INTRODUCTION This study describes the hydrology and hydroecology of Arroyo Alamar, in Tijuana. Arroyo Alamar is being considered for development by the Instituto Municipal de Planeación (IMPlan). The project entails the river channel improvement from the intersection with Tecate toll road to its confluence with the Tijuana River. The length of the channel improvement project is 10 km. The watershed of Arroyo Alamar is part of the Tijuana river basin. The Tijuana river basin encompasses the County of San Diego and the municipalities of Tijuana, Rosarito and Tecate, in Baja California. The hydrology study comprises the analysis of Cottonwood Creek and its major tributaries, including the hydrologic effect of Morena and Barrett dams, on the U.S. side. Cottonwood Creek takes the name of Tijuana River at its confluence with Tecate Creek, inside California, and later changes its name to Arroyo Alamar as it enters Mexico.