This 1946 entry in the "Movietone Adventures" series of shorts was in Technicolor when originally released. Narrated by Lowell Thomas, it is a trip from Medicine Hat in southern Utah down the rapids of the San Juan River ending with a view of the Rainbow Bridge, hence the title and the use of Tehnicolor. It was nominated for an Oscar for Best Short Subject, One-Reel.
Streaming availability for US region, via JustWatch.

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