Franklin T. Heitmuller

Local- and watershed-scale controls on the spatial variability of natural levee deposits in a large fine-grained floodplain: Lower Panuco Basin, Mexico

This study examines spatial variations in natural levee deposits within the lower reaches of a large coastal plain drainage system. The Panuco basin (98,227 km 2 ) drains east-central Mexico, and is an excellent setting to examine the influence of watershed and local controls on the morphology and sedimentology of natural levees. Although many fluvial systems in the U.S. Gulf Coastal Plain have been investigated, the rivers in the Mexican Gulf Coastal Plain have received comparatively little attention. Lateral and downstream characteristics of natural levee morphology and sediment texture are considered within the context of meandering river floodplain deposits. Data sources include total-stations surveying, sediment samples of surficial levee deposits, topographic maps (1:50,000), and aerial photographs (1:40,000). The slope of natural levees average 0.0049 m/ m, whereas the texture (D84) of levee deposits averages 0.12 mm. Natural levee characteristics vary due to local- and watershed- scale controls. The lateral reduction in levee height displays a curvilinear pattern that coincides with an abrupt change in sediment texture. The downstream pattern of natural levee texture exhibits the influence of local-scale perturbations superimposed upon a larger watershed-scale trend. Disruption to the fining trend, either by tributary inputs of sediment or reworking of Tertiary valley deposits, is retained for a limited distance. The influence of the channel planform geometry on levee morphology is examined by consideration of the radius of curvature (Rc) of meander bends, and is inversely related to natural levee width. This suggests that the planform geometry of river channels exerts a control on the dispersal of flood sediments, and is responsible for considerable local variability in the floodplain topography. The average width of natural levees increases with drainage area, from an average of 747 m in the Moctezuma to an average of 894 m in the Panuco. However, in the lower reaches of the Panuco valley the width of natural levees rapidly decreases, which is associated with fining of the suspended sediment load. Thus, the reduction in natural levee width signifies an abrupt change in the directionality of cause- effect relationships at the watershed-scale. Findings from this study elucidate linkages between meandering river channels and floodplains for a large lowland alluvial valley. D 2003 Elsevier Science B.V. All rights reserved.

Historical Channel Adjustment and Estimates of Selected Hydraulic Values in the Lower Sabine River and Lower Brazos River Basins, Texas and Louisiana

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Overbank sedimentation from the historic A.D. 2011 flood along the Lower Mississippi River, USA

In this study, we document sedimentary characteristics of overbank flood deposits associated with the epic A.D. 2011 flood along the Lower Mississippi River (southern USA) and directly compare the findings to sedimentation from a comparable flood event in 1973, with the general purpose of understanding how extreme floods contribute to floodplain depositional patterns and accretion rates of embanked fluvial systems. The thicknesses of the 2011 flood deposits averaged 138 mm along natural levee crests, 9 mm on meander scrolls, and 3 mm in backswamps. These thicknesses are considerably less than those documented for the 1973 flood, sampled at the same locations. We contend that less sedimentation in 2011 occurred because the flood was not supplied with much upstream sediment from the Missouri River. Further, the 2011 sediments are coarser than in 1973, indicating that the higher 2011 flood levels were associated with more energetic overbank flows that flushed fine-grained sediments downstream within the narrow embanked floodplain corridor. The largest recorded flood in North American history is only marginally preserved in the embanked floodplain stratigraphy of the alluvial valley of Earth’s third largest fluvial system.

Magnitude and Frequency Relations of Bankfull Flows and Bed-Material Entrainment in the Llano River Watershed, Central Texas, USA

Bankfull channel geometry and bed-material entrainment are examined in the Llano River watershed in central Texas, which is characterized by mixed alluvial-bedrock boundaries and a highly variable flow regime. Field data, including cross-section surveys and sediment samples from 17 sites, are coupled with one-dimensional hydraulic models and flood-frequency analyses to compute recurrence intervals of bankfull and entrainment conditions. Recurrence intervals of flows that entrain median-sized bed material (d 50) are consistently less than 1 year, whereas flows that define bankfull morphology range from one to four years (median of 1.75 years). Recurrence intervals of flows responsible for incipient entrainment of relatively coarse bed material (d 84) are typically less than 1.5 years, but complete mobilization of this size fraction occurs less frequently than bankfull flows. At multiple sites, morphologic surfaces defining bankfull stage are discontinuous and set within a macro-channel, which is formed by high-magnitude flows with recurrence intervals greater than five years. Finally, the frequency of bankfull flow in the lowermost, confined, sandbar-dominated channels is similar to the frequency of the mobilization of relatively coarse low-flow channel gravels; but discontinuous, sandy bank deposits indicate that sediment exchange with channel bars is the most important process for development of bankfull indicators.

Introduction to the Physical Geography of Medium-Sized Rivers, Focusing on the Southeastern and South-Central United States

and four in the forthcoming issue (Vol. 32, No. 6), resulted from a special session on the geo-morphology and physical geography of medium-sized watersheds at the meeting of the Southeastern Division of the Association of American Geographers (SEDAAG) held in November 2010 in Birmingham, Alabama. Medium-sized rivers are gener-ally large enough to contain multiple zones, in which different geomorphic pro-cesses may be dominant. In their headwaters, known as Zone 1, low-order streams dominate and have a unique signature in terms of water quantity, water quality, hillslope sediment, and nutrients provided from local contributing areas (Schumm, 2005). As tributaries converge into higher-order rivers, the Zone 2 or transfer zone takes on greater relative importance. Geomorphic investigations in this zone require integration of multiple inputs from the larger drainage area and consideration of lag times, from initial landscape disturbance to changes in sediment delivery and channel and floodplain morphology. Generally, sediment input can equal output if a stream is stable in the transfer zone, but it is usually disrupted when anthropogenic disturbances are prevalent, a theme that is explored in a number of the papers in this issue. In medium-sized watersheds, one substantial problem encountered in efforts to relate landscape condition to channel and floodplain change is that a wide variety of disturbances can be occurring, as was the case in many of the rivers discussed in this issue. In some cases, the dominance of a particular activity or the timing of geomor-phic and sedimentary responses may suggest the drivers of change to the system. The downstream transmission of fluvial disturbances in Zone 2 can eventually result in a diversity of sedimentation patterns and rates in Zone 3, the lowermost depositional zone in the areas of lowest gradient.Although the initial common theme was scale of watershed, an unplanned com-mon theme of human impacts was central to most of the papers and has become a recurring theme in geomorphology (James and Marcus, 2007). Human disturbances were associated with early and recent land cover and land use changes. The three papers that focus on mining in the southeastern and south-central United States are important additions to the literature, in that mining is more often associated with the western states. Lessons learned from the changes in morphology and sedimentation associated with these disturbances provide insights into the needs and strategies

Bed-material entrainment and associated transportation infrastructure problems in streams of the Edwards Plateau, central Texas

Printed on recycled paper The Texas Department of Transportation (TxDOT) commonly builds and maintains low-water crossings (LWCs) over streams (fig. 1) in the Edwards Plateau in Central Texas. LWCs are low-height structures, typically constructed of concrete and asphalt, that provide acceptable passage over seasonal rivers or streams with relatively low normal-depth flow. They are designed to accommodate flow by roadway overtopping during highflow events. The streams of the Edwards Plateau are characterized by cobbleand gravel-sized bed material and highly variable flow regimes. Low base flows that occur most of the time occasionally are interrupted by severe floods. The floods entrain and transport substantial loads of bed material in the stream channels. As a result, LWCs over streams in the Edwards Plateau are bombarded and abraded by bed material during floods and periodically must be maintained or even replaced.