The Red River of the North drains much of eastern North Dakota and northwestern Minnesota and flows north into Manitoba, Canada, ultimately into Lake Winnipeg; therefore, water quality is an International concern. With increased runoff in the past few decades, phosphorus flux (the amount of phosphorus transported by the river) has increased. This is a concern, especially with respect to Lake Winnipeg, an important inland fishery and recreational destination. There is pressure at the State and International levels to reduce phosphorus flux, an expensive proposition. Depending on the method (controlling sources, settling ponds, buffer strips), control of phosphorus flux is not always effective during spring runoff. This work represents a first step in developing a causal model for phosphorus flux by examining available data and changes in concentration over time. Total phosphorus concentration data for the Red River at Emerson, Manitoba, and at Fargo, North Dakota-Moorhead, Minnesota, were summarized and then analyzed using WRTDS (Weighted Regressions on Time, Discharge, and Season) to describe total phosphorus changes over time in two analysis periods: 1970-1993 and 1993-2012. Total phosphorus concentration increased in the first period at Emerson, Manitoba, indicating phosphorus was likely being transported to streams during runoff events. A very different pattern occurred at Fargo-Moorhead with declines in concentration, except at high discharge. While concentration continually changes, during the second period it decreased during spring runoff at Emerson and Fargo-Moorhead and during the growing season at Fargo-Moorhead, perhaps because of improved agricultural practices and declines in some uses of phosphorus.