Abstract

An examination of data from two continuous stage and discharge streamgages and one continuous stage-only gage on the Middle Mississippi River was made to determine stage-discharge relation changes through time and to investigate cause-and-effect mechanisms through evaluation of hydraulic geometry, channel elevation and water-surface elevation data. Data from discrete, direct measurements at the streamgages at St. Louis, Missouri, and Chester, Illinois, during the period of operation by the U.S. Geological Survey from 1933 to 2008 were examined for changes with time. Daily stage values from the streamgages at St. Louis (1861–2008) and Chester (1891–2008) and the stage-only gage at Cape Girardeau, Missouri (1896–2008), throughout the historic period of record also were examined for changes with time. Stage and discharge from measurements and stage-discharge relations at the streamgages at St. Louis and Chester indicate that stage for a given discharge has changed with time at both locations. An apparent increase in stage for a given discharge at increased flows (greater than flood stage) likely is caused by the raising of levees on the flood plains, and a decrease in stage for a given discharge at low flows (less than one-half flood stage) likely is caused by a combination of dikes in the channel that deepen the channel thalweg at the end of the dikes, and reduced sediment flux into the Middle Mississippi River. Since the 1960s at St. Louis, Missouri, the stage-discharge relations indicated no change or a decrease in stage for a given discharge for all discharges, whereas at Chester, Illinois, the stage-discharge relations indicate increasing stage for a given discharge above bankfull because of sediment infilling of the overflow channel.

Top width and average velocity from measurements at a given discharge for the streamgage at St. Louis, Missouri, were relatively constant through time, with the only substantial change in top width resulting from the change in measurement location from the Municipal/MacArthur Bridge to the Poplar Street Bridge in 1968. The average bed elevation appeared to be lowering with time at both measurement locations at St. Louis. Flow in the Horse Island Chute overflow channel for the streamgage at Chester, Illinois had an effect on top width and average velocity from measurements, and this effect changed with time as the inflow channel to Horse Island Chute filled with sediment. Top width from measurements at a given discharge was consistent through time at the Chester streamgage when adjusted to remove the part of the flow through Horse Island Chute. Average velocity from measurements at a given discharge appears to be increasing with time, possibly as a result of a series of dikes built or extended in the channel immediately upstream from the Chester streamgage; however, the average bed elevation for all discharges less than bankfull at the Chester streamgage fluctuate around an average value from 1948 to 2000, and the fluctuations appear to be related to the occurrence of moderate and large floods.

Daily stage and discharge values available for the streamgage at St. Louis, Missouri, from 1861 to 1932 display distinct, fixed relations that change slightly with time before operation by the U.S. Geological Survey, indicating daily discharge was obtained from the daily stage value during this timeframe. A sudden and substantial reduction of about 24 percent at the upper end of the ratings for discharge at a given stage occurred between 1932 and 1933 when the U.S. Geological Survey began operating the streamgage. This change likely is the result of the change to Price AA current meters from other, less-accurate methods used for discharge measurements before 1933. Based on modeling results for the Middle Mississippi River by the U.S. Army Corps of Engineers and the findings of this study, the accuracy of the historic record before 1933 is questionable, and needs to be examined further.

The difference in daily water-surface elevation between St. Louis, Missouri, Chester, Illinois, and Cape Girardeau, Missouri, also were examined from the 1890s onward. From the 1890s to the 1930s, the water-surface elevation differences between St. Louis and Chester and Cape Girardeau decreased, whereas the water-surface elevation difference between Chester and Cape Girardeau was nearly constant. The Kaskaskia cutoff in 1881 is the likely cause for the steady decrease between St. Louis and Chester, and the channel adjusted to the cutoff between the 1880s and 1930s. Other small but abrupt fluctuations in the water-surface elevation differences between the gages appear to be related to large flood events, or smaller flood events during extended periods of low flow.

Cross sections extracted from measurements made at the streamgages at St. Louis, Missouri, and Chester, Illinois, were examined for changes with time. All of the cross sections displayed substantial variability through time, likely resulting from the effects of temperature, seasonal variations, and rising and falling stage. The cross sections for both measurement locations at the St. Louis streamgage and for the Chester streamgage indicated substantial variability at a given discharge range, but the variability appears to decrease after the early 1970s at the Chester streamgage for cross sections in the 100,000 cubic feet per second and 400,000 cubic feet per second discharge ranges, possibly because of work done on a dike field immediately upstream from the Chester streamgage. Cross sections from measurements made during the flood of 1993 also indicated the substantial variability of these sections with changing discharge in a single flood event. Substantial movement of bed sediments is apparent in the cross sections at the St. Louis streamgage during the 1993 flood, whereas the cross-section bed elevation steadily lowered until the 1993 flood peak at the Chester streamgage, and then rose to a level somewhat lower than before the flood.

Posted October 21, 2009