The stacks are some of the most popular characteristics of the steamboats. They rise high above the boat, sometimes 30 feet or more.
Most of the boats have two stacks, but some have only one. Often the stacks can be lowered to pass low-clearence-bridges. But at some bridges even this does not work, because the clearence is to low at all, e.g. at the Arkansas River, where only smaller boats like the Delta Queen, can go through.
In the old times, when the boats still ran with coal or wood, the smoking stacks could already be seen from far distances, so the people waiting at the next landing knew very early, when the steamboat will come. But of course this was not the reason, why the stacks were rising that high in the sky: Steamboats, that did the transportation of cotton had to be very careful to prevent the cotton (and the whole boat) from burning down, so the sparkling of the fire had to be kept as far away as possible from the cotton.