70 ILCS 1205/9.3-4
Whenever revenue bonds are issued under Section 9.3-1 of this Article, all revenue derived from the operation of the recreational facility shall be deposited in a separate fund specifically designated by reference to the type of facility of the park district. This fund shall be used only in paying the cost of operation and maintenance of the specific facility, in providing an annual depreciation fund, and in paying the principal of and interest upon the revenue bonds of the district issued under Section 9.3-1 of this Article.
A depreciation fund is a fund for such replacements as may be necessary from time to time for the continued effective and efficient operation of the facility. Such fund shall not be allowed to accumulate beyond a reasonable amount necessary for the purpose and shall not be used for extensions to the facility.
70 ILCS 1205/9.3-4