Sisalde is a river valley holding rivers such as O Bolaños and Sisalde which, along with the proximity of the sea, help to generate humid and favourable environmental conditions for the development of flora and fauna. The humid ocean climate creates abundant rain in November and December, and frequent fog throughout the year, but it maintains a pleasant temperature in all seasons. The coastal zone is rugged, with steep inlets and headlands that generate a strong contrast between high mountains and deeper valleys.
This route manages to combine the essence of the Atlantic coast with its many beaches and the inland route of the River Sisalde, a riverside walk little touched by man, characterized by the presence of indigenous river bank flora and riparian forests typical of areas untouched and far from man. These riparian woods constitute the formations that line the rivers, and their root systems are indispensable for the conservation and retention of the banks, which are at risk of being swept away by the river. Although they are resistant to water, they are very vulnerable to human activity; and the alteration of the course of the river or its modification might mean they cannot develop and are displaced by less specific introduced species.
This environment is ideal for fauna, the presence of Otters (Lutra lutra) stands out. Although they are difficult to see due to their nocturnal habits, it is possible to find tracks and traces on the banks as their presence is considered habitual. The ornithological richness of this area brightens up the route with the presence and the trills of different species. Amongst the Reeds, and in the river bank area, there are species which are in danger of extinction such as the Reed Bunting (Emberiza schoeniclus), and also species that indicate the environmental quality such as the Common Kingfisher (Alcedo athis).
At the Rexedoiro Dam, Meicende and the River Bolaños pond it is possible to see the Eurasian Coot (Fulica atra), Common Moorhen (Gallinula chloropus) and Grey Heron (Ardea cinerea), a large-sized wading bird that stands motionless while searching for fish.
Seagulls are also abundant, mainly Black-headed and Yellow-legged Seagulls, birds associated with fishing villages. Favoured by this humid environment, there are abundant amphibian species such as the European tree frog (Hyla arborea) and the Marble Newt (Triturus marmoratus).
In the rest of the valley Iberian woods with Atlantic influence are common, such as those formed by Quercus robur. The Pedunculate Oak is very demanding in relation to humidity and deep ground, and it is characterized by lobulated deciduous leaves, and a slow-growing but robust trunk which can reach up to 40 metres in height. The Sweet Chestnut (Castanea sativa) is also important, forming long-living deciduous woods of big trees with rounded tops when they are not husbanded by man. General species such as Red Foxes (Vulpes vulpes) live there, as well as a great variety of forest birds.