I. Straight channels tend to develop sinuousity. Any perturbation tends to enlarge, either erosional by bank cutting or depositional by formation of bars attached to channel sides. These rivers will meander if flows sufficiently strong and/or bank material sufficiently weak to allow channel migration. So it is hard to get a perfectly straignt channel in nature.

At the same time there is an upper limit on how much sinuousity can occur because if too sinuous meander loops will touch a get cut off (ox bow lakes can form this way). Hence there is a zone, the meander belt or channel belt, along a river valley where the active meandering channel will tend to be found. The channel freely meanders within this zone through time, but the width of the belt is set by the sinuousity of the channel. Over time the channel belt can migrate, if, for example, the channel tends to migrate to the right or left over time, but generally the belt stays more or less fixed until the river avulses, i.e. abandons its channel at a point, during a flood, and after the flood receeds the river follows a new course.

Río Socopo flowing off eastern slope of Venezuelan Andes. View is up river (towards WNW). Octover 24, 1984. Note point bars along inner bends. The channel belt is defined as the region between the red lines wihtin which the active channel tends to meander. Photo from R.H. Meade, USGS

II. Meandering processes and deposits

A. As meander belts migrate they incise along the cut bank on the outside of a bend and deposit a point bar along the inner part of the bend. The point bars are seen in white in the photo above. As the channel continues to migrate, the old position of a point bar is preserved topographically as a system of ridge and swales referred to as scroll bars that can be seen out across modern flood plains and in ancient sedeimtnary deposits (below).

	Side-looking radar (SLAR) image of the flood plain between the Rio Japurá and Rio Solimoes (Amazon River basin), taken in 1971/1972. Flow is towards the lower right. Note the many scroll bars marking the former positions of channels (point bars) across the flood plain. Note these are large rivers with very large meander wavelengths, so that scorsll bars are also very large. SLAR does not see vegetation, overwise this view would only show rain forest.
	Scrolls marks on flooplain between Ríos Apure and Apurito, Orinoco River basin. View towards south, May 13, 1986. Photo by R.H. Meade, USGS
	Scolby Fm., Jurassic, Burniston Wyke, Yorkshire, England. Shows scroll bars (curving in towards the left) on wave cut terrace. Photo by C. Paola, Univ. of Minnesota
	Allen Ridge Sandstone, Cretaceous, central Wyoming. This view, taken on the class field trip shows a series of lateral accretion surfaces formed by migration of a point bar.
	Same photo as above highlighting in yellow and orange individual lateral accretion surfaces and the basal scour (in red) cut as the channel migrated from right to left in this photo.

B. Channel fills tend to fine upward due to decreased flow depth and resultant decrease in shear stress, so that the flow is only capable of carrying finer and finer material as channel depth gets reduced.

C. Levees can build during floods as the river rises, and comes out of its confined channed. As the water flows overbank, there is flow expansion, a reduction in shear stress and any sediment in the flow will start to deposit.

	New overbank sand from the 1993 flood on upper Mississippi River, at Slim Island, MO, at mile 267. River flow is to the upper right. Note how flow expansion leads to build up of levees. Photo by R.H. Meade.
	Guadalope-Mataranya deposits (Oligocene) at Bot, Spain. Sandy channel fill scours into fine grained overbank muds. Off to the left of the channel there is deposition above a marked red paleosol horison. This beige deposit above the paleosol is the levee deposits associated with the scoured channel to the right.

D. At times the levees are breached locally during a flood, a process referred to as a crevasse splay. Water shoots out of this gap and, via flow expansion, slows down and deposits its sediment, referred to as a crevasse splay deposit.

	Missouri River at Stigerse Island, near LEavenworth, KS. Levee break and new overbank sediment from flood of 1993. River flow is toward bottom of page.
	Crevasse splay sand from levee break at Bryants Creek MO crossing floodplain of upper Mississippi River near mile 260. Photo by R.H. Meade, USGS
	Crevasse splay deposits after flood of May, 1978 on Poder River, SE Montana. Photo by R.H. Meade, USGS.
	CLICK HERE (click next page to return) - to go to large version of this figure. Shows an isolith map (in this case contour map of % sandstone seen in well logs). Note high sandstone abundance maps out as a channel and the digitate crevasse splay deposit is not quite as sandy. ABesides sandstone the toerh lithology is overbank mudstone.

E. Fining upwards sequences take place as the channel migrates and is filled in by progressively finer and finer grained sediment.

F. Avulsion - Over long time scales (centuries to thousands of years) river avulsion takes place whereby rivers leave their channel belt at a point, presumably during a flood, and move to another part of the alluvial basin. This results in the abandonment of channel belts. In the rock record this can be seen by abrupt tops of sand bodies, representing the channel belts.

Allen Ridge Sandstone, Cretaceous, along the Platte River, Wyoming. A single storey sandstone lense can be seen in the center of the photo. This represents a channel belt that was abandoned. See version below.

As above, except some of the surfaces are highlighted. The red surface is the scoured base of the sand body, showing the scour into the underlying flood plain (slope former) as the river first avulsed to this spot on the flood plain. The yellow lines are some of the lateral accretion sets formed by migrating bars. Purple is the top of the sand body which was abandoned when the river once again avulsed to some other place on the flood plain.

III. Braided Streams (Click to go to this section).