Holocene paleohydrology and paleofloods in the Driftless Area

ABSTRACT The study of Holocene paleohydrology and paleofloods represents one of James C. Knox’s most enduring contributions to our understanding of the geology and physical geography of the Driftless Area. His work on these subjects resulted in over 20 journal articles, refereed book chapters, field-trip guidebooks, and unpublished reports over a period of 40 years. By systematically amassing a wealth of radiocarbon-dated morphologic, stratigraphic, and sedimentologic observations, he was able to quantitatively document changing hydrologic conditions in the region over the past 11.7 ka. He extended these empirical results to establish a broadly applicable theoretical perspective about the profound hydrologic and geomorphologic impacts of even modest changes in climate. This theory, grounded in field-derived data collection, detailed sedimentological analysis, statistical methods, and contextual analysis of supplementary paleoenvironmental evidence, has important implications for our understanding of changing flood magnitudes and frequencies in response to ongoing climate change. Knox pioneered novel methods for reconstructing past hydrologic variability. His work on the cross-sectional geometry of paleomeanders provides a direct proxy for estimation of high-frequency, low-magnitude bankfull floods. His analysis of overbank gravels facilitates reconstruction of an early through late Holocene time-series of large, infrequent floods. His attention to sandy beds occurring within uninterrupted, fine-grained, overbank depositional sequences enables continuous magnitude and frequency analysis of floods to be extended hundreds, even thousands, of years beyond the modern gaging record. We demonstrate this with statistical correlation of a gaging record to >0.25 mm sand contents at one of Knox’s former sites. By quantifying hydrologic change in the Driftless Area over millennial timescales, Knox’s work demonstrates conclusively the non-stationarity of flood magnitudes and frequencies, a result that has significance for fluvial geomorphology, paleohydrology, water resource management, and flood mitigation. The scientific and societal value of these results continues to increase in relevance for the future.