Sentence:  During the trip Selkirk decided that the route through Illinois territory to Indiana and the eastern United States was the best route for goods from England to reach <WSD>Red</WSD> River and that the United States was a better source of supply for many goods than either Canada or England . 
Meaning of the word marked with <WSD> tags:  a tributary of the Mississippi River that flows eastward from Texas along the southern boundary of Oklahoma and through Louisiana