I took in a sunset over the Delaware River from Beverly (I think. I get my river towns confused.) yesterday and low tide happened to coincide with it. Towards the middle of the river I noticed a sandbar which brings my to the titular question. I half remember topographic maps of shorelines and sandbar formation, but I keep getting stuck. I vaguely remember something about currents. Help me out, Dopers.
Slightly related follow-up: a few miles north of sandbar in Beverly is Burlington Island, an island in the middle of the Delaware. Is the island a big sandbar, glacial leftovers, or some combination?
Currents carry sand down-stream. When the current is forced to bend around a curve or some other obstacle, it must slow down. When this happens, the sand its carrying drops out.
Now, there may be more than one way for sandbars to form, but that is deffinitely one such way they can form.
Another way in which they can form is when investors and bar owners recognize that the sand demographic in an area is likely to support a business serving Jell-O shots and imported beer to sand… thus, a sandbar is born.
Let me clean up Trigonal Planar’s start on the formation of point bars, found where streams bend.
Moving water carries clastics, which are fragments of preexisting rocks, downstream. The load carrying capability of the moving water is dependent upon its velocity.
The simple model of point bar formation is that when a stream must round a bend, for all of the water that enters the bend to exit together, that water which must travel the furthest (around the outer edge) must travel faster than that which travels the shorter (inside edge) route. That inner bank water slows and loses some load carrying capability, and sand particles are dropped, adding to the accreting sand bar on the tighter side of the bend.
The more complex model describes the movement of the water as helical, but you still have the lowering of velocity, and subsequent sedimentation, on the inner, shorter, part of the stream’s cut.