Most approaches to interpreting alluvial records in drylands fall into one of two categories: (1) The "arroyo problem," wherein workers study cycles of streambed aggradation and degradation in broad, unconstricted alluvial valleys; and (2) paleoflood hydrology, where alluvial sequences in constricted bedrock canyons are interpreted as paleoflood deposits from streams with stable channel grade and geometry. Both approaches can be valid in their end-member settings, but there is confusion about how the two record types relate in a single drainage. We address this disconnect in Buckskin Wash, an ephemeral stream that consists of a broad alluvial reach draining into a tightly constricted slot canyon. By employing detailed sedimentology, stratigraphy, and geochronology in both the alluvial and constricted reaches of the watershed, we test the hypothesis that the slot canyon deposits are anticorrelated to valley-fill deposits upstream, implying that arroyo cutting is driven by episodic flooding. Alluvial reach deposits are characterized by stratal packages representing incremental, long-term aggradation bound by erosion surfaces representing channel entrenchment. At least four packages younger than ~3 ka are present, the youngest spanning ~0.7 - 0.15 ka. Each is composed of interfingering imbricated gravels, laminated sands, and massive silty clays. Constricted reach deposits consist of five discrete packages, each composed of tabular beds of laminated silty sand that were deposited relatively rapidly. The oldest package dates to ~1.9 - 1.1 ka, whereas the rest of the deposits are younger than ~0.15 ka. Traditional paleoflood techniques would suggest that the constricted reach deposits record a ~1000-year absence of paleofloods followed by ~100 years of frequent, high-magnitude flooding that indeed correlate to arroyo cutting upstream. We argue instead that the constricted reach deposits record an episode of higher preservation potential. Transport of sediment from the alluvial reaches during historic arroyo cutting likely led to a pulse of sediment storage and changed stage-discharge relations in the slot canyon downstream, allowing even moderate floods to overtop existing deposits and be preserved. This new interpretation suggests that, because preservation may be a function of episodic sediment loading from upstream, constricted-reach deposits may not accurately record the paleoflood history of a stream.

The Mississippi Alluvial Valley (MAV) contained vast bottomland hardwood forests into the 20th century. Humans cleared forests, and altered hydrology, yet the MAV remains important for North American waterfowl and other wildlife. To estimate standing crops of aquatic invertebrates as food in hardwood bottomlands for wintering waterfowl, I quantified dry mass of invertebrates in naturally flooded forests (NFFs) and greentree reservoirs (GTRs) during winters 2008–2010. The MAV had greater invertebrate mass in NFFs (x̄ = 18.39 kg/ha; SE = 2.815 [CV = 15.3%]) than GTRs (x̄ = 5.16; SE = 0.492 [CV = 9.5%]), compared with lesser masses in Mississippi Interior Flatwoods’ GTR (x̄ = 2.26; SE = 0.320) and NFF (x̄= 1.45; SE = 1.305). Invertebrate diversity was greatest in NFFs and in depths from 10–40 cm. Flooding GTRs ≤ 40 cm and managing naturally dynamic hydrology may benefit invertebrates, ducks, and associated bottomland hardwood communities

Winter fallow corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production systems are susceptible to erosion and agrochemical transport. This research determined the effects of Cover Crop Minimum Tillage (CCMT) on erosion and agrochemical transport from corn-soybean rotations at field scale, while assessing impacts to agroeconomics and irrigation in Mississippi’s Delta Region. CCMT did not affect total suspended solids (p = 0.53), total inorganic phosphorus (TIP) (p = 0.30), or total nitrogen (TN) (p = 0.25) loads, but did reduce TIP (p = 0.018), TN (p = 0.011), and nitrate-nitrite (p = 0.007) concentrations. An economic loss of $281/ha with no effect on yield (p = 0.09), irrigation use efficiency (p = 0.38), or consumptive water use (p = 0.83) was observed. CCMT will not improve profitability of corn-soybean rotations in the Delta and transitioning from fallowing to CCMT will have varying effects on erosion and agrochemical transport.

Waste rice in harvested fields and natural seeds in moist-soil wetlands are important foods for waterfowl in the Mississippi Alluvial Valley (MAV). Waste rice declines significantly between harvest and late autumn. I conducted experiments in 19 rice fields in Arkansas and Mississippi during autumns 2003 and 2004 to evaluate the ability of common post-harvest practices to conserve waste rice between harvest and late autumn. Greatest mean abundance of waste rice in late autumn occurred in standing stubble (x&95 = 105 kg/ha; CL = 72.84, 150.16 kg/ha) followed by burned (x&95 = 72 kg/ha; 49.57, 105.81 kg/ha), mowed (x&95 = 67 kg/ha; 46.65, 97.42 kg/ha), rolled (x&95 = 51 kg/ha; 35.54, 73.076 kg/ha), and disked stubble (x&95 = 48 kg/ha; 33.26, 68.41 kg/ha). I recommend leaving stubble or burning fields to create interspersion of stubble and water after flooding. Additionally, I estimated abundance of moist-soil seed in 72 management units on public lands throughout the MAV for autumns 2002-2004. Mean seed abundance was 496 kg/ha (SE = 62). I recommend increased active management of moist-soil wetlands to mitigate decreased availability of waste rice in the MAV.

Mammalian herbivory of bottomland hardwood seedlings has been listed as one of the primary causal factors of failed afforestation efforts in the Lower Mississippi Alluvial Valley (LMAV). This study examined the biological and economic effects of mammalian herbivory on recently afforested carbon sequestration sites in the LMAV. Selected seedlings of six planting mixes were observed through the first year following planting to monitor seedling survival, growth, and mammalian herbivory. It was determined that greater than 10% of selected seedlings were browsed by various mammalian herbivore species, and some species mixes were browsed in excess of 50%. Financial analyses compared alternative afforestation strategies and determined to what extent herbivore-induced seedling mortality could reduce investment returns of landowners engaged in afforestation activities. In the presence of extreme mammalian herbivory, landowner returns can be reduced by hundreds of dollars per acre and could prevent further afforestation activities in the LMAV.

Sediment pulses are defined as large amounts of loose sediment that are suddenly deposited in river corridors due to the action of external factors or processes of natural or anthropogenic origin. Such factors and processes include landslides, debris flows from tributaries, volcanic eruptions, dam removal projects, and mining-related activities. Their occurrence is associated with a surplus in sediment load to downstream reaches, and therefore, with severe channel aggradation and degradation, significant floodplain deposition, increase in flood frequency, damage of infrastructure, and impairment of aquatic habitats. The main objective of this research is to develop a better understanding of the fundamental mechanisms that govern the propagation of these sediment-flow hazards in alluvial sand-bed rivers. Specifically, the study presented herein is divided into three separate parts to achieve this overarching goal. First, a component intended to improve the numerical modeling of morphodynamic processes in alluvial sand-bed rivers by proposing a novel solution methodology that applies either the decoupled or the coupled modeling approach based on local flow and sediment transport conditions. Secondly, a detailed numerical analysis to characterize the behavior of fine-grained sediment pulses (i.e. composed of granular material in the sand size range) in alluvial sand-bed rives by identifying the properties of these types of pulses, as well as the characteristics of riverine environments, that are most relevant to their downstream migration. And lastly, a case study application to assess the effect of the magnitude, duration, and frequency of severe hydrologic events on the overall propagation behavior of fine-grained sediment pulses in alluvial sand-bed rivers. Ultimately, this research aims to contribute towards reducing the uncertainty associated with the impact of these phenomena, and hence, improving the resilience of rivers corridors. / Ph. D.

Alluvial fan surfaces in the Atacama Desert of northern Chile preserve evidence of recent, precipitation-driven, surface flows. Determining the hydrologic characteristics of these flows is important for understanding the effects of rare yet significant storms in the region. Flow reconstruction, runoff analysis, and comparison with climatological data yield surface activation recurrence intervals of ~1-20 years for three small fans and associated catchments proximal to Iquique and Antofagasta. Relatively short-lived and intense precipitation events (1-3 hour, > 4 mm/hr) are required to mobilize and transport the largest surface grains. Modeled discharges provide minimum constraints on the rates of precipitation that yield surface-forming flows in the hyper-arid region. The results of this study aid in understanding the evolution of various surfaces in the region. In particular, results provide a clear indication of the ability of a particular storm event --i.e., precipitation rate to activate a surface. / Master of Science

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Le but de ce travail est de tester une approche multi-traceurs permettant de caractériser l'origine (éléments majeurs, Sr, Br, 87Sr/86Sr, δ18O, δ2H) et les temps de résidence (3H/3He, CFC, SF6) des eaux, pour identifier l'origine et évaluer le devenir des contaminants dans une nappe alluviale superficielle et peu profonde, la nappe de la Vistrenque. Les molécules étudiées sont les triazines, le métolachlore, le diuron, la carbamazépine, le sulfaméthoxazole, le diclofénac et l'ibuprofène. L'aquifère étudié est alimenté par une recharge directe, occasionnant le lessivage des pesticides des sols, et une recharge latérale provenant de l'aquifère karstique adjacent entraînant une dilution des eaux de la nappe en triazines. Localement, une contribution des eaux de surface (cours d'eau locaux, eau importée du Rhône) à la recharge de la nappe est mise en évidence entraînant également une dilution des eaux de la nappe en triazines mais au contraire une contamination en COE, quoique, en faibles concentrations. Les âges apparents des eaux alluviales échantillonnées, déterminés principalement par le couple 3H/3He, varient entre 1.4 et 22 ans. Le couplage de l'âge des eaux à leurs teneurs en triazines montre une persistance de ces molécules dans le milieu souterrain, et une atténuation de leur signal d'entrée soulignant l'efficacité de leur interdiction en 2003. Finalement, les eaux de la nappe alluviale montrent des rapports de dégradation des triazines variant entre 0,3 pour les eaux influencées par les eaux de surface et 4,8 pour celles montrant des âges apparents de 22 ans, suggérant une augmentation de ce rapport avec le temps de transfert des pesticides dans le système. / The aim of this work is to test a multi-tracer approach allowing the characterization of groundwater origin and residence time in a shallow alluvial aquifer, the Vistrenque aquifer, in order to identify the origin and the fate of contaminants therein. The selected compounds for the study are triazines, metolachlor, diuron, carbamazepine, sulfamethoxazole, diclofenac, and ibuprofen. The studied aquifer is mainly fed by 1) a direct recharge inducing pesticides leaching from soil layers and unsaturated zone and 2) by a lateral recharge from the karst adjacent aquifer, which induces triazines dilution in the alluvial aquifer. A local contribution of surface water (local streams and imported Rhône River water) was evidenced in the alluvial groundwater inducing also triazines dilution but EOCs contamination nevertheless at low concentrations. The apparent age of the alluvial groundwater samples varies between 1.4 and 22 years. Linking groundwater age to triazines contents allowed to highlight, first, the persistence of these compounds in the alluvial groundwater and, second, the decreasing of their input signal in relatively recent groundwater samples in accordance with their forbidding in 2003. Finally, the Vistrenque alluvial groundwater showed triazines degradation ratios varying from 0.3 for groundwater influenced by surface water infiltration to 4.8 for groundwater characterized by relatively older apparent residence time on the order of 22 years. This suggests an increasing ratio with the transfer time of these compounds in the alluvial aquifer system.