The directors elected as provided in the next preceding section hereof shall meet on the second Tuesday of the first calendar month following their election, at the county seat of the county most centrally located in the district, and proceed to organize by electing from their own number a president, secretary and treasurer, and shall enter upon the duties of their offices. As soon as practicable thereafter they shall define the boundaries of the drainage district, and shall file the portion thereof relating to each county in which said district is located with the county clerk of such county. Said boundary line shall include all lands subject to overflow by the highest flood water.