Better for "riverbed" in Latin is "alveus" or "canalis". The "cursus" is the "flow" or "course" or "direction of flow" that could potentially carry off what you throw in. The river's course sculpts and follows the riverbed but the riverbed won't carry off anything.
Nec "alveus" nec "canalis" est "cursus" quod solus cursus auferendi sensum impertit.
I'm writing in Latin hoping for correction, and not because I'm confident in how I express myself. Latinè scribo ut ab omnibus corrigar, non quod confidenter me exprimam.