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spec: Natural banks downstream from engineered banks often erode because water deflected off the hard , engineered surfaces hits softer , natural surfaces with more force.Stream Restoration vs. EnhancementStream restoration means returning an ecosystem to a close approximation of its condition prior to disturbance . The Phobos and Deimos images were obtained by the Viking Orbiter spacecraft in 1977 ; the Gaspra image is the best of a series obtained by the Galileo spacecraft on October 29 , 1991 .