Problems of Shapour River Salinity Rising Over Recent Prolonged Streamflow Reduction Period and Solutions of River Salinity Management: An Originally Freshwater River Intensively Salinized by Natural Salinity Sources

Abstract The Shapour river with catchment area of 4254 km2 is a major river system in southern Iran. While the upstream river flow (the upper Shapour river) is fresh, it becomes extremely salinized at the downstream confluence of Shekastian salty tributary and the entering nearby Boushigan brine spring. The river then passes through the Khesht plain and finally discharges into the Raeisali-Delvari storage dam, which went into operation in 2009. Over the 2006–2019 period, reduced precipitation and over-utilization of freshwater resources resulted in ~ 72% streamflow reduction in the Shapour river. Consequently, the ratios of unused salty/brine water of Shekastian tributary and Boushigan spring to fresh-outflow of the upper Shapour river increased by ~ 3 times and river salinity fluctuation domain at the Khesht plain inlet dramatically increased from 2.1-4.0 dS m− 1 to 3.7–26.0 dS m− 1. It also resulted in disappearance of most river aquatic species and caused major economic damages in the middle Shapour river. On the seasonal time-scale, consecutive processes of salt accumulation during irrigation season of the Khesht plain date orchards and then salt drainage during rainy season have adjusted salinity fluctuation domain from 3.7–26.0 dS m− 1 at the plain inlet to 5.2–8.9 dS m− 1 at the plain outlet. In the lower Shapour river, storage/mixing of fresh/salty inflow waters in the Raeisali-Delvari reservoir has adjusted strong salinity fluctuation domain from 0.9–10.7 dS m− 1 at the reservoir inlet to 3.6–5.5 dS m− 1 at the reservoir outlet. Success of the Raeisali-Delvari reservoir for salinity adjustment is due to its suitable location on the Shapour river, by being situated on downstream of all of the main river tributaries with natural saline/fresh sources of water. Therefore, construction of storage dam on proper site in conjunction with controlled freshwater utilization are viewed as effective measures for salinity management of subjected rivers to natural salinity sources.