Results are presented of computer simulations of light ion channelling in crystals which indicate four intervals of initial row impact parameters governing the dechannelling behaviour. The first (smallest impact parameters) causes 90% to 100% dechannelling in the collision with the first row, the second causes dechannelling in collisions with subsequent rows mainly due to the periodicity of the row potential, the third similarly due to subsequent collisions mainly due to thermal vibrations, and the fourth interval, associated with largest impact parameters, is the domain of electron multiple scattering.