Navigation districts in Texas were first given legislative authorization in 1909, but subsequently many changes were made in the law. Navigation districts provide for the construction and improvement of waterways to aid navigation, and district commissioners have the authority to go on land in the district and acquire rights-of-way for necessary improvements. The normal district is within a single county, but provision is also made for districts embracing parts of two counties. A district can be established by a two-thirds vote of resident property-tax payers (if it is organized under Article III, Section 52); in this case the district is headed by a three-member board with four-year terms for its members. A navigation district can also be established by a simple majority vote (if organized under Article XVI, Section 59), in which case it is headed by three commissioners, each serving a six-year term. A district board is appointed by the county commissioners court. The amount of indebtedness allowed depends upon the provision under which the district was organized. Any district may levy a maintenance tax of up to ten cents per $100 valuation. Self-liquidating navigation districts with bonds that are voted on but never issued may become self-supporting through the implementation of tolls, rents, fees, and other charges to pay for construction costs. There is no provision for any form of state administrative control for any of the districts other than the examination and registration of bonds by the attorney general and comptroller. The county assessor-collector and treasurer also serve the district. Some navigation districts, such as the Harris County Navigation District, have more extensive governmental organization. These districts serve populations of at least 100,000 and their administrators oversee the operation and development of deepwater ports. Such districts are headed by a board of five commissioners serving five-year terms. These districts may own and operate docks, warehouses, wharves, grain elevators, and other improvements necessary to deepwater shipping. In 1992 twenty-four navigation districts were registered with the Texas Water Commission. See also WATER AGENCIES AND PROGRAMS.