Interdisciplinary and unconventional research methods offer important insights into geomorphic, hydrologic and hydroclimatologic characteristics of large floods that are often difficult or impossible to resolve in the framework of conventional flood analysis. Four detailed studies of modern floods, historical floods, and paleofloods in western and central Arizona demonstrate the benefits of analyzing recent and historical extreme floods within the conceptual framework of paleoflood hydrology and flood hydroclimatology. Analysis of the hydroclimatological and paleohydrological context of extreme flooding in Arizona during the winter of 1993 provides a detailed analog to the likely climatic, meteorologic, and hydrologic conditions associated with the largest events in the regional paleoflood record. Investigation of the distribution of relict high-water evidence from extreme floods on the lower Verde River in 1993 improves the accuracy of the river's paleoflood record and reveals interesting hydrological phenomena of extreme floods in the Verde River Basin. A multidisciplinary study of the extraordinarily large Bronco Creek, Arizona, flood of August 1971, shows the original estimate to be significantly overestimated because of complex flow behavior of an extreme flood and the related dynamic morphological response of a high-gradient alluvial channel. The approach to this study is a template for similar analyses of extreme floods and extraordinary flood discharge estimates. A similar, more comprehensive application of paleoflood research methods is demonstrated by the compilation of a detailed regional chronology of flash-flooding in small desert drainage basins (7-70 km²) in western Arizona. The occurrences of large, recent and historical floods were documented with nearly annual resolution, and a 1200-year regional paleoflood record was compiled. Comparison of these records to conventional regional flood-frequency relations indicates that the regional equations are probably inaccurate because of data limitations. The study presents a viable approach to developing a quantitative assessment of regional flood frequency in areas with no conventional data on real floods. The results of each of these studies extend the spatial and temporal scope of the paleoflood and historical flood record of the lower Colorado River Basin and provide further support for the concept of a regional upper limit to flood peak magnitudes.

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Between 1865 and 1915, arroyos developed in the southwestern United States across diverse hydrological, ecological and cultural settings. That they developed simultaneously has encouraged the search for a common cause-- some phenomenon that was equally widespread and synchronous. There are few southwestern streams for which we have even a qualitative understanding of timelines and processes involved in initiation and extension of historic arroyos. Tucson's Santa Cruz River, often cited in the arroyo literature, offers a unique opportunity to chronicle the arroyo legacy and evaluate its causes. The present study reconstructs both the physical and cultural circumstances of channel entrenchment along the Santa Cruz River. Primary data include newspaper accounts, notes and plants of General Land Office surveys, eyewitness accounts, legal depositions, and repeat photography. On the Santa Cruz River, arroyo initiation and extension happened during relatively wet decades associated with frequent warm episodes in the tropical Pacific (El Niño conditions). Intensified El Niño activity during the period 1864-1891 may be symptomatic of long-term climatic change, perhaps indicative of global warming and destabilization of Pacific climate at the end of the Little Ice Age. During this period all but one of the years registering more than three days with rain exceeding 2.54 cm (1 in) in Tucson were El Niño events. The one exception was the summer of 1890, when the central equatorial Pacific was relatively cold but when prevailing low-surface pressures and low-level winds nevertheless steered tropical moisture from the west coast of Mexico into southern Arizona. In the twentieth century, catastrophic channel widening was caused by floods during El Niño events in 1905, 1915, 1977 and 1983. The Santa Cruz River arroyo formed when climatic conditions heightened the probabilities for occurrence of large floods in southern Arizona. Inadequate engineering of ditches that resulted in abrupt changes in the longitudinal profile of the stream further augmented probabilities that any one of these floods would initiate an arroyo. In the future, changing flood probabilities with low-frequency climatic fluctuations and improved flow conveyance due to intensified land use and channel stabilization will further complicate management of the arroyo in an increasingly urbanized floodplain.