40 C.F.R. § 81.274
The Mountain Counties Intrastate Air Quality Control Region consists of the territorial area encompassed by the boundaries of the following jurisdictions or described area (including the territorial area of all municipalities (as defined in section 302(f) of the Clean Air Act, 42 U.S.C. 1857h(f) ) geographically located within the outermost boundaries of the area so delimited):
In the State of California: Amador County, Calaveras County, Mariposa County, Nevada Country, Plumas County, Sierra County, Tuolumne County.
El Dorado County-all of El Dorado County except that portion within the drainage area naturally tributary to Lake Tahoe including said Lake.
Placer County-all of Placer County except the following described area:That portion of Placer County within the drainage area naturally tributary to Lake Tahoe including said Lake, plus that area in the vicinity of the head of the Truckee River described as follows: commencing at the point common to the aforementioned drainage area crestline and the line common to Townships 15 North and 16 North, Mount Diablo Base and meridian (M.D.B. & M.), and following that line in a westerly direction to the northwest corner of Section 3, Township 15 North, Range 16 East, M.D.B. & M., thence south along the west line of Sections 3 and 10, Township 15 North, Range 16 East, M.D.B. & M., to the intersection with the said drainage area crestline, thence following the said drainage area boundary in a southeasterly, then northeasterly direction to and along the Lake Tahoe Dam, thence following the said drainage area crestline in a northeasterly, then northwesterly direction to the point of beginning.
40 C.F.R. §81.274