Reasons may include: a) free current; b) narrow fairway; c) sand banks; d) variable hydraulicity; e) strong streams; f) modifications of the river bed; g) cliffs; h) fords; i) other.
Par exemple: a) courant libre; b) chenal étroit; c) bancs de sable; d) hydraulicité variable; e) courants forts; f) modification du lit du fleuve; g) rochers; h) hauts-fonds; i) autre.