En descente vs. en aval
The French expressions 'en descente' and 'en aval' both relate to downward movement, but they have distinct uses and connotations.
En descente
'En descente' refers to a downward slope or motion, often used in the context of physical movement or terrain.
La route en descente est dangereuse par temps de pluie.
(The downhill road is dangerous in rainy weather.)
Les skieurs débutants trouvent les pistes en descente intimidantes.
(Beginner skiers find downhill slopes intimidating.)
Le cycliste a accéléré en descente.
(The cyclist accelerated on the downhill stretch.)
En aval
'En aval' specifically refers to the downstream direction of a river or water flow, or figuratively to later stages in a process.
Le village en aval est menacé par les inondations.
(The downstream village is threatened by floods.)
L'usine de traitement des eaux est située en aval de la ville.
(The water treatment plant is located downstream from the city.)
Dans la chaîne de production, le contrôle qualité se fait en aval.
(In the production chain, quality control is done at a later stage.)
Summary
While both 'en descente' and 'en aval' involve downward direction, 'en descente' is more general and often relates to physical slopes or descents, whereas 'en aval' is specific to downstream movement in water contexts or later stages in processes.