Water allows seeks its lowest point and flows downhill. The gradient is actually the slope, and is calculated as the drop of the stream, per unit distance. It can be the change in height from one point to the next divided by distance in feet or miles. A steep gradient means that because of the force of gravity, the water will be flowing rapidly. This type of stream carves a V-shape out of the landscape below and can transport clay, sand, silt, pebbles and even gravel due to the fast velocity. A low gradient stream has water that flows slowly and sluggishly and is only capable of transporting fine sediments a short distance.
A stream's gradient is the amount of elevation drop over a given lateral distance, usually feet of drop per mile(s), or meter(s) per kilometer(s). A steep gradient would be closer to a 1:1 ratio of elevation to distance, whereas a lesser gradient would be .5:1 ratio, or less.