Its catchment area, covering 238 km² and reaching a maximum altitude of 932 m, features a wide variety of geological formations, dominated by marl-limestone (flysch); it is primarily agricultural and pastoral, with extensive areas of heathland on the hills (over 50% of the catchment’s total area). Its water, which is neutral to slightly alkaline and often clouded by rain, remains of good quality as far as St Pée-sur-Nivelle; it deteriorates further downstream.

Due to an impassable dam, migratory fish—including salmon, sea trout, eels and shad—can only access the first 18 kilometres of the river.

The activities relating to the monitoring of migratory fish at the site are the subject of annual reports :

Reports : 2024 - 2023 - 2022 - 2021 - 2020 - 2019 - 2018 - 2017 - 2016 - 2013 - 2012 - 2011 - 2010 - 2009 - 2008 - 2007 - 2006 - 2005 - 2004 - 2003 - 2002 - 2001 - 2000 - 1984 - 1983