Incised channels and gully erosion in Northern Iberian Peninsula: Controls and geomorphic setting

Gully erosion has scarcely been reported in the Cantabrian Cordillera (Northern Iberian Peninsula), even though this phenomenon is relatively frequent in this mountain range. A total of 133 permanent channels with evidences of gully erosion have been mapped in a 71.8 km2 study area on the northern slope of this mountain range. The underlying Ordovician unit comprises quartzite, sandstone and slate, which are partially mantled by different surficial deposits. The relief is fairly steep, with average slopes of 27° and hillsides with level drops of the order of 700 m and generally vegetated by Atlantic heaths, a result of the degradation suffered by the natural vegetation following forest fires. The mapped channels present different morphological features – shallow and deep channels, some of them with associated ravines – and are usually related to headcut in first and second-order, pre-existing channels, although some of them are located on unchanneled hillsides with no evidence of pre-existing channels. Moreover, lower reaches of first and second-order channels are often incised into bedrock. A field description and morphometric study has been carried out on channels affected by gully erosion, based on their cartography, Digital Terrain Models and several layers of thematic mapping. This study describes some of the relations between relief factors, the type of substratum and the development of gully erosion: i) there are robust correlations between morphometry, relief and erosion intensity; and ii) gullying is strongly influenced by the presence of a deep regolith. The erosion forms observed and the described relations suggest that gully development is associated to landsliding processes, which are ultimately controlled by ground water movement through a porous regolith. Regional uplift and deforestation are argued to be the most likely causes of channel incision and gullying.