Whenever the county legislative authority of a county with a population of one hundred twenty-five thousand or more deems it for the interest of the county to construct or to aid the United States in constructing a canal to connect any bodies of water within the county, such county may construct such canal or aid the United States in constructing it and incur indebtedness for such purpose to an amount not exceeding five hundred thousand dollars and issue its negotiable bonds therefor in the manner and form provided in RCW
36.67.010. Such construction or aid in construction is a county purpose.