CHARACTER AND ORIGIN OF MISSOURI FLOODWATERS

Floodwaters are dynamic assemblages of stormwaters, surface waters, groundwaters, solutes and particulates, whose amounts and relative proportions depend on antecedent conditions and timing relative to peak flow. Discharge at a given site conforms to a characteristic curve, called a “hydrograph”, that over time features a rapid rise to a peak representing maximum flow, followed by a gradual recession. On small rivers these curves are good approximations of flux curves calculable from diffusional or darcian flow. Oxygen isotope data show that flood peaks in natural streams are typically dominated by locally-derived waters, particularly groundwater, and that flood peaks are not well described by convolution integrals of precipitation, but rather are threshold phenomena related to pulses in hydraulic head. Overland flow dominates only in urban settings or when unusually intense rainfall overwhelms infiltration mechanisms, as during the destructive May 2000 flash flood at Union, Missouri.

Discharge variations are strongly related to topologically similar pulses of solute and particulate concentrations. Solutes define chemical hydrographs, or “chemographs”, that are typically inverted and slower responding relative to the discharge hydrograph for most but not all constituents. Thus, minimum concentrations for most ions coincide with or postdate maximum streamflow, and then slowly increase. Particulate concentrations, however, rapidly attain peak values while discharge is rising, and then quickly attenuate while streamflow remains high. Concentrations of bacteria and certain ions such as Fe, Pb and Hg also rapidly attain positive peaks, indicating that they are more closely associated with the particulate load than with the dissolved load. Accordingly, floodwaters are dangerous not only because of high stages and great energy, but also because of their poor water quality and their capability to deposit large quantities of sediment in unwanted areas.