#Riffle river series

Human activities, such as channelization or dredging, could modify the riffle–pool forms. Furthermore, riffle–pool morphology is also important in terms of mesoscale habitats, as different physical conditions would determine different habitat type. The morphology of riffle–pool sequences is considered as a fundamental process of channel adjustments in both vertical and planform dimensions. The riffle–pool sequences are commonly placed in both straight and meandering rivers with coarse-grained channels of <2% slope. Riffles represent the topographically shallow section of an undulating channel bed, while pools are the topographically deep areas of the channel bed ( Figure 2A). These vertical undulations in bed elevation are generally referred to as riffle–pool sequences.

#Riffle river series

At the reach scale, the channel thalweg is modified into a series of bathymetric lows and highs.

Īlluvial river channels commonly exhibit a dynamic, three-dimensional variation in the planform, cross-sectional, and longitudinal aspects. In addition, the riffle–pool undulation on thalweg profiles is also likely to be indicative of longer-term morphological adjustment and enables an analysis of downstream trends in the riverbed form resistance. With hydrographic data exceeding a century in timespan recorded by the USACE (from the 1870s to 2013), the LMR is an ideal site to study geomorphological response over decadal scales. Therefore, given the complex morphodynamic behavior in the LMR region, understanding long-term (decadal scale) variations regarding to river thalweg morphology due to anthropogenic disturbance is an important task. However, they also cautioned that a solid conclusion could not be drawn because the comparison of the modeling results is based on a very limited number of field measurements. In a recent modeling study, Wang and Xu found that, proportionally, more riverbed materials were carried downstream in the Mississippi mainstem than to the outflow channel. After the completion of these structures, different studies discussed whether the ORCS blocked sediments and resulted in sediment deficiency, or stabilized the channel gradient and caused riverbed aggradation. Due to preventing the main channel of Mississippi River from suddenly changing its course (avulsion) into the Atchafalaya River, the first phase of the Old River Control Structure (ORCS) was completed in 1963 by the United States Army Corps of Engineers (USACE). Fluvial geomorphological responses to human impacts are highly dynamic and complex, and the lowermost Mississippi River (LMR) is a notable example ( Figure 1). Different human activities, such as channelization, flood control, or dredging for navigation affected several large drainage systems such as the Nile, Mississippi, Indus, Yangtze, and Euphrates rivers. Large alluvial rivers in the world usually have immense socio-economic importance. Future management strategies of this and similar river systems should consider recent riverbed changes in dredging, sediment management, and river engineering. In sum, this study examines decadal-scale geomorphic responses in a low-lying large river system subject to different human interventions, as well as natural flood events. Temporal variations of the same riffles and pools reveal that aggradation and degradation might be heavily controlled by similar factors to the thalweg variations (i.e., sediment supply, backwater effects).

Results from riffle–pool sequences reveal that the averaging W s ratios (length to channel width) are 6–7, similar to numerous previous studies. Furthermore, the change pattern of thalweg profiles appear to be controlled by backwater effects, as well as the Bonnet Carré spillway opening. Different flood events that supplied sediment might be the cause of upstream aggradation from 1963–1975 and net aggradation along the entire reach from 1992–2004. Following the completion of the ORCS, reductions in sediment input to the LMR resulted in net degradation of the thalweg profile patterns since the mid-1960s except for the 1992–2004 period. Extensive aggradation was observed during 1950s to 1960s, as the Atchafalaya River was enlarging before the completion of the Old River Control Structure (ORCS). Historic hydrographic surveys from the mid-1900s to the early 2010s were used to document the thalweg morphology adjustments, as well as the riffle–pool sequences. The lowermost Mississippi River (LMR) is one of the largest deltaic systems in North America and one of the heavily human-manipulated fluvial river systems.