The canals of Amsterdam were as brightly gay last week as a field of Dutch tulips. The occasion was the opening of the new Amsterdam-Rhine Canal, a 45-mile short cut across The Netherlands that will bring Amsterdam's river traffic 25 miles and 20 hours closer to Germany. In the age-old competition for the rich river traffic, the sister port of Rotterdam, sitting near the North Sea, has always had the advantage. Now, with the opening of the largest inland navigation lock in Europe, and the completion of the canal which was first planned back in 1915, Amsterdam hopes to double its 23 million tons of shipping in the river. Despite the two-city rivalry, all The Netherlands celebrated, and Queen Juliana herself was at the helm of the first ship to pass through the canalthe royal yacht.