We examined the influence that wetland brown water had on the toxicity of copper to aquatic species in Swamp Creek basin, Wisconsin. Copper was added to Swamp Creek water and to laboratory-prepared reconstituted water and tested for toxicity using daphnids (Ceriodaphnia dubia and Ceriodaphnia reticulata), larval fathead minnows (Pimephales promelas), and larval walleye (Stizostedion vitreum). The toxicities of copper in Swamp Creek water, which contained dissolved organic carbon (DOC), were significantly lower than in water with no DOC. Therefore in successive experiments, using cultured C. dubia as the test organism, we experimentally reduced DOC in Swamp Creek water. We established that increased concentrations of DOC and hardness increased the odds of survival of C. dubia in the presence of potentially lethal concentrations of copper. In the Swamp Creek watershed which contains brown water and in watersheds where DOC concentrations are substantial, DOC, and hardness could be the principal factors in developing site-specific criteria involving copper and perhaps other metals.