The Han River is the main water resource for the Seoul metropolitan area (Korea) with twenty million people relying on it, and its eutrophication is of great concern for preserving drinking water quality. In this study, long-term trends in biological oxygen demand (BOD), chemical oxygen demand (COD), total phosphorus (TP), total nitrogen (TN), and suspended solids at a downstream site of the river (St. Gui) are presented from 1989 to 2006 using data from the Ministry of the Environment. Longitudinal distributions in TP, TN, and chlorophyll-a concentration were measured in the downstream reaches between the Paldang Dam and Haengju Bridge. The long-term average BOD was $1.82{pm}0.67;mg/L$ and showed a decreasing trend, whereas COD did not vary consistently with a long-term average of $3.46{pm}0.87;mg/L$, and consequently, the BOD/COD ratio decreased. This pattern can be interpreted as an increasing trend in the nonbiodegradable organic matter/biodegradable organic matter ratio, which can be attributed to enhanced sewage treatment. The long-term record for concentration did not show a consistent delate trend, whereas the seasonal variation was remarkably large with high concentrations during the flood season. In contrast, phytoplankton density was higher during low-flow seasons. It seemed that hydraulic residence time was the major factor controlling phytoplankton, as is typical in a lotic environment, which overwhelmed the effects of other factors such as temperature, nutrients, and solar radiation. In conclusion, BOD has decreased in the lower Han River system, but nonbiodegradable organic matter and phosphorus concentrations have not decreased.