A 1905 treaty between the United States and Mexico called for the elimination of bancos. A banco is a curve in a river channel, a bend oftentimes resembling a horseshoe or oxbow. Some are so large as to be unnoticeable except from the air or by surveys. Bancos usually form where the ground is level, the drop or grade is slight, and the river is sluggish. During floods, since the water encounters resistance in these curves, the channels frequently shift. Where the Rio Grande is the boundary between the United States and Mexico the shifts have had international repercussions. Boundary commissioners described the Rio Grande border as having three divisions. From El Paso to Rio Grande City, the fall was steep and the banks usually solid. From Rio Grande City to the Gulf of Mexico, a straight distance of 108 miles, the river meandered through 241 miles of curves (bancos) to reach the coast. The soil was alluvial, the fall measured in inches. River channels frequently twisted back upon themselves, creating cut-offs and confusions about where the border was. By 1970 American and Mexican boundary commissions had sliced through 241 bancos by straightening the river. Those bancos that protruded into Mexico went to Mexico, the others to Texas. More than 30,000 acres of land changed hands, most of it in the lower Rio Grande valley. The United States got 18,505 acres, and Mexico received 11,662.