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Recent research has indicated that numerous lowland rivers in North America and Europe had multithread, anabranching channel patterns prior to widespread human modification of landscapes and that contemporary human activities have transformed many anabranching rivers into single-channel meandering rivers. Although lowland rivers in the upper midwestern United States are predominantly meandering, anabranching occurs locally. Whether anabranching of these river systems was uncommon in the past or intensive agriculture has resulted in near eradication of anabranching channel patterns remains uncertain. This article quantifies the prevalence of river anabranching within several watersheds in the upper midwestern United States prior to widespread European settlement in the early to mid-1800s based on General Land Office survey plat maps and field notes. It also compares this prevalence to the contemporary occurrence of river anabranching. Results show that anabranching reaches were somewhat more common historically (22 percent more reaches in the past compared to the present), but were still relatively rare, constituting only about 1 percent of the total length of streams in the study watersheds. Analysis of the geographic distribution of anabranching reaches reveals that historical spatial patterns generally are different than modern patterns. Thus, only a handful of historical anabranching reaches have persisted through time to the present. The findings place contemporary river anabranching in the upper midwestern United States within the context of past conditions and suggest that human modification of landscapes throughout this region has not substantially modified channel patterns of rivers. 

Abstract Relatively little is known about the geomorphological characteristics of floodplain secondary channels and the potential for floodplain flows to mobilize bed material within these channels. This study examines the geomorphological characteristics (channel form, material properties, wood jams) and bed‐material mobilization potential of secondary channels on the floodplain of a meandering river in Illinois, USA. It also compares these attributes to those of the main channel. Results show that secondary channels are at most about one‐third the size of the main channel but also vary in size over distance. Channel dimensions tend to be greatest near the proximal connection of secondary channels to the main channel, suggesting that flow from the main channel is effective in producing scour where it enters secondary channels. The beds of secondary channels consist mainly of mud in contrast to sand and gravel on the bed of the main channel, implying that secondary channels do not convey bed material from the main channel onto the floodplain. Secondary channels connected to the main channel at both ends have more abundant active wood jams than those connected only at the proximal end. Flow from the main channel enters secondary channels at sub‐bankfull stages, but maximum mobilization of cohesive bed material in secondary channels only occurs during flows that exceed the average bankfull stage in the main channel. Overall, secondary channels are active conduits of flow, sediment, and large wood on floodplains and can contribute to floodplain sediment fluxes through entrainment of bed material.