeter includes parts of the municipalities of Varese, Induno Olona, Malnate, Vedano Olona, Lozza, Castiglione Olona, Lonate Ceppino, Gornate Olona, Castelseprio, Cairate, Solbiate Olona, Fagnano Olona, Gorla Maggiore, Gorla Minore, Olgiate Olona, Marnate, and Castellanza, as well as areas of neighboring municipalities on the morainic plateaus within the Olona's hydrographic basin, such as Busto Arsizio and Tradate.
Hydrology
In its valley stretch, the Olona receives numerous tributaries rising from surrounding hills, including the Bevera, Lanza (also known as Ranza, Anza, Clivio, or Gaggiolo), Fogascè (or Gerre), Quadronna, Selvagna, Mornaga, Riale delle Selve, Marubbio, Valdessera, Riale San Pancrazio, Riale di Torba, Riale di Castelseprio, Rile, and Tenore.
The Valle Olona hosts several wetlands: The Stagno Buzonel, located on the valley floor between Castelseprio and Lonate Ceppino, fed by the Bozzone stream, which then joins the Olona. The Stagno di Cairate, at the border of Cairate and Lonate Ceppino, sustained by springs. The Refreddo or Fontanile Crotto, a stream originating at Crotto Valle Olona (on Castelseprio's valley floor) and merging with the Olona downstream.
Geology
Formed during the Riss glaciation by the erosive force of a glacier from Lake Lugano, the Valle Olona exhibits the characteristic U-shaped cross-section typical of glacially carved valleys. After the ice retreated, rivers and streams began to flow, initially filling parts of the alluvial zones with gravel and sand, and later depositing earthy sediments during repeated floods, fostering today's vegetation.
Climate
The Valle Olona experiences a continental climate with warm, moderately rainy summers and high humidity. Winters are cold, with weeks of frost, and occasional snow days.
Ventilation is generally poor, varying by location. In the northern valley, near the Prealpi Varesine, light breezes may clear the air, while in the Alto Milanese, especially in winter, fog persists due to stagnant conditions.
Climatological data from the (named after Milan Malpensa Airport, itself derived from ) for 1961–1990 include: +MILANO MALPENSA (1961–1990)MonthSeasonYearJanFebMarAprMayJunJulAugSepOctNovDecWinterSpringSummerFallAvg. max. temperature (°C)6,18,613,117,021,325,528,627,624,018,211,26,97.217.127.217.817.3Avg. min. temperature (°C)−4,4−2,50,44,39,012,615,314,811,56,40,7−3,6−3,54.614.26.25.4Absolute max. temperature (°C)21,0 (1982)24,4 (1990)25,4 (1990)28,0 (1975)30,7 (1979)34,3 (1965)37,0 (1983)35,8 (1974)33,9 (1988)28,1 (1986)22,8 (1964)21,1 (1967)24.430.737.033.937.0Absolute min. temperature (°C)−18,0 (1985)−15,6 (1987)−12,2 (1971)−6,1 (1970)−5,2 (1979)0,6 (1974)4,7 (1974)4,7 (1979)0,5 (1976)−5,3 (1974)−13,6 (1988)−15,2 (1973)−18,0−12,20.6−13,6−18,0Cloudiness (okta per day)4,84,64,44,74,94,43,63,93,84,14,94,64.74.74.04.34.4Precipitation (mm)67,577,199,7106,3132,093,366,897,573,2107,4106,354,6199.2338.0257.6286.91 081,7Rain days66891096867861827232189Mean relative humidity (%)787669737474747374778080787273.77775.2Pressure at 0 m a.s.l. (hPa)1 0191 0171 0161 0141 0151 0161 0161 0151 0181 0201 0161 0171 017,71 0151 015,71 0181 016,6Wind (direction-m/s)N 3,3N 3,3N 3,4N 3,5N 3,3N 3,2N 3,1N 3,0N 3,1N 3,1N 3,4N 3,33.33.43.13.23.3
History
The Middle Ages
Since the Valle Olona ends at Castellanza, the nearby Legnano, just south along the Olona, was strategically chosen by the Lombard League to position the Carroccio—a symbol of municipal autonomy—during Frederick Barbarossa’s fifth and final Italian campaign. This conflict with the medieval communes of Northern Italy culminated in the communal victory at the Battle of Legnano on May 29, 1176, followed by the Peace of Constance on June 25, 1183. The latter agreement saw the Emperor recognize the Lombard League, granting administrative, political, and judicial concessions to the communes, effectively ending his bid to dominate Northern Italy.
At that time, the borough of Legnano offered easy access from the north to the Milanese countryside, positioned at the Valle Olona's outlet. This gateway required staunch defense against military incursions targeting Milan, facilitated by an ancient Roman road, the , linking Mediolanum (modern Milan) to Verbanus Lacus (Lake Maggiore) and onward to the Simplon Pass (Latin: Summo Plano).
The Valle Olona was also the heart of the Contado del Seprio, with ruins of its capital preserved in Castelseprio. In 1287, the entire Seprio was annexed to the Lordship of Milan, which became the Duchy of Milan in 1395. The Valle Olona and Seprio remained under Milanese rule until the Napoleonic era, when they were incorporated into successive Napoleonic puppet states extending beyond modern Lombardy. After Napoleon's defeat at the Battle of Waterloo, they joined the Kingdom of Lombardy–Venetia, a dependency of the Austrian Empire.
Since the Middle Ages, this Lombard region was heavily exploited due to the Olona and its tributaries, enabling the construction of numerous mills. These harnessed water power for grinding grain, forging metal, pressing oil, and sawing timber from local resources like wood, wheat, and oilseeds such as rapeseed and colza. By 1610, the was established to regulate water use, overseeing 116 Olona mills—equipped with 463 waterwheels—spanning the river's course to Rho.
The 19th Century and Industrialization Between 1826 and 1828, during the neoclassical reconstruction of Milan's Spanish-era Porta Comasina (renamed Porta Garibaldi in 1860), four colossal statues by Giambattista Perabò were added atop the gate. These allegorically represent Lombardy's major rivers: Po, Adda, Ticino, and Olona.
By the mid-19th century, amid early industrial development, the Valle Olona's economic exploitation evolved. Mills were replaced or supplemented by modern industrial complexes, including cotton mills (e.g., Cantoni Cotton Mill in Legnano, in Solbiate Olona, and in Fagnano Olona), tanneries (Conceria Fraschini in Varese), paper mills ( in Cairate and Molina in Varese and Malnate), brick and lime kilns, and spinning or dyeing plants. These used advanced waterwheels, far more efficient than traditional mill wheels.
The valley and its Olona conurbation became one of Italy's leading industrial zones. Intensive water use—for power and processing—severely polluted the Olona, earning it the title of Italy's most polluted river. This dubious distinction was rivaled only by rivers like the Lambro, Seveso, Lura, Mella, and Arno, which also traverse Lombardy's industrial heartlands.
Following the industrial boom of the 1970s and subsequent economic crises—exacerbated by frequent flooding that crippled local industries—the Olona's water quality has steadily improved. Enhanced by treatment plants, it now rates as "sufficient" or better between Varese and Castellanza.
Protected Areas
Parco Valle del Lanza The is a locally protected area near the Switzerland border, spanning the provinces of Varese and Como. It occupies the western foothill arc of Lombardy, between the Adda and Ticino rivers, centered on the valley of the Lanza stream (also called Gaggiolo or Ranza). Rising on the southern slopes of Monte San Giorgio in Canton Ticino, the Lanza joins the Olona in Malnate.
Parco Rile Tenore Olona
The (commonly "RTO") safeguards the valley across Lozza, Castiglione Olona, Gornate Olona, Castelseprio, and Lonate Ceppino. It also covers the western morainic plateau, including Gazzada Schianno, Morazzone, Caronno Varesino, and Carnago, where the Rile and Tenore streams flow. Established in 2006 at Castiglione Olona’s initiative, the park is headquartered there.
Parco del Medio Olona The protects the valley in Fagnano Olona, Gorla Maggiore, Solbiate Olona, Gorla Minore, Olgiate Olona, and Marnate, encompassing the Tenore stream area in Fagnano Olona and eastern woodlands in Gorla Maggiore, home to the . Founded in 2006, it is based in Fagnano Olona.
Parco del Bosco del Rugareto
The , a locally protected area, extends east of the Valle Olona to the Tradate pine forest. Covering about 1,400 hectares, it spans Cislago, Marnate, Gorla Minore in Province of Varese, and Rescaldina in Province of Milan. Its woodlands host three streams: the Fontanile di Tradate, (or Bozzentino), and , with the latter maintaining steady flow even in dry periods.
Parco Archeologico di Castelseprio The preserves significant archaeological finds from prehistoric times through the Middle Ages to the Early Modern period. It includes ruins of the fortified settlement and borough of Castelseprio, plus the nearby . The nearby Torba monastery, managed by the FAI, forms part of this archaeological complex.
Designated a UNESCO World Heritage Site on June 26, 2011, the park emerged after historian rediscovered the site in the 1950s.
Nature
Flora
The Valle Olona's slopes are largely cloaked in woodlands. On the valley floor, small groves alternate with cultivated fields, heathlands, meadows, and disused industrial sites where climbers and shrubs have proliferated since operations ceased.