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Amistad International Reservoir is located at the Texas–Mexico border, and is fed by four main tributaries: the middle Rio Grande (MRG), the Pecos, the Devil’s, and the Rio Conchos from Mexico (Fig. 1). This reservoir is among the largest reservoirs in the western US, and it was built to hold 6.7 billion m3 (5.5 million acre-ft.) of water. The structure was completed in 1968, and the Reservoir was filled near its capacity by 1972 (Fig. 2b). The storage declined to 3.1 billion m3 by 1985, backed up to over 4.0 billion m3 for much of 1986 through 1992, then depleted to as low as 1.5 billion m3 during the last decade, following the drought which started in 1994. Salinity of the Rio Grande at Amistad prior to reservoir construction averaged 560 mg L-1 (Fig. 2a). Starting in 1975, salinity reached 700 mg L-1, and remained at that level through 1983. This was followed by a steep increase in salinity which peaked in 1988, and again in 1996. Salinity of the outflow increased to 945 mg L-1 during 1988, and during February of that year, it reached the federal secondary drinking water standard of 1,000 mg L-1. There is a concern that salinity may exceed the limit with a greater frequency in the future. This problem of salinity increase at Amistad was noted a decade ago (Miyamoto et. al., 1995). In the meantime, a reconnaissance survey was carried out for identifying salt sources entering the Pecos River (Miyamoto et al., 2005). The report indicates that the Pecos River had been salinized largely due to saline water intrusion and through the reduction in streamflow that is needed for diluting the saline water intrusion. The flow of the MRG below El Paso has also declined, and saline irrigation returnflow has deposited large quantities of salts in the reach between El Paso and Presidio. Consequently, bank salinity is extremely high in the MRG below El Paso. The Rio Conchos from Mexico has historically provided the largest inflow into Amistad. According to the data from the US Section, International Boundary and Water Commission (USIBWC), salinity of this flow when it enters the Rio Grande has been steadily increasing in the recent decades. These signs do not bode well for maintaining low salinity at Amistad. This study was conducted to identify the influence of tributaries on salinity fluctuation at Amistad Reservoir. This type of assessment may be useful for developing salinity control and water management strategies. The data shown in Fig. 2 indicate that the first salinity peak appeared during the high storage period under a seemingly normal inflow situation; and this will be the focus of this study. The second peak appeared in 1996 during a low flow and low storage period. In this instance, the increase in salinity is certainly drought-related.