The Río Mojotoro is a river in the province of Salta, Argentina, formed at the confluence of the Vaqueros River and the Wierna River, flowing through the Siancas Valley in La Caldera Department. It serves as the natural northern border of the municipality of Salta, the provincial capital, and drains a catchment basin of approximately 835 square kilometers (322 square miles). The river is part of the upper basin of the Bermejo River system, and upon crossing into Jujuy Province, it takes on the name Lavayén (Río Lavayén), eventually flowing as a tributary into the San Francisco River. The basin sits on an alluvial plain, giving the river a character shaped by the sediment-rich landscapes typical of northwestern Argentina's subtropical foothills and valleys.