40 C.F.R. § 81.121
The Four Corners Interstate Air Quality Control Region (Colorado-New Mexico-Utah) has been revised to consist 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 Colorado: Archuleta County, Dolores County, La Plata County, Montezuma County, San Juan County.
In the State of New Mexico: San Juan County (in its entirety); Rio Arriba County (that portion lying west (Pacific slope) of the Continental Divide, and all portions of the Jicarilla Apache Indian Reservation lying east (Atlantic slope) of the Continental Divide); Sandoval County (that portion lying west (Pacific slope) of the Continental Divide, and all portions of the Jicarilla Apache Indian Reservation lying east (Atlantic slope) of the Continental Divide); McKinley County (that portion lying west (Pacific slope) of the Continental Divide); Valencia County (that portion lying within the Zuni and Ramah Navajo Indian Reservations).
In the State of Utah: Emery County, Garfield County, Grand County, Iron County, Kane County, San Juan County, Washington County, Wayne County.
40 C.F.R. §81.121