Geochemical variations during flash flooding, Meramec River basin, May 2000

Severe flooding in the Meramec River basin followed an extraordinary rainfall event on May 7, 2000. Precipitation measurements for the 13-h event ranged from 12.7 to 39.9 cm over a 4100 km2 region centered near Union, Missouri. Sample collections for isotopic and chemical analyses and field measurements of water temperature, specific conductivity, turbidity, and pH were made from three rivers during the course of the event. Relative to pre-storm values, flood water underwent a three to 10-fold decrease in conductivity, a 100-fold or more increase in turbidity, and pH fluctuations over a range of 1.0 unit. Concentration of major ions varied inversely (Ca, Mg, Na, Cl, SO4) or directly (K) proportional to discharge. Oxygen isotope measurements were used to separate each discharge hydrograph into pre-event and event water components. Event water dominated during peak runoff on each of the rivers, a condition atypical of floodwaters in this region and the result of overland flow or rapid subsurface delivery of precipitation. The Bourbeuse River showed the largest event water component during this exceptional event, with storm water making up essentially 100% of the flow for more than 24 h.