The canal or waterway shall be and remain under the control of the city and immediately upon its completion the city shall establish outer dock lines lengthwise of said canal or waterway on both sides thereof in such manner and position that not less than two hundred feet of the width thereof shall always remain open between such lines and beyond and between which lines no right shall ever be granted to build wharves or other obstructions except bridges; nor shall any permanent obstruction to the free use of the channel so laid out between said wharf or dock lines excepting bridges, their approaches, piers, abutments and spans, ever be permitted but the same shall be kept open for navigation.