River salinization is a widely recognized by-product of water resource development in arid regions.  However, salinization in the Lower Pecos River is often attributed to natural upwelling of high-salinity groundwater.  Consequently, ecological effects of salinization are commonly overlooked.  Pre-development salinity of the Lower Pecos River was not recorded, so I review a suite of hydrological changes that affected streamflow salinity.  Four factors contributed to salinization:  (1) reduced flood frequency and magnitude, (2) diminished base-flow magnitude, (3) increased evapotranspiration, and (4) increased prevalence of saline groundwater discharge.  Salinization was a cumulative effect of water resource development and was highest where cumulative impacts were greatest.  Higher, fresher base flow historically diluted saline groundwater inflows.  Perennial flows and periodic floods exported salts, maintaining a salt balance.  Salinity was low enough to support more than 30 native fishes.  Salinization was associated with changes to the fish fauna.  Fewest native fishes (six) persist in the most saline reach and fewest nonnative fishes (four) have become established.  Salinization has affected fish faunas throughout western North America.  It is an indicator of severe ecological degradation and can be expected wherever rivers of arid regions are developed extensively for agriculture.