Braid bar

Braid bars, or mid-channel bars, are river landforms typically present in braided river channels.  These formations have many names, including medial, longitudinal, crescentic, and transverse bars, as well as the more colloquial sandflat.  Braid bars are distinguished from point bars due to their presence in the middle of a flow channel, rather than along a bank of the river channel.
Formation.
Braid bars often originate from remnants of point bars or the growth of mid-channel unit bars in braided rivers. These features typically form in rivers with a high sediment load, within channels characterized by a large bed load and easily-eroded bank material.  There are several mechanisms of formation.  One explanation is that flow in the river may be redirected over a point bar, wherein the bar may be separated from the bank and thus become isolated within the channel.  This forms a small ‘island’ of sediment, which can evolve into a braid bar.  Alternatively, an obstacle such as a wedged log can result in the formation of a mid-channel bar if sediment is deposited along the feature.
As sediment is transported downriver, material can alternatively accrete onto the existing bar or erode away. Therefore, such features tend to be ephemeral and fluid, and can change shape quite frequently. Braided rivers can have variable, erratic flows, which can lead to successive periods of greater deposition or erosion.  Entire bars can be created, destroyed, or migrate across a channel over the span of several years.
These formations are often composed of poorly-sorted sand and/or gravel.  The generally coarse material is the result of the processes common to braid bar formation, and more broadly, braided rivers.  Braided rivers are defined by relatively high flow power, as well as significant rates of erosion.  These high rates of flow will transport fine-grained materials further downstream, while coarser material remains near the eroded bank and is eventually deposited onto mid-channel bars.
Growth and Evolution.
There are 6 stages for braid bar growth and morphological evolution.
Classification.
There is some fluidity in the distinction between braid bar and island.  Compared to a braid bar, an island is regarded as having a more permanent and potentially larger structure that can influence, to some degree, the path of the river.  Vegetation can be present on a braid bar, but is suggested that, on islands, vegetation has a significant stabilizing impact. This is in part due to the nature of many braided rivers, where variable flow discharge can create a hostile environment for any significant vegetation. In addition, the elevation of the island in regards to bankfull discharge should also be considered; an island will likely not be entirely submerged when bankfull discharge occurs, whereas a braid bar may be.