Idaho Code § 31-144
Twin Falls county is described as follows: beginning with the intersection of the middle of the channel of the Snake river with the north and south center line of section twenty-eight (28), township ten (10) south, range twenty-one (21) east;
Eastern boundary. Thence south on said center line of section twenty-eight (28), to the point of intersection of the north line of the right of way of the Minidoka & Southwestern Railroad Company, which point is one hundred (100) feet distant, at right angles, from the center of the main track of the line of road of said railroad company as the same is now located; thence in a southwesterly direction along the north line of said railroad right of way, to a point where said line intersects the south line of the canal right of way of the Twin Falls Land & Water Company, which point of intersection is one hundred (100) feet distant, at right angles, from the center line of the main canal of said Twin Falls Land & Water company; thence south to the south line of section thirty-six (36), township ten (10) south, range twenty (20) east; thence west to the southwest corner of section thirty-six (36); thence south on the section line to the south line of township eleven (11); thence west to the southeast corner of township eleven (11) south, range eighteen (18) east; thence south on the range line to the south line of the state of Idaho;
Southern boundary. Thence west along the south line of the state of Idaho to the thirty-eighth (38th) meridian of longitude west from Washington;
Western boundary. Thence north along said meridian to the intersection of the center of the main channel of Snake river;
Northern boundary. Thence up the center of the main channel of Snake river to the point of beginning.
County seat-Twin Falls.
Idaho Code § 31-144