quality conditions (e.g., turbidity, temperature, dissolved oxygen) that exist in the headwaters can never be realized in the far downstream reaches. Beyond typical longitudinal patterns, there are also large differences among the subbasins within the Mississippi drainage basin. Any comprehensive evaluation of Mississippi River water quality must consider these differences along the river’s length and across the river’s watershed.

This chapter examines issues associated with evaluating Mississippi River water quality. It describes some key features of the river and how its hydrologic and watershed characteristics affect water quality monitoring. The chapter reviews past and existing monitoring programs on the Mississippi River mainstem. It discusses the value of river system monitoring in tracking changes in water quality and the importance of monitoring in achieving Clean Water Act goals. It also discusses challenges of using data and information from monitoring programs to help meet Clean Water Act objectives. Finally, this chapter offers recommendations for enhanced state and federal efforts to improve monitoring efficiency, reduce data gaps, and strengthen implementation of the Clean Water Act.

The mainstem Mississippi River exhibits markedly different hydrology, sediment loads, and other features between its upstream and downstream portions. These upstream-downstream differences are driven in large part by inputs from the Mississippi’s two main tributaries, the Missouri and Ohio Rivers, which enter the Mississippi at St. Louis, Missouri, and Cairo, Illinois, respectively. The Missouri River is the longest tributary of the Mississippi, and its flow is about two-thirds of the upper Mississippi River above St. Louis. It carries a suspended sediment load several times that of the upper Mississippi River (Meade, 1995). The dams constructed on the Missouri River have reduced the Missouri’s total sediment contribution to the Mississippi by more than half since 1953 (Meade and Parker, 1985; Meade et al., 1990). As the Mississippi flows southward, the waters it receives from the Illinois and Missouri Rivers more than double its discharge (Meade, 1995). Downstream, the Ohio River is the Mississippi’s largest tributary with respect to discharge, carrying almost twice the discharge of the upper Mississippi River above St. Louis (Table 2-1). Just as the river’s discharge doubles when it receives the waters of the Missouri, its discharge more than doubles again as it receives the waters of the Ohio River (Meade, 1995).

Downstream of the Mississippi River’s confluence with the Ohio River, the river takes on a very different character than in its upstream reaches. In the Mississippi’s lower reaches, the river becomes much deeper and wider