At the start of the video there is (an almost) top down view and the bottom of the stream is visible.
The light transmits through the water and the plants in the water reflect back clearly.
The camera is then moved to a look further into the distance, the stream is then at a shallow angle, and nothing beneath the service of the water is visible. This is due to the Fresnel effect and the angle of incidence.
There is not much if any specular reflection (perhaps a shadow of the building to start with - but this is still distorted) in the water, and as the camera moves to look further down the stream the reflection becomes diffused.
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