The Monday Creek river located in Southeastern Ohio (Athens, Hocking, and Perry Counties) is deeply affected by acid mine drainage generated by multiple high-sulfur coal sources within its 300 km2 watershed. The Monday Creek Watershed Restoration Group has been monitoring the water composition, pH, conductivity, temperature, and water discharge at several points since 1997. In addition, some of the main sources of acidity in this watershed have been investigated previously (Pigati and López, 1999; López et al., 1999, López and Stoertz, 2001). The data show that concentrations and loadings are higher at the sources during the high flows of late winter and spring. In comparison, along the river, concentrations are lower and loadings are higher during high flows due to dilution effects by runoff. For reactive elements like iron and aluminum, these variations in concentrations and flow produce transient variations in saturation indexes of minerals like ferrihydrite, and hydroxilated aluminum sulfates and hydroxides along the flow path. Regions of the river that are supersaturated with respect to a given mineral at low flow can be undersaturated at high flow conditions. During high flow, increased transport of sediments and dissolved contaminant loadings, as well as dissolution processes should enhance the transport of contaminants downstream.