Joann Mossa

Sediment dynamics in the lowermost Mississippi River

Abstract There is much to be gained from investigating sediment dynamics in the lower Mississippi system, the largest river in terms of discharge and sediment load in North America. Such work can improve conceptual knowledge concerning downstream changes at the lower end of large river systems and can be applied to manage sediment diversions for wetland restoration in south Louisiana. Suspended sediment dynamics in the lowermost Mississippi River system in Louisiana are characterized using three approaches: (1) temporal changes in discharge-suspended sediment relationships showing interannual variations and the effects of floods over short timescales; (2) empirical relationships between discharge and suspended sediment variables at various locations; and (3) downstream changes in discharge-suspended sediment relationships. Interpretation of this data set is enhanced with other secondary data regarding processes, morphology, and bed materials. Upstream, near Old River, LA, empirical relationships show nonlinearity, particularly in fine sediments, with decreased concentrations at highest discharges. During high discharge years, suspended sediment concentration peaks precede discharge crests by 40–85 days. The lead generally decreases with decreasing discharge maxima so that in low discharge years sediment peaks and discharge crests closely coincide in time. Downstream, near Belle Chasse, LA, fine bottom materials are resuspended and the timing of sediment peaks and discharge crests is coincident, regardless of flow magnitude. Conceptually, results suggest caution when generalizing about the relative timing of the sediment wave and flood wave and their downstream progression. These phenomena are influenced by local bed material and hydraulic conditions, and depend on the causative factors of sediment peaks. From an applied perspective, diversions should be managed differently depending upon where they are constructed along the river and upon the magnitude of the annual maximum flow. During high discharge years, when concerns for navigation and water supply are minimal, flow should be diverted on the rising limb upstream, near Old River, and during the discharge crest downstream near New Orleans.

Channel planform and land cover changes on a mined river floodplain

Abstract The Amite River in southeastern Louisiana, USA, is a disturbed floodplain and channel, where combined gravel and sand extraction has exceeded 10 million tons per year. To examine relationships between mining and channel change along this river, land cover data from a 55-km floodplain reach with variable mining intensity were compiled from photographic, map and documentary sources for two different time periods. These data were manipulated using a geographic information system and analysed using nonparametric statistics, producing statistically robust, moderate correlations between the degree of floodplain mining and change in channel position. The approach and results have application in research, planning and management concerning floodplain disturbances and channel instability.

Fossil Tree Casts in South Louisiana Soils

ABSTRACT Cylindrical structures up to 70 cm or more in diameter and 100 cm or greater in depth are found in soils on upland landscapes in south Louisiana. The cylinders are in two stratigraphic units differing in age and lithology, the late Pliocene or early Pleistocene Citronelle Formation and the overlying Sicily Island Loess (thermoluminescence-dated at 75,000-95,000 B.P.). By morphology and stratigraphy we infer that the cylinders are fossil tree-root casts preserved by pedogenesis and diagenesis. The cylinders can be divided into circumferential zones including a reduced core and an indurated rim that are morphologically, physically, and chemically distinct from each other and from the surrounding soil. In the Citronelle Formation, the rim can be subdivided into an inner rim, a black hand and an outer rim; a transition zone between the core and the rim also may be present. Differences in time of development and lithology have resulted in differences in the morphological, physical, and chemical properties of the cylinders, but their effects could not be separated. Our model for these tree-root casts explains how they were formed and preserved.

Participatory Student Field Guides and Excursions.

Abstract This paper discusses the motivation, implementation and impressions of involving students in written and oral field guide contributions in two geography classes. This approach is especially useful in situations where the instructor is knowledgeable about the area but not an authority, such as in the case of a recent appointment. Various aspects of the experience, including the superior quality of participatory field guides and excursions as compared with an exclusively instructor‐generated product, prove beneficial for those involved and for outside parties. Students also learn more from participating, obtain written and oral communication opportunities, and acquire self‐esteem through contributing.

Thalweg variability at bridges along a large karst river: the Suwannee River, Florida

Abstract Geomorphologists and engineers have different perspectives and approaches for examining river channels and the changes that occur during floods. The field-oriented approach typically adopted by geomorphologists has little predictive ability and design usefulness. In contrast, the empirical approach adopted by engineers is based on predictive equations or models that often differ greatly from reality. Such equations are not based on comprehensive field data and often fail to consider a number of site conditions, especially geology and geomorphology. Yet, in order for geomorphic techniques to be useful to the design and planning of engineering structures such as bridges, it is important that sufficient observations exist in order to characterize long-term and short-term changes in bottom topography and scour potential. Six gaging stations on the Suwannee River, a large river draining karst terrain in the southeastern US, were used to examine the temporal variability in thalweg elevation, the deepest point in a given cross-section. The cross-sections have maximum thalweg variability of just a few meters, despite the occurrence of several large floods, suggesting that the bottoms are fairly stable. Historical approaches can be applied to design the length and depth placement of pilings by providing information on site conditions not considered in engineering equations, such as response of bottom materials to various flow conditions, and thus have potential benefits to public safety and cost effectiveness.

Historical changes of a major juncture: Lower Old River, Louisiana

Old River is one of the most important river junctures in North America. It connects three large, navigable rivers – the Mississippi, Atchafalaya and Red – which support economies and environments critical to United States of America. Starting in the 1800s, the Atchafalaya River began receiving more flow through this juncture, and by the mid-1900s, an avulsion was likely so the Old River Control Project was authorized to prevent diversion of the Mississippi River to the Atchafalaya. This paper examines the historical changes of Lower Old River, the main juncture prior to 1963 before the flow became controlled by structures, including its flow changes, human modifications, and whether and how it was changing as the Atchafalaya received more flow. In contrast to some reports that suggest that bidirectional flow in Old River was just a few years in duration, occasional flow from the Red River is documented: starting in the 1880s and persisting for several decades through 1945. This paper identifies three transitions in the transformation from bidirectional to unidirectional flow, the first of unknown cause in the late 1910s, the second likely instigated by the flood of record in 1927, and the third immediately following a local artificial cut-off in 1944-1945. Also, changes in flow and geometry in Lower Old River are related to changes in the adjacent Mississippi and Atchafalaya Rivers.

Pit Avulsions and Planform Change on a Mined River Floodplain: Tangipahoa River, Louisiana

Floodplain mining creates a vulnerable landscape comprised of pits, bare land and stock piles with minimal vegetation. During floods, when stages can exceed bankfull by several meters, all or part of the channel may divert into a pit, re-routing the flow and transforming the planform and profile of alluvial landscapes. Using geospatial data from the mined Tangipahoa River floodplain for 1980 to 2004, the objectives of this study are to (1) assess if pit characteristics such as proximity to channel and size affect the potential for diversion; and (2) interpret relationships between pit diversions, channel changes, and floods. Of the 56 pits mapped in the floodplain in 1980, six had been captured by 2004. Captured pits were closer to the main channel than those which were not captured, but the areas of captured and noncaptured pits were not significantly different. However, avulsions into large pits caused more channel planform change than those into smaller pits. Other findings are that point bar areas increased 72% overall, more so in the mined reach, and that degradation exceeded 6 m in the mined reach, in contrast with aggradation of 1 m roughly 20 valley km downstream. Results will be useful in understanding landscape vulnerability and can be applied to river management and restoration.

Oxbow Lakes as Indicators of River Channel Change: Leaf River, Mississippi, USA

This study uses oxbow lakes to ascertain the extent and nature of change in a rivers form, focusing on the effects of post-1950s floodplain gravel extraction on the geomorphology of the Leaf River in southeastern Mississippi, USA. The river and oxbow lakes of a 5 km reach were examined for the effects of local and upstream floodplain and in-channel gravel mining. Data from 12 cross-sections suggests that the present-day river is 35% wider and 12% shallower than the cross-sectional geometry in cutoffs that pre-date 1955. Field measurements reveal that the Leaf River was relatively stable from at least 1911 to the early 1940s, but has since undergone an average of 2.65 m of degradation in the study reach. Survey data from a nearby USGS stream gage show that degradation began abruptly in the mid 1970s, most likely as a result of in-stream mining and pit avulsions on a major tributary that deprived downstream areas of bed load and created profile adjustments.

Capstone portfolios and geography student learning outcomes

Due to increasing demands regarding student learning outcomes and accreditation, a capstone portfolio was added to assess critical thinking and communication skills of geography majors at a large public university in the USA. The portfolio guidelines were designed to be adaptable to a flexible curriculum where about half of the requirements within the major are structured. This paper discusses the portfolio requirements and assessment rubrics developed following multiple portfolio iterations, gives guidance to instructors, and synthesizes potential benefits for students, departments and employers.

The Promise for Geomorphic Discovery in the South.

Abstract Until recently a neglected region, renown for limited exposures, the southern United States is becoming an area of geomorphic discovery. This article presents an overview of current geomorphic research in this region, suggesting that despite past problems, academic geographers have made significant contributions to the geomorphology of the South in recent years. Research topics published on the region are highly varied, with some spatial bias as well as topical bias toward fluvial systems. Yet because of the limited historical work, much is unknown about these unique landscapes and their susceptibility to natural and human disturbances, and much benefit is to be gained from both applied and theoretical study of them.