Influences of historical river channels on current flood events: implications of geomorphic legacies for biocomplexity of coupled human and natural systems in large rivers

S.V. Gregory, L.R. Ashkenas, and R.C. Wildman. Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97331

The spatial distribution of floods in large rivers reflects both existing hydrologic conditions and influences of historical channel morphology. Human activities in river floodplains and attempts to conserve or restore floodplain functions are determined in part by legacies of river history. We mapped the Willamette River and its side channels for 1850, 1895, 1932, and 1995. In addition, we mapped the distribution of the 1996 flood, the 21st largest flood year on record since 1850. For the 1996 flood, we designated each channel as either new or historical channel. Approximately half of the flood channels were reoccupied historical channels. The proportion of channels that were historical legacies was a function of larger-scale valley landscapes. Patterns of human systems (population, structures, land value) were linked to historical channel conditions. Urban land uses reduced legacies of past channels, while agricultural landscapes tended to maintain greater proportions of historical channels. As a result, riparian conditions and floodplain dynamics are shaped by interactions of human systems and their tendencies to edit or obliterate geomorphic and hydraulic legacies of historical river systems. These biophysical and socioeconomic constraints can be incorporated in spatially explicit systems for developing conservation and restoration programs in large rivers.