ABSTRACT
Floodplains are comprised a variety of depositional environments formed by distinct fluvial processes (Fig. 1). Natural levees are formed from numerous flood deposits that create sinuous ridges of coarse sediments along river channels. Sediment sorting during overbank conditions results in lateral fining of flood deposits. This results in a landform that slopes toward lower lying floodplain bottoms, and is the highest component of floodplain topography.[1,2] Understanding natural levees is important to a variety of disciplines, including earth sciences, engineering, and archaeology. Because natural levees are aggregate flood deposits and represent the sediment and stream flow regime of a river over medium time scales (102-3 yr), earth scientists study natural levees for insight into the pattern of flood sedimentation and how it has varied over time. Although natural levees do not represent a significant component of floodplain construction, they may become a ‘‘permanent’’ part of the floodplain in the event of a channel cutoff, and the recognition of buried natural levee deposits is essential to understanding valley-fill chronologies and floodplain evolution.[3] Geotechnical properties of natural levees are important to proper river engineering, particularly design of artificial flood control levees (dikes) and for bank stability structures. Because of their topographic and drainage characteristics natural levees are significant to floodplain hydrology. Indeed, natural levees were utilized by prehistoric civilizations for settlement and agriculture, and archaeologists are interested in active and abandoned natural levees because of their excellent archaeological potential.[4]
LOCATION
Although natural levees are most associated with lowland meandering river floodplains, natural levees form on any type of river floodplain that regularly floods and includes braided, straight, anastomosing, estuarine, crevasse, and deltaic distributary channels. Along active meandering rivers natural levees extend into floodplain bottoms where they overlay older channel deposits or backswamps. Natural levees tend to be
large along cutbanks, but smaller natural levees often form on the inside of meander bends (burying point bar deposits) in rivers with low rates of lateral migration.[5] In non-meandering channels natural levees are approximately the same size on either side of the river.[1]
FORM AND PROCESS
Natural levees are formed by the process of overbank flood sedimentation, but an understanding of natural levee formation should also consider the overall floodplain style and mechanism of floodplain inundation. The thickness of individual flood deposits varies from several millimeters to tens of centimeters, and decreases laterally in thickness and particle size away from the channel bank.[6,7] Natural levee sedimentation involves several processes, which is in part dependent upon the amount of inundation within the floodplain bottoms. Upon exiting the channel and flowing onto the floodplain there is an abrupt reduction in flow velocity, which results in immediate deposition of coarser sand and silt, which is transported along the floodplain surface as bedload.[8] At upper flow regime (high velocity) this results in planar bedding, while at lower flow regime coarse sediment is transported along the surface by saltation (bouncing), forming ripples, and crossstrata. The distance that coarser sediments are transported across the floodplain surface as bedload is not far, perhaps one channel width, in part because floodplain vegetation results in flow resistance. Where overbank flood deposits are transported into floodplain bottoms previously inundated by sources such as crevasse, conduit, groundwater, or local precipitation, natural levee sedimentation is dominated by turbulent diffusive mechanisms (Fig. 1).[9] Essentially, this sedimentation process occurs in floodplains that have flood basins with high water levels adjacent to the channel. During a flood event this produces a steep lateral sediment concentration gradient between the channel and flood basin and sediment quickly falls out of suspension.[10] This mechanism results in steep (high gradient) natural levees.[9] Alternatively, during flood events in which floodplain bottoms are not significantly inundated (water level remains low), advective
K arst-N
euse
sedimentation processes dominate. This process is characterized by flood waters flowing down the levee flank to a low lying bottomland not significantly inundated (water level is low relative to river stage). This process is associated with natural levees having a lower gradient and a curvilinear morphology. At the distant margins of natural levees, slack-water deposition of clay is the dominant mode of sedimentation. In large river valleys the distal portions of natural levees also experience slackwater sedimentation and very low rates of vertical accretion from other mechanisms of floodplain inundation. Crevasse and sloughs connected to the main-stem channel, or smaller tributaries draining adjacent terraces may inundate floodplain surfaces, including the backslope of levee surfaces (Fig. 1). These mechanisms result in very thin clay laminations. Sedimentation rates are influenced by sediment supply and sediment size, as well as flood characteristics such
as frequency, seasonality, duration, and magnitude. The amount of time required for natural levees to form probably requires several hundred to several thousand years and can be investigated by analysis of soils and sediments exposed at channel cutbanks.
