Cal. Health and Safety Code - HSC § 6830. Section 6830.
There may be annexed to a district any of the following territory which is in the same county as the district or which is within another county but in the natural watershed area of the district:
(a)
Any territory contiguous to the district.
(b)
Any territory any point of which touches any point of the district.
(c)
Any territory separated from the district by a “separating barrier,” which term includes a street, road, highway, railway line, railway crossing, railway
right-of-way, watercourse, lagoon, or other natural barrier.
(d)
Any territory not contiguous to the district that will, in the opinion of the district board, be benefited by inclusion in the district.
Any territory specified in this section may consist of one or more separate parcels of land, but it is not necessary that all parcels shall constitute in the aggregate one tract of land.