The powers granted by this act to any board of park commissioners shall not be construed to have been exhausted by any one use of the same, but said commissioners may, from time to time, proceed with further enlargements or extensions: Provided, however, That all such enlargements or extensions lie within the district or territory, the property in which shall be taxable for the maintenance of the park systems under the control of such board of park commissioners, or within public waters or rivers adjoining or being a part of such district or territory.
70 ILCS 1230/6