AN ANALYSIS OF THE SUSPENDED SEDIMENT RATING CURVE PARAMETERS IN THE UPPER MISSISSIPPI RIVER BASIN AT THE MONTHLY AND ANNUAL LEVELS

Suspended sediment rating curve parameters were analyzed to investigate the relationship of suspended load and discharge in the Upper Mississippi River Basin (UMRB) at the annual and monthly levels. The rating curve parameters were obtained from the power function of load and discharge: Load = a × (Discharge)^b [(Discharge)^b]. The function was solved by ordinary least squares regression on its logarithmic form.

The annual rating coefficient a and exponent b ranged from 0 to 0.25 (kg/s)(s/m³)^b [(kg/s)(s/m^3)^b] and from 0.91 to 4.27, respectively. The monthly rating coefficient a and exponent b ranged from 0 to 0.239 (kg/s)(s/m³)^b and from 0.09 to 3.72, respectively. The intercept ln(a) and slope b of the logarithmic graph of suspended load and discharge were negatively correlated. This correlation was stronger for rivers categorized as having high discharge (> 218 m³/s [m^3/s]). This study also showed negative correlations between the rating coefficient a and stream discharge at annual and monthly levels, indicating that in large rivers, the rating curve tends to have a smaller intercept and larger slope. Smaller values of a and b in winter compared to other seasons suggested a low supply of sediment into streams due to frozen ground and the inactive state of streams in transporting sediment during winter months. The dominant shape of annual sediment rating curves in the region was convex, suggesting a transport-limited system for sediment transport in the basin. The transport-limited system indicates the potential of a flow to entrain additional sediment (possibly of larger grain sizes) during high discharge due to its higher competence. The apparent contradiction between the transport-limited condition and the findings of Meade and Moody (2010) is attributed to different approaches to the issue (trend of mean suspended load over time versus sediment rating curve). The results of this thesis also suggested that the UMRB has remained transport-limited after the flood in 1993, although this merits further investigation.