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.[1] Along >4000 km of the Mississippi River system, we document that climate, land-use change, and river engineering have contributed to statistically significant increases in flooding over the past 100-150 years. Trends were tested using a database of >8 million hydrological measurements. A geospatial database of historical engineering construction was used to quantify the response of flood levels to each unit of engineering infrastructure. Significant climate-and/or land use-driven increases in flow were detected, but the largest and most pervasive contributors to increased flooding on the Mississippi River system were wing dikes and related navigational structures, followed by progressive levee construction. In the area of the 2008 Upper Mississippi flood, for example, about 2 m of the flood crest is linked to navigational and flood-control engineering. Systemwide, large increases in flood levels were documented at locations and at times of wing-dike and levee construction.

Accurate mapping of stream channel networks is important for measuring hydrologic parameters, for site planning in construction projects, and for use in hydrologic models. This article compares five existing and two new methods for extracting stream channel networks for use in topographic mapping. In order of increasing accuracy, these methods are: (1) blue lines on USGS 1:24,000 topographic maps (64.6 percent underrepresentation), (2) placing stream heads using a constant flow-accumulation area to mimic USGS blue lines (47.8 percent underrepresentation), (3) constant flow-accumulation area equal to the mean for identified channel heads (30.3 percent combined under-and overrepresentation), (4) variable flow-accumulation area estimated by multiple linear regression (28.9 percent combined under-and overrepresentation), (5) variable flow-accumulation area estimated by a slope-power relationship (23.6 percent combined under-and overrepresentation), (6) identifying stream cells using logistic regression (12.7 percent combined under-and overrepresentation), and (7) extracting stream channel head locations from digital orthophotoquads (DOQs) (nearly 100 percent accurate, but only applicable under ideal conditions). Methods 2-6 require 10 m resolution digital elevation models that can be acquired directly in many areas or can be derived from 1:24,000 hypsography where available; Methods 4 and 6 are new methods developed in this paper.Using DOQs, while extremely accurate, is labor intensive and can be applied only in a small minority of locations where vegetation cover does not obscure channel head locations. We conclude that identifying stream cells using logistic regression has the broadest applicability because it can be implemented in an automated fashion using only DEMs while still achieving accuracies for mapping low-order streams that are far superior to existing USGS maps.

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Abstract:This study used stream gauge records to assess the impact of levees on flood levels, providing an empirical test of theoretical and model predictions of the effects on local flood response. Focusing upon a study area in Illinois and Iowa for which levee records were available, we identified 203 gauges with ≥ 50 years hydrological record, including 15 gauges where a levee was constructed during the period of record. At these sites, step-change analysis utilizing regression residuals tested levee-related stage changes and levels of significance and quantified the magnitudes of stage changes. Despite large differences in stream sizes, levee alignments, and degree of floodplain constriction, the post-levee rating-curve adjustments showed consistent signatures. For all the study sites, stages for below bankfull (non-flood) conditions were unaffected by levee construction. For above bankfull (flood) conditions, stages at sites downstream of their associated levees also were statistically indistinguishable before versus after levee construction. However, at all sites upstream of levees or within leveed reaches, stages increased for above bankfull conditions. These increases were abrupt, statistically significant, and generally large in magnitude -ranging up to 2.3 m (Wabash River at Mt. Carmel, IL). Stage increases began when discharge increased above bankfull flow and generally increased in magnitude with discharge until the associated levee(s) were overtopped. Detailed site assessments and supplementary data available from some sites helped document the dominant mechanisms by which levees can increase flood levels. Levee construction reduces the area of the floodplain open to storage of flood waters and reduces the width of the floodplain open to conveyance of flood flow. Floodplain conveyance often is underestimated or ignored, but Acoustic Doppler Current Profiler (ADCP) measurements analysed here confirm previous studies that up to 70% or more of the total discharge during large floods (~3% chance flood) can move over the floodplain. Upstream of levees and levee-related floodplain constriction, backwater effects reduce flow velocities relative to pre-levee conditions and, thus, increase stages for a given discharge. The empirical results here confirm a variety of theoretical predictions of levee effects but suggest that many one-dimensional model-based predictions of levee-related stage changes may underestimate actual levee impacts.