Final answer:
Friction decreases the speed of an avalanche as it opposes motion between the avalanche and the slope.
Step-by-step explanation:
The effect of friction on the rate of movement of an avalanche is that it decreases the speed of the avalanche. Friction is an external force that always opposes motion between surfaces, and therefore, an increase in friction will result in a decrease in speed. In the case of an avalanche, which involves massive amounts of ice and snow sliding down a slope, the friction between the moving material and the ground, as well as internal friction within the snow itself, serves to slow the avalanche down. This is analogous to an object which, when sliding down a rough horizontal surface, grinds to a halt due to friction. Reducing friction would allow the avalanche to travel faster.