Ayasse Stream
Edited by:
Massimo Martini
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Introduction
The Ayasse stream flows the entire length of the Champorcher Valley, the first right-hand side valley of the Dora Baltea as it enters the Aosta Valley from the south. It rises from Lake Misérin, at an altitude of 2,580 metres, at the foot of the valley head enclosed by the Rosa dei Banchi and the Bec Costazza, and flows theentire length until it joins the Dora Baltea in the municipality of Hône, just south of the Bard bridge. With a course of over 23 kilometres, it is the main right-bank tributary of the lower Aosta ValleyAosta in this section, and its catchment area, covering almost 110 km², drains the entire valley, including the upper reaches of the Mont Avic Nature Park.
Description
The upper reaches of the Ayasse stream have their source in theamphitheatre between the Col de la Balma and the Bec Cotasse: here, the main branch, fed by Lake Misérin, flows for around 7.5 kilometres before being joined by the Laris stream, which flows from thelake of the same name, and the Torrent du Bois, whose tributaries in turn rise from other lakes in the upper valley, including the Vercoche and the Gelé. The entire catchment area, running from west to east, covers approximately 110 km², with its highest point on Mont Glacier (3,185 m) and its lowest at the confluence with the Dora Baltea (335 m); only a tiny fraction of the area, less than one per cent, remains covered by residual glacial features. The flow regime is typical of Alpine streams: maximum flows in spring and summer due to snowmelt, and minimum winter flows sustained by perennial springs in the upper valley.
Historically, the waters of the Ayasse have sustained the valley’s agro-pastoral economy through a network of irrigation channels and various mills, some of which survive as historic structures along the middle and lower reaches. The steep gradient of the mountainous sections has profoundly shaped the landscape, giving rise to significant erosional features in the final stretch of the course, near Hône. The clear waters of the stream are home to trout populations, including the ‘marmoratus’ phenotype native to the Aosta Valley, making the stretch upstream of Champorcher a recognised area of interest for fish fauna and the site of a regulated fishing reserve.
In the upper reaches, the catchment borders Piedmont to the south, the Cogne Valley to the west, the Clavalité Valley to the north-west and the Mont Avic Valley to the north: in thisdirection, the basin’s territory partly overlaps with the Mont Avic Nature Park, the existence of which ensures the conservation of the alpine habitats traversed by the watercourse, from high-altitude pastures to the coniferous forests at the valley floor.