Sentence:  Thomas Douglas , fifth Earl of Selkirk , a noble humanitarian Scot concerned with the plight of the crofters of his native Highlands , conceived a plan to settle them in the valley of the <WSD>Red</WSD> River of the North . 
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