Meandering rivers that anabranch in the upper Midwest (USA): Prevalence, morphological characteristics, and power regimes

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Channel planform is the result of present and past hydrologic, sedimentary, and geologic conditions, and represents an adjustment of fluvial systems to these conditions. An important yet overlooked aspect of planform diversity of rivers in intensively managed agricultural landscapes of the upper Midwest (USA) is the co-occurrence of single-thread meandering and multi-thread anabranching reaches. Recent work on the role of river corridors in critical-zone dynamics of intensively managed landscapes has revealed the existence of juxtaposed meandering and anabranching reaches, yet the prevalence of anabranching reaches within meandering rivers in the upper Midwest is unknown. In addition, the morphological characteristics of these reaches and whether the stream power domain of these reaches differs from that of meandering reaches are also unclear. This paper quantifies the prevalence, planform morphology, and stream power domain of anabranching reaches along numerous lowland meandering rivers in the upper Midwest. Anabranch prevalence is explored for five representative watersheds using Google Earth aerial imagery and airborne LiDAR data. Less than 1 % of the total stream length in these watersheds (∼42,000 km) exhibits anabranching, but no watershed is devoid of anabranching reaches. Most anabranching reaches are characterized by a two-channel looping configuration, although complex anabranches with more than three channels exist at a few locations. Mean bifurcation angles (70° ± 29°) of the anabranching reaches are higher than those of braided rivers (40°-60°), with maximum angles as high as 150°, whereas mean confluence angles (60° ± 28°) fall within the range of junction angles in stream networks (45°-75°). Stream power analysis indicates that the occurrence of anabranching reaches along meandering rivers in the upper Midwest is significantly related to local increases in channel slope. The stream power domain of most anabranching reaches in the upper Midwest falls within that of meandering rivers, although some reaches have power similar to that of braided or wandering gravel-bed rivers. When considered collectively, multi-thread reaches juxtaposed with meandering reaches do not fit neatly into existing classifications of anabranching rivers.