ABSTRACTThe effect of hydrologic and chemical processes on salinization of stored waters was determined for two small floodwater‐retarding structures located in western Oklahoma. One structure, already designed to accommodate a large influx of sediment, was further overdesigned hydrologically by upstream diversion of approximately one‐half the inflow. Over a 2‐year period, the total salinity of stored waters increased approximately 22 times and the stored water volume decreased to 1/33 its initial volume in the overdesigned structure, while both volume and salinity of stored waters remained comparatively stable in the other structure. The lack of sufficient dilution by better quality surface runoff and the increased residence time of water in the impoundment apparently caused most of the salinity increase. The bulk of the salt load entering the over‐designed structure, to be concentrated later by evaporation, was associated with base rather than storm inflow. After base inflow ceased, substantial losses of salt load and stored water occurred concurrently. The loss was not adequately explained by chemical precipitation in association with evaporation. Seepage and evaporation‐associated variables appeared to account for much of the hydrologically unexplained loss of stor