Gradation processes and channel evolution in modified West Tennessee streams; process, response, and form

Abstract

Channelization of alluvial channels in West Tennessee has increased energy conditions along main stems and tributaries and initiated systematic trends in channel adjustment. Gradation processes and adjustment trends are a function of the magnitude and extent of an imposed disturbance on a stream channel and the location of the adjusting reach in the fluvial network. Degradation at a site is described by power-decay equations. Exponents denoting the nonlinear rate of downcutting with time decrease with distance upstream from the area of maximum disturbance, and generally range from-0.002 to-0.04. Aggradation occurs in reaches immediately downstream from the area of maximum disturbance and in upstream reaches following overadjustment by degradation, and also can be described by power equations. Aggradation rates increase linearly with distance downstream and reach a maximum of 0.12 meters per year. Adjustment of channel width by mass-wasting processes follows degradation and continues through the aggradational phases. Bank instabilities are induced after downcutting creates bank heights and angles that exceed the critical conditions of the material. Piping in the loess-derived bank materials enhances bank-failure rates. Development of the bank profile is defined in terms of three dynamic and observable surfaces: vertical face (70° to 90°), upper bank (25° to 50°), and slough line (20° to 25°). Both the vertical face and upper bank sections represent major failure planes, and masses of failed bank material often come to rest on the upper bank. The slough line develops from additional flattening and downslope movement by low-angle slides and fluvial reworking, and is the initial site of reestablishing riparian vegetation and stable bank conditions. A six-stage, semiquantitative model of channel evolution in disturbed channels is developed by quantifying gradation trends, by interpreting process-response relations during stages of bank-slope development, and by interpreting changes over space as changes over time.

Disturbance, stream incision, and channel evolution: The roles of excess transport capacity and boundary materials in controlling channel response

“…Entire lengths of trunk streams were channelized in southeastern Nebraska and in West Tennessee. By the 1930s most streams tributary to the Missouri and Mississippi Rivers in the loess area of the midwestern United States had been dredged and straightened ( 1965; Piest et al, 1976;Lohnes et al, 1980;Simon, 1994). In many parts of the region these activities were conducted periodically throughout the 1960s and 1970s as additional tributaries were channelized and because some previously dredged channels filled with eroded sediment from upstream reaches (Lohnes et al, 1980;Simon, 1994).…”

“…Work was undertaken in southeastern Nebraska, Mississippi, West Tennessee and west-central Illinois around 1910 (Moore, 1917, Speer et al, 1965; around 1920 in western Iowa (Lohnes et al, 1980). In many areas, this work increased channel gradients by about an order of magnitude (Moore, 1917;Simon, 1994). Entire lengths of trunk streams were channelized in southeastern Nebraska and in West Tennessee.…”

“…In contrast, smaller tributary streams near the Mississippi River bluff have cut only into loess materials, have silty beds, and no source of coarsegrained material. To reduce erosional forces and stream power for a given discharge without a coarse-grained sediment supply for downstream aggradation, channel widening (stage IV) was the only mechanism for the siltbed streams to recover (Simon, 1994). The silt-bed channels were the deepest and most rapidly-widening channels in West Tennessee and may take hundreds of years to recover.…”

Legacy effects on sediments in river corridors

“…Channelization is now widespread throughout high-income countries and has been done on rivers as diverse as steep, gravel-bed channels of the European Alps (Habersack and Piégay, 2008) (Fig. 5) and lowland, sand-bed channels such as those in Tennessee, USA (Simon, 1994). More than 340,000 km of river channels were channelized during the first 150 years of European settlement in the United States (Schoof, 1980), for example, and channelization along mainstem rivers in Britain varies from 41% in London to 12% in less densely populated regions (Brookes, 1988).…”

Local flood hazards assessed from channel morphometry along the Tisza River in Hungary

“…Thus, the flood hazard is closely related to channel planform and dyke configuration. Downstream aggradation is a common adjustment of a meandering river after channelization (Simon, 1994(Simon, , 1995 and often manifested in lateral accretion Table 1 Locations of water gauges along the Tisza River with basic data (from Lászlóffy, 1982;Bogárdi, 1974;VITUKI, 1990VITUKI, -2005Kovács, 2007 and other water management sources).…”