Multicomponent chemical reaction analysis constitutes a powerful tool for understanding the origin and provenance of evaporated surface waters, as well as the mineralogy of evaporite deposits. In this chapter, we apply such modeling to better understand the springs and saline lakes of the Sierra Nevada Mountains in the USA. We then develop quantitative models of the reaction accompanying the evaporation of seawater to the point of nearly complete desiccation, assuming reaction occurs in both closed and open systems.