The board shall have power and authority to straighten, widen, deepen and improve any and all rivers, watercourses or streams, whether navigable or otherwise, flowing through or located within the boundaries of such district; and the beds of any streams or rivers which may be changed, shall become the property of the district, and the board shall have the power to sell and dispose of the same, or exchange the same or any portion thereof for other lands.
RCW 85.24.285