* Erosion: Bedrock channels form when a stream's erosive power is strong enough to cut through solid rock. This typically occurs in the upper reaches of a stream where:

* Steeper gradients: The stream has a steeper slope, giving it more energy to erode.

* Higher velocity: The water flows faster due to the steeper slope, increasing its erosive power.

* Less sediment: There is usually less sediment being transported in the upper reaches, allowing the stream to focus its energy on eroding the bedrock.

* Downstream changes: As a stream flows downstream, it typically:

* Gradualizes: The gradient becomes less steep.

* Slows down: Velocity decreases.

* Carries more sediment: The stream picks up sediment from upstream, which can cushion the flow and reduce its ability to erode bedrock.

Exceptions:

While bedrock channels are most common near the headwaters, there are exceptions:

* Resistant bedrock: If a stream encounters a particularly resistant bedrock layer further downstream, it may carve a channel through it even with a lower gradient.

* Tectonic uplift: Areas experiencing tectonic uplift can have rejuvenated streams with increased gradients even downstream, potentially forming bedrock channels.

In summary: Bedrock channels are primarily a feature of the upper reaches of streams where the erosive power is greatest. However, specific geological conditions can lead to their formation further downstream.