Stream depth - shallow and deep
A deep stream is basically a narrow extension of the pond itself. The prime reason for adding a deep stream is to create an area for the fish to swim up. Deep streams do not provide the filtration benefits of shallow streams. Also, the water flowing through the gravel in streams only two or three inches deep adds sound as well as interesting ripples.
In addition, we have added large, deep, still areas in streams for interest and the effect. A stream that can change direction and go from rapid moving water to deep, still waters is much more interesting than a long stream with the same flow from one end to the other.
Ideally, when a stream is long, we combine a deep area of the
stream where the water enters the pond and a shallow section closer
to the waterfalls. This allows the fish to swim partially up the
flowing stream (an activity that Koi enjoy, like their salmon
cousins).