New Mexico Statutes Chapter 4. Counties § 4-24-1. Original county boundaries

That all of that portion of Rio Arriba county, New Mexico, comprised within the following boundaries, as hereinafter described, shall form and constitute a new county, to be hereafter known as the county of San Juan, to wit: commencing at the state line of Colorado, running along the San Juan river to where the San Juan crosses range line between ranges seven and eight, [west; thence south on said range line] to the north line of Bernalillo [McKinley] county; thence west to the line of Arizona; thence running north on the Arizona line to the state of Colorado; thence east to the place of beginning; also to include all the settlements on the San Juan river below the mouth of the Los Pinos river.