A district may be organized pursuant to this part. A district may include any contiguous territory in one or more counties, contiguous to a navigable river or to any channel connected with a navigable river, but a municipal corporation shall not be divided in the formation of a district, and territory situated in a county in which there is located a duly organized and existing port district, created pursuant to Part IV of this division, may not be included. Territory situated within the boundaries of any port or harbor district, organized or designated and existing under the provisions of any city charter, shall not be included. Any territory, annexed to a municipal corporation, which is in a district, becomes a part of the district.