Kelso was Henry Kelsey who joined the service of the Hudson’s Bay Company at age 17 in 1688 and rose to become a governor of the Company. At the time, and for centuries, the Company set itself up in forts on Hudson’s Bay and let the Canadian First Nations bring the furs down to the Bay for trade. Very occasionally, a Bay man would explore inland. In 1690 young Henry Kelsey joined a group of First Nations travelling into what must have been the Canadian heart of darkness. His journals were preserved in the Company archives and rediscovered in the 20th century. He is believed to have travelled southwest from the Bay to the Grand Rapids of the Saskatchewan River, near the modern town of The Pas, and then west and south onto the prairie. He is believed to have been the first European Canadian to reach the prairie from the Bay.
Stan Rogers discussed the process of writing Northwest Passage in a radio interview in 1982 and admitted that he had been unsure of Kelsey’s name and had guessed Kelso while recording the song. He never said if he believed that Kelsey himself had described the prairie as a “sea of flowers” or what brought that image to his mind – since he would himself have only seen the farmlands that the prairies have become.
Kelsey kept a journal, and his only descriptive references to the prairie are as a bleak heath of short round grasses. This indicates that he saw the short sere grasses of the high plains, rather than the tall grass prairies of more fertile regions. It is also not untypical of 17th century aesthetic sensibilities toward nature. It was only in the late 18th and the 19th centuries, through the Romantic movement, that Europeans and European Americans began to see nature as beautiful in itself.
The image of the prairie as a sea or ocean of grass and flowers was employed by the American poet William Cullen Bryant to describe the edge of the plains in southwest Illinois in the early 19th century, and adopted by many later poets and writers, including the 19th century Canadian poet and essayist Charles Mair. The engineer and inventor Sanford Fleming described the prairies that way on arriving at the edge of Red River Valley near the modern town of Ste. Anne, along the Dawson Road from Lake of the Woods in 1870. Fleming and Mair were in the last generation to see the prairies that way, before the slaughter of the last great herds of bison and the breaking of the prairie to agriculture.
In reaching for the beautiful and true meaning of exploration, Rogers transcended geographical and historical accuracy to take us off the asphalt road and into the sea of flowers.

From: http://sea-of-flowers.ca/wp/?p=6