Variability in the discharge of the Mississippi River and tributaries from 1817 to 2020

There are conflicting predictions of climate change effects and landuse on the discharge of the Mississippi River-the largest river in North America. Are discharges becoming higher or lower, and if they did change, then when? To address these uncertainties I compiled a two-hundred-year long dataset of the annual average, minimum, and maximum discharges at five stations draining the Mississippi River watershed: at Clinton, IA, Herman, MO, St. Louis, MO, Louisville, KY, and Vicksburg, MS. A spline/Lowess analysis tested for trends and inflection points. All three discharge metrics increased, and the minimum annual discharge increased faster than either the annual maximum discharge or annual average discharge. A regression analysis of variations in average discharges from 1950 to 2020 at these five locations demonstrates correlations to the air pressure differentials represented in the North Atlantic Oscillation (NAO) Index for January, February and March. The longest data set, for the Mississippi River at Vicksburg, demonstrates a similar direct relationship with the NAO Index from 1826 to 1969. After 1969, however, the relationship between discharge and the NAO Index is insignificant even though the range of Index values overlap for the two intervals. A breakpoint and rise in discharge ca. 1970 is consistent with well-documented land cover and land use changes occurring then that resulted in reduced evapotranspiration as homogenous cropping systems were established, and a higher percent of precipitation was routed into groundwater and baseflow. The Bonnet Carre Spillway at New Orleans, LA, is being opened more frequently to reduce flood threats as the river's stage increasingly reaches the threshold for opening it. Significant water quality impairments in the coastal zone will appear or be sustained with these openings. These data may be useful for climate change assessments through modeling or synthetic assessments in combination with other data sets.

Automated summary

Conflicting predictions of climate change effects and land use on the discharge of the Mississippi River are discussed. The study compiled a two-hundred-year dataset to analyze the trends in discharge. The results show that all three discharge metrics increased, with the minimum annual discharge increasing faster. The study also found correlations between average discharges and air pressure differentials represented in the North Atlantic Oscillation (NAO) Index. However, after 1969, the relationship between discharge and the NAO Index became insignificant. The study highlights the importance of land cover and land use changes in influencing discharge and discusses the potential implications for water quality in the coastal zone.