The slope of a stream channel expressed as the vertical drop of a stream over a specified distance is gradient. Portions of the lower Mississippi River have very low gradients of 10 centimeters per kilometer or less. By contrast, some mountain stream channels decrease in elevation at a rate of more than 40 meters per kilometer-a gradient 400 times steeper than the lower Mississippi(Figure 57). Gradient varies not only among different streams but also over a particular stream's length. The steeper the gradient, the more energy available for stream flow. If two streams were identical in every respect except gradient the stream with the higher gradient would obviously have the greater velocity.