Selenium is a pervasive problem in wetlands of the Salton Sea, California, where it can bioaccumulate in food webs and influence development and hatching success in oviparous animals such as the endangered Yuma Ridgway’s rail (Rallus obsoletus yumanensis). We compared water chemistry, selenium partitioning and concentrations in biota between wetlands managed with Colorado river water and unmanaged wetlands fed by irrigation drain water during spring 2023 and 2024 to better understand factors driving selenium hazards for nesting rails.  In addition to drain and river sources, chemical signatures showed that each wetland had unique contributions of groundwater, geothermal, and Salton Sea water.  We found selenium partitioning and concentrations in biota followed a spatial gradient from water inlet to outlet that was independent of water source or wetland type and aligned with patterns of dissolved selenium in water, suggesting that wetland processes decreased selenium bioavailability as water flowed toward the outlet.  These results coupled with those from on-going rail diet studies will be used to populate predictive selenium bioaccumulation models to clarify the role of wetland processes and food web complexity as drivers of selenium hazards for Salton Sea rails.