SEASONAL VARIATION IN THE STAGE-DISCHARGE RELATIONSHIP OF THE MIDDLE MISSISSIPPI RIVER

An analysis was undertaken to quantify the seasonal variation in the stage-discharge relationship along the Middle Mississippi River (MMR). Three rated and twelve unrated gaging stations along the MMR between Saint Louis, MO and Thebes, IL where analyzed for seasonal variation. This analysis utilized daily stage and discharge data (1942 - 2002) to construct a rating curve for each month. A second-order polynomial regression equation was developed for each month's rating curve. The rating curves for the unrated stations were developed by extrapolating discharge from the rated station based on cumulative drainage basin area. For each month, the regression equation was used to calculate the stage for the smallest monthly maximum discharge for period of record analyzed.

The maximum seasonal variation in stage for the tested discharge condition (generally just above bankfull) ranged from 0.3 to 1.1 m. The gaging stations located along the alluvial section of the MMR generally showed the greatest seasonal variation. The maximum and minimum stages for these stations generally occurred during the summer and winter to early spring, respectively. In contrast, the bedrock-floored section showed no or an inverse trend.

Current river gaging and flood forecasting methods utilize averaged annual rating curves, in which stage-discharge relationship is assumed invariant throughout the year. The results of this investigation suggest this is not a valid assumption. The observed systematic variations in the stage-discharge relationship have been most commonly attributed to one of two mechanisms: (1) intra-annual variations in vegetation along channel banks and on the floodplain, and (2) variations over time in bedforms and bedform-induced roughness. The results suggest one or both of the mechanisms may be resulting in the seasonal variation along the MMR.