The flow of a river is the amount of water in the river. The gradient of a river is defined as grade measured in by the ratio of drop in elevation of a stream per unit of horizontal distance (in other words, the "steepness" of a river). A river's gradient is often measured in feet per miles or meters per kilometer.
The flow of the river is dependent upon the position along the stream in terms of the gradient. Near the top of a high gradient, the flow will be less than that near the bottom.
Likewise, the steeper the river's gradient, the faster the flow of water (due to the pull of water down the gradient by gravity). Likewise, lower gradients would result in slower flows of the river.