text: Water flowing through a bend in a river must follow curved streamlines to remain within the banks of the river.  The water surface is slightly higher near the concave bank than near the convex bank.  (The "concave bank" has the greater radius. The "convex bank" has the smaller radius.)  As a result, at any elevation within the river, water pressure is slightly higher near the concave bank than near the convex bank.  A pressure gradient results from the concave bank toward the other bank.  Centripetal forces are necessary for the curved path of each parcel of water, which is provided by the pressure gradient.
  The primary flow around the bend approximates a free vortex – fastest speed where the radius of curvature of the stream itself is smallest and slowest speed where the radius is largest.  The higher pressure near the concave (outer) bank is accompanied by slower water speed, and the lower pressure near the convex bank is accompanied by faster water speed, and all this is consistent with Bernoulli's principle.
  A secondary flow is produced in the boundary layer along the floor of the river bed.  The boundary layer is not moving fast enough to balance the pressure gradient and so its path is partly downstream and partly across the stream from the concave bank toward the convex bank, driven by the pressure gradient.  The secondary flow is then upward toward the surface where it mixes with the primary flow or moves slowly across the surface, back toward the concave bank.  This motion is called helicoidal flow.
  On the floor of the river bed the secondary flow sweeps sand, silt and gravel across the river and deposits the solids near the convex bank, in similar fashion to sugar or tea leaves being swept toward the center of a bowl or cup as described above. This process can lead to accentuation or creation of D-shaped islands, meanders through creation of cut banks and opposing point bars which in turn may result in an oxbow lake. The convex (inner) bank of river bends tends to be shallow and made up of sand, silt and fine gravel; the concave (outer) bank tends to be steep and elevated due to heavy erosion.