Sentence:  Since the hazards of poor communication are so great , p can be justified as a habitable site only on the basis of unusual productivity such as <WSD>is</WSD> made available by a waterfall for milling purposes , a mine , or a sugar maple camp . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)