In geologic terms, the stream will erode down through its bed to achieve the base level of erosion throughout its course.

Gradient The gradient of a stream is a critical factor in determining its character and is entirely determined by its base level of erosion.

Such rivers form when the base level of erosion is rapidly lowered, so that the river begins downcutting into its channel faster than it can change course (which rivers normally do on a constant basis).

The base level of erosion is the point at which the stream either enters the ocean, a lake or pond, or enters a stretch in which it has a much lower gradient, and may be specifically applied to any particular stretch of a stream.