Part IV: Glossary

Alluvial fan: A geological formation formed when rivers deposit sediment at the point of transition from relatively fast-moving water to relatively slow-moving. Deltas are one commonly occurring form of alluvial fan and the Mississippi River delta is an example of this kind of geological formation.

Batture: A Cajun French term for the low-lying areas outside of a levee system.

Bayou: A slow-moving waterway usually formed as a distributary of a river system within an alluvial fan.

Birdfoot Delta: The furthest extent of the Mississippi River system where fresh water enters the Gulf of Mexico. This delta is so called because of the resemblance of its distributary system to the foot of a bird.

Channel migration: The movement of a river channel across its flood plain over time, primarily driven by a combination of bank erosion and the formation of sediment bars in the river channel.

Clay: Sediment belonging to the smallest category of grain size, less than 2 microns in diameter. Chemically, clay generally combines various clay minerals and metal oxides.

Delta: A particular type of alluvial fan that occurs at the point where a river enters the sea (or some other large body of still water). Deltas are characterized by river systems with networks of distributary channels and accumulate large amounts of sediment over time.

Dewatering: The shrinking of clay sediments upon drying. Dry clays have significantly less volume when dry than when wet and they expand significant when water is added.

Distributary: A small branching channel of a main river that occurs within an alluvial fan. In the context of river deltas, distributaries are often referred to as bayous.

Erosion: The removal and transportation of rock and sediment by weathering forces, including rain, wind, and the action of colluvial/alluvial systems.

Floodway: A human-made channel that can be opened to allow the flow of water away from a flooding river system.

Glacial period: Also referred to as an “Ice Age,” these are prehistoric periods of relatively cold climate in which much of the Northern Hemisphere is covered by continental glaciers and global sea level was much lower than at present.

Holocene: The current geological epoch, which began around 10,000 years ago, in which global temperatures and sea levels have been relatively high. The Holocene is an example of an interglacial period.

Ice Age, see Pleistocene

Interglacial period: Time periods in which global temperatures and sea levels have been relatively high and relatively little of the Northern Hemisphere has been covered by continental glaciers. The Holocene, which is the current geological epoch, is an example of an interglacial period.

Jetty: A human-made structure extending into the channel of the river to accelerate the flow of water in the channel, scouring away sediment and preventing new sedimentation.

Levee: A human-made bank structure (cf. Natural levee) that acts as a barrier in preventing flooding from the channel of a river or a coastal waterway.

Lower Mississippi Valley: The flat and low-lying lower course of the Mississippi River valley extending from Cape Girardeau, Missouri, to the Gulf of Mexico. The Lower Mississippi Valley formed through the prograding of the Mississippi River delta over millions of years.

Natural levee: An elevated bank structure adjacent to a river channel formed through repeated overbank flooding and the sorting of sediments according to water velocity. Natural levees, such as those in New Orleans, are frequently the locations of human settlement.

Pleistocene: The prehistoric geological epoch, which began around 2.5 million years ago and ended 10,000 years ago, sometimes referred to as the “Ice Age.” During much of this period, large portions of the Northern Hemisphere were covered by continental glaciers, though there were dozens of alternations between glacial and interglacial phases.

Progradation: The formation of new coastal land surfaces through the deposition of sediment within a river delta.

Reclamation: The drainage of wetlands for the purposes of settlement or agricultural activities through the construction of levees and the pumping of water outside of that levee system.

Retrogradation: The loss of coastal land surfaces through sea level rise, coastal erosion, and geological subsidence.

Ridge: An elevated land surface in a coastal wetland, often the remains of a natural levee of a distributary channel (bayou) or a river system.

Ridge restoration: The enhancement of natural ridge features in coastal wetlands through the dredging and/or pumping of sediment and the encouragement of the growth of stabilizing vegetation.

Riprap: The stabilization of coastal land surfaces and protection from erosion through lining with stone, concrete, or other energy-resistant materials.

Saltwater intrusion: The inundation of coastal freshwater habitats with marine saltwater, leading to the death of plant communities and the acceleration of coastal erosion through the loss of anchoring roots systems and vegetation buffers.

Silt: A fine-grained sediment that is smaller than sand particles but larger than clay particles. Like clay, silt remains suspended in flowing water.

Sediment: Particles of stone resulting from the weathering and erosion of bedrock. Sediment can be transported by wind and flowing water, forming various geological formations.

Sedimentation: The accumulation of sediment in a geological deposit. In the context of a delta, sedimentation leads to the formation of new land surfaces.

Spillway, see Floodway

Storm surge: A localized short-term rise in sea level caused by the proximity of a tropical storm.

Subsidence: The sinking of a land surface at a continental margin due to the weight of accumulation sediment, in addition to sediment dewatering. Subsidence causes a relative increase in sea level.

Tributary: A stream that feeds into a larger steam or river, thus increasing the total amount of water in its channel. The Missouri and Ohio Rivers are both large tributaries of the Mississippi River.