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Because lower Missouri River management began in the early 1800s, a challenge for present-day ecosystem restoration efforts is a lack of quantitative data on pre-management river hydrology and long-term (100+ yr.) river response to changing management practice and intensity. We address this challenge and report new results from a study spanning 200 years of lower Missouri River hydrology, encompassing natural, channelization-only, and channelization with reservoir release regimes (Ehlmann & Criss, Geology, forthcoming, Nov/Dec 2006).

Rayleigh fractionation is an important, open system process that involves the progressive removal of a fractional increment of a trace substance from a larger reservoir. A consistent relationship, such as a distribution coefficient, equilibrium constant, or a fractionation factor, is maintained between the reservoir and each increment at the instant of its formation, but, once formed, each increment is thereafter removed or otherwise isolated from the system. The mathematical constraints of this process include the distribution coefficient along with a statement of material balance in the shrinking system. These constraints may be combined into a single differential equation that may be integrated to a well-known relationship discovered by Rayleigh (1902). Examples of this common process include condensation, distillation, and the formation of crystals from a melt or a solution. As shown in the following, Rayleigh fractionation explains many different characteristics of meteoric waters, and provides a first-order explanation of the MWL