Whenever a county of this state, acting pursuant to RCW 86.12.010 through 86.12.033, shall make an improvement in connection with the course, channel or flow of a navigable river, thereby causing it to abandon its existing channel, bed, bank or banks for the entire distance covered by said improvement, or for any part or portion thereof, or by said improvement shall prevent a river from resuming at a future time an ancient or abandoned channel or bed, or shall construct improvements intended so to do, all the right, title and interest of the state of Washington in and to said abandoned channel or channels, bed or beds, bank or banks, up to and including the line of ordinary high water, shall be and the same is hereby given, granted and conveyed to the county making such improvement: PROVIDED, HOWEVER, That any such gift, grant or conveyance shall be subject to any right, easement or interest heretofore given, granted or conveyed to any agency of the state.