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Data from the 1803-1806 Lewis and Clark expedition and nineteenth century stage records are used to quantitatively benchmark natural, premanagement hydrology of the lower Missouri River and assess the magnitude and timing of hydrologic change over nearly two centuries. Data show doubling in daily stage variability from the nineteenth century to 2005. Annual maximum stages have, at some sites, become more extreme, and their seasonality is less regular. Observed changes adversely affect riverine habitat and flood levels; their timing, beginning as early as 1900, suggests that channelization is the major driver. Keywords: basin management, Missouri River, hydrology, channelization, Lewis and Clark.

Flood stages for constant discharge have increased 2-4 m over the past century at numerous locations in the Mississippi River basin. However, no increases are observed on rivers such as the Meramec and the upper Missouri, which have been spared extensive river engineering projects. Flood-stage increases on the middle Mississippi River and lower Missouri River are mostly attributable to channelization.