Fluvial Systems

Meander

Figure 18.34 Meandering channel 

A meander is a bend in a stream. Vertical channel cutting is typical of the early stages of stream system evolution and hence, meandering channel pattern is negligible. However during later stages as base level is achieved and  channel equilibrium is approached, lateral migration of the stream channel is more prevalent. Meanders grow both laterally and in the down stream direction. As water flows into a meander it takes on a helical or spiral flow which determines where erosion and deposition is concentrated. Centrifugal force draws water toward the outside bank (cut bank) causing erosion. Sediment eroded from the outside bank is deposited on the inside bank and transported downstream.

Point Bar

Figure 18.35 Point bars (white) and Bar and Swale Topography on an meandering channel (Courtesy USGS DDS21) Click mage to enlarge

As  water rounds a meander, the water swings toward the outside bank where erosion is concentrated and then spirals toward the inside banks. As the water spirals toward the inside of the meander it is slowed by frictional drag imposed by the bed of the channel. This causes deposition of alluvium on the inside bank to form a bar. A point bar forms on the inside bank of a meander and rising from the channel as an accumulation of alluvium. As the channel meander continues to erode laterally, a succession of bars with intervening swales form called bar and swale topography. 

Neck & Cutoff

A neck is the upland between opposing meanders of a stream. A cutoff occurs when the neck between river meanders is eroded away and the meanders join to shorten the length of the channel. The slope of the channel increases as well when the river shortens its length.

Mark Twain aptly described the process and effect of river cutoffs when he wrote:

"The Mississippi is remarkable in another way--its disposition to make prodigious jumps by cutting through narrow necks of land, and thus straightening and shortening itself. More than once it has shortened itself thirty miles at a single jump! These cut-offs have curious effects: they have thrown several river towns into the rural districts, and built up sand bars and forests in front of them. The town of Delta used to be three miles below Vicksburg: a recent cutoff has radically changed the position, and Delta is two miles above Vicksburg."

~ Life on the Mississippi ~

Oxbow lake & meander scar

Figure 18.36 Oxbow lake (Courtesy USGS)

A river cut-off results in a portion of the river isolated from the new channel called an oxbow lake.   Oxbow lakes are typically crescent shaped - like that of an oxbow. Groundwater seeping into the oxbow maintains the lake. Some oxbows will drain or silt up due to deposition during floods. The remnants of the oxbow is identified as a meander scar.  Wetland and marshes are often found in the scar. 

Cut-off and Oxbow Formation on a Stream Table (Courtesy stevekny)
(A stream table is used to model and simulate stream flow.)

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