Its catchment area, covering 238 km² and reaching a maximum elevation of 932 m, contains a wide variety of geological formations, dominated by marl-limestone (flysch). It is mainly agricultural and pastoral, with large areas of heathland on the hills (more than 50% of the catchment’s total area). The water, which is neutral to slightly alkaline and often clouded by sediment eroded by rainfall, hasof good quality as far as St Pée-sur-Nivelle, but the quality degrades further downstream.

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

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

Reports salmon stock : link to HAL (since 2024)

 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

Monitoring reports on the population of allis shad, sea trout and salmon : link to HAL (2005 to 2014)